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123 E. Quaia (ed.), Radiological Imaging of the Kidney, Med Radiol Diagn Imaging, DOI 10.1007/978-3-642-54047-9_5, © Springer-Verlag Berlin Heidelberg 2014 Abstract The improvement of computed tomography (CT) imaging technique, especially with the advent of multidetector CT scanners (MDCT), has deeply influenced the assessment of the urinary system. MDCT allows larger volume cover- age in shorter scan times, submillimetric slice thickness with an upgraded longitudinal resolution along the z-axis, and higher contrast and spatial resolution. Several CT protocols have been developed in order to define the different aspects of renal and urinary pathologies. Unenhanced CT is principally used for the detection of urinary stones, hemorrhage, calcifications and, as precon- trast scan, to quantify contrast enhancement in renal tumors. Multiphasic CT is employed in the detection and characterization of renal tumors. CT angiography repre- sents the first-line imaging technique in the assessment of renal vasculature. Finally, CT urography has now replaced excretory urography in the assessment of renal and extra- renal urinary tract. 1 General Concepts A. Rossi, A.G. Gennari, R. Angileri, P. Martingano, M. Cavallaro, and E. Quaia Since its introduction in the early 1970s, computed tomogra- phy (CT) has undergone tremendous improvements in tech- nology, performance, and clinical applications. CT has evolved from conventional to spiral and, lastly, to multide- tector technology which allows faster scanning and isotropic spatial resolution. Nowadays, multidetector-row CT (MDCT) is considered a well-established modality for the study of the urinary tract allowing a detailed evaluation of the kidneys, renal vessels, pelvis, calyces, ureters, peri- and pararenal spaces, and surrounding organs. Computed Tomography Alexia Rossi, Antonio Giulio Gennari, Roberta Angileri, Paola Martingano, Marco Cavallaro, Roberto Pozzi-Mucelli, Giulia Zamboni, Livia Bernardin, Alberto Contro, and Emilio Quaia A. Rossi (*) • A.G. Gennari • R. Angileri P. Martingano • M. Cavallaro • E. Quaia (*) Department of Radiology, Cattinara Hospital, University of Trieste, Strada di Fiume 447, 34149 Trieste, Italy e-mail: [email protected]; [email protected] R. Pozzi-Mucelli • G. Zamboni L. Bernardin • A. Contro Department of Radiology, G.B. Rossi Hospital, University of Verona, Piazzale L.A. Scuro 10, 37134 Verona, Italy Contents 1 General Concepts ............................................................... 123 1.1 Technical Developments of MDCT ..................................... 124 1.2 Technical Parameters ........................................................... 125 1.3 Unenhanced and Contrast-Enhanced CT ............................. 131 1.4 CT Nephrogram Alterations................................................. 144 2 Multidetector CT Urography and CT Angiography ...... 149 2.1 Multidetector CT Urography ............................................... 149 2.2 CT Angiography .................................................................. 160 2.3 Dual-Energy CT ................................................................... 160 References .................................................................................... 163

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Page 1: [Medical Radiology] Radiological Imaging of the Kidney || Computed Tomography

123E. Quaia (ed.), Radiological Imaging of the Kidney, Med Radiol Diagn Imaging,DOI 10.1007/978-3-642-54047-9_5, © Springer-Verlag Berlin Heidelberg 2014

Abstract

The improvement of computed tomography (CT) imaging technique, especially with the advent of multidetector CT scanners (MDCT), has deeply influenced the assessment of the urinary system. MDCT allows larger volume cover-age in shorter scan times, submillimetric slice thickness with an upgraded longitudinal resolution along the z-axis, and higher contrast and spatial resolution. Several CT protocols have been developed in order to define the different aspects of renal and urinary pathologies. Unenhanced CT is principally used for the detection of urinary stones, hemorrhage, calcifications and, as precon-trast scan, to quantify contrast enhancement in renal tumors. Multiphasic CT is employed in the detection and characterization of renal tumors. CT angiography repre-sents the first-line imaging technique in the assessment of renal vasculature. Finally, CT urography has now replaced excretory urography in the assessment of renal and extra-renal urinary tract.

1 General Concepts

A. Rossi, A.G. Gennari, R. Angileri, P. Martingano, M. Cavallaro, and E. Quaia

Since its introduction in the early 1970s, computed tomogra-phy (CT) has undergone tremendous improvements in tech-nology, performance, and clinical applications. CT has evolved from conventional to spiral and, lastly, to multide-tector technology which allows faster scanning and isotropic spatial resolution. Nowadays, multidetector-row CT (MDCT) is considered a well-established modality for the study of the urinary tract allowing a detailed evaluation of the kidneys, renal vessels, pelvis, calyces, ureters, peri- and pararenal spaces, and surrounding organs.

Computed Tomography

Alexia Rossi, Antonio Giulio Gennari, Roberta Angileri, Paola Martingano, Marco Cavallaro, Roberto Pozzi-Mucelli, Giulia Zamboni, Livia Bernardin, Alberto Contro, and Emilio Quaia

A. Rossi (*) • A.G. Gennari • R. Angileri P. Martingano • M. Cavallaro • E. Quaia (*) Department of Radiology, Cattinara Hospital, University of Trieste, Strada di Fiume 447, 34149 Trieste, Italye-mail: [email protected]; [email protected]

R. Pozzi-Mucelli • G. Zamboni L. Bernardin • A. ControDepartment of Radiology, G.B. Rossi Hospital, University of Verona, Piazzale L.A. Scuro 10, 37134 Verona, Italy

Contents

1 General Concepts ............................................................... 1231.1 Technical Developments of MDCT ..................................... 1241.2 Technical Parameters ........................................................... 1251.3 Unenhanced and Contrast-Enhanced CT ............................. 1311.4 CT Nephrogram Alterations................................................. 144

2 Multidetector CT Urography and CT Angiography ...... 1492.1 Multidetector CT Urography ............................................... 1492.2 CT Angiography .................................................................. 1602.3 Dual-Energy CT ................................................................... 160

References .................................................................................... 163

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1.1 Technical Developments of MDCT

MDCT involves the use of a multiple-row detector array instead of the traditional single-row detector array used in single-slice spiral CT (Coppenrath and Mueller-Lisse 2006).

In MDCT scanners, each individual detector is seg-mented in the z-axis direction. Different section width must be available to adjust the optimum scan speed, longitudinal resolution, and image noise for each application. With a single-slice CT detector, different collimated slice widths are obtained by prepatient collimation of the X-ray beam (Flohr et al. 2005). For a very elementary model of a two-slice CT detector consisting of m = 2 or two detector rows, different slice widths can be obtained by prepatient colli-mation if the detector is separated midway along the z-extent of the X-ray beam (Flohr et al. 2005). For m > 2, this simple design principle must be replaced by more flex-ible concepts requiring more than m detector rows to simul-taneously acquire m slices. Different manufacturers of MDCT scanners have introduced different detector designs (Flohr et al. 2005). Different slice widths can be obtained combining several detector rows electronically to a smaller number of slices according to the selected beam collima-tion and the desired slice width. The fixed array detector consists of detector elements with equal size in the longitu-dinal direction. The adaptive array detector comprises detector rows with different sizes in the longitudinal direc-tion, so different slice widths may be obtained by appropri-ate combinations of the detector rows. Dual-source CT (DSCT) has been recently introduced. It is equipped with two X-ray tubes and two corresponding detectors; using the z-flying spot technique, each detector acquires 64 overlap-ping 0.6 mm slices per rotation. The main advantage of DSCT is the improved temporal resolution which is partic-ularly useful in cardiac imaging. In abdominal applications, DSCT is beneficial for the examination of obese patients; indeed, its power reserve reaches 200 kW when both acqui-sition systems are simultaneously used. Additionally, both X-ray tubes can be operated at two different kV and mA settings, allowing the acquisition of dual-energy datasets (Flohr et al. 2006).

Technical specifications of different CT scanner are reported in Table 1.

Consequently MDCT allows larger volume coverage in shorter scan times and thinner slices with an improved lon-gitudinal resolution along the z-axis (patient axis). Subsecond gantry rotation and large detector size permit rapid volume coverage, not only facilitating multiphase scanning with short breath-holds but also minimizing respi-ratory motion artifacts. Due to short scan times, the kidneys can be depicted in well-defined (dynamic) phases of con-trast enhancement, and renal lesions can be characterized more precisely. Thinner slices allow better spatial resolu-tion. Nevertheless, it is advisable to choose a reconstruc-tion thickness of 3–5 mm as a compromise between spatial resolution and contrast-to- noise ratio. On the basis of radiological findings, reconstructions in other planes and slice thickness, down to the submillimeter range, can be added (depending on scanner type and number of detector rows available).

Narrow collimation, used in most of MDCT scanners, produces isotropic voxel, so the arbitrarily reconstructed planes come close to the image quality in the original scan plane. Thanks to all these improvements, MDCT has further increased the efficacy of CT technique and expanded its applications in the urinary tract. Among these new applications, the evaluation of the urinary tract – MDCT urography – is the most innovative.

CT has been used to effectively evaluate many urinary tract disorders including characterization of renal masses that are indeterminate or suspicious at US, cystic lesions (Bosniak 1991; Curry 1995; Silverman et al. 2008), tumors, pseudotumors, calcifications, urinary tract calculi (Primak et al. 2007; Türkbey et al. 2010), genitourinary trauma (Goldman et al. 2004; Srinivasa et al. 2009), arteriovenous malformations, and renal infection (Dalla Palma et al. 1997). Indications for CT investigation of the kidney include uroli-thiasis, tumor diagnosis and staging, renal trauma, and vas-cular disease. In children, special indications for CT of the kidney are polytrauma and tumor staging. MDCT of the kidney has become a very valuable tool in urology, but a careful protocol strategy is mandatory.

Table 1 Technical specifications of different MDCT scanners

CT scanner 4-slice CT 16-slice CT 64-slice CT 256-slice CT 320-slice CT Dual-source CT

Detector collimation (mm) 4 × 1 16 × 0.5 64 × 0.5 2 × 128 × 0.6* 320 × 0.5 2 × 32 × 0.6*; 2 × 64 × 0.6*4 × 1.25 16 × 0.625 64 × 0.65

16 × 0.75 2 × 32 × 0.6*

Slice width (mm) 1.3 0.8–1.0 0.5–0.8 0.6 0.5 0.6Spatial resolution (mm) 1 0.6 0.33–0.60 0.4 0.35 0.30–0.33Gantry rotation time (s/rotation) 0.5 1.375–0.420 0.28–0.40 0.27 0.35 0.27–0.33*Double z-sampling

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1.2 Technical Parameters

1.2.1 ImagingWith the advances in CT technology, the imaging techniques to evaluate the kidneys and urinary tract have progressively changed. State-of-the-art X-ray CT tube/generator (Fig. 1a, b) combinations provide a peak power of 60–120 kW, usu-ally at various user-selectable voltages, for example, 80, 100, 120, and 140 kV. The spatial resolution of the CT images may achieve 24 lines per cm with an image matrix size of 512 × 512, 768 × 768, or 1,024 × 1,024 pixels. Modern MDCT systems use solid-state detectors (Fig. 1c, d) consisting of a radiation-sensitive solid-state material (cadmium tungstate, gadolinium oxide, or gadolinium oxysulfide with suitable dopings), which converts the absorbed X-rays into visible light. The light is then detected by a Si photodiode. The resulting electrical current is amplified and converted into a digital signal. CT detectors must provide different slice widths to optimize scan speed, longitudinal resolution, and image noise for each application.

Nowadays, the visualization of the acquired images at the workstation is supported by simple visualization inter-faces and the radiologist can review the images on axial, coronal, sagittal, and multiplanar reformation (MPR), which describes the option of arbitrary plane reconstruction from volumetric datasets. These planes can be chosen in an orthogonal plane (sagittal, coronal), in an oblique plane, or even in a curved-planar reconstruction (e.g., for course of vessels). Coronal images are useful in the evaluation of the kidney because they can be depicted in one single image rather than in multiple axial sections. Although normal coronal images cannot depict the entire ureter in a single image, the evaluation of the urinary tract is, anyway, sim-ple. Moreover, the coronal plane is suitable for reconstruc-tion of an “in situ” perspective resembling the view of the abdominal or urologic surgeon. Maximum intensity projec-tion (MIP) images are more indicated in the evaluation of the urinary tract, in relation with its opacification with con-trast media. MIP depicts the structures of highest CT den-sity within a volume of interest. Volume rendering technique

X-ray tube

Collimator

Detector

Detector tilex

z

High-voltagegenerator

Bi-directionalanti-scatter gird

To the focal spot

a

c d

b

Fig. 1 (a–d) Geometry of the X-ray tube scanning in the multidetector CT (MDCT). (a) Modern 256-row MDCT scanner; (b) basic system components of a modern MDCT scanner. (c) MDCT detector array with evidence of a single detector structure with bidirectional

anti- scatter grid; x corresponds to the rotating axis; z corresponds to the patient long axis. (d) A last generation detector with bidirectional anti-scatter grid (Courtesy of Philips)

Computed Tomography

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(VRT) is an image-processing option that emphasizes regions of selected CT density range, thereby accentuating specific tissues or organs.

Image quality of any given CT system is determined by a number of scanning parameters that the user can modify: (a) collimation, (b) pitch (Fig. 2), (c) reconstruction interval, and (d) timing of scanning after contrast injection. Optimizing these parameters to achieve the best possible image quality for a specific MDCT examination requires thorough understand-ing of their interrelationships. Collimation is related to spatial resolution, image noise, and length of coverage. Spatial reso-lution may be increased by means of a decrease in the collima-tor width, but decreased collimator width also results in increased image noise and decreased length of coverage. The reduced coverage encountered with narrow collimation, how-ever, can potentially be overcome with an increase in pitch.

The effective detector row thickness is simply the sum of the widths of the contributing detector rows for each chan-nel. In multidetector-row CT scan acquisitions, the effective detector row thickness of all channels must be identical. Effective detector row thickness is an important parameter because the reconstructed section thickness cannot be smaller than the effective detector row thickness. The term detector configuration describes a given scan acquisition mode in terms of the number of z-axis data channels being used and the effective detector row thickness of each data channel (Saini 2004). The beam collimation is simply the product of the number of data channels being used and the effective detector row thickness (Saini 2004).

In single-slice helical CT, pitch (p) is defined as the table feed during a 360° rotation of the tube-detector apparatus (mm/rotation) divided by the collimator width in millimeter (collimated beam width). It shows whether data acquisition occurs with gaps (p > 1) or with overlap (p < 1) in the longitu-dinal direction. In MDCT the beam collimation is different from the slice collimation, and the beam collimation depends on the number of detector rows. In particular, in MDCT there

are two distinct versions of pitch, depending on whether the entire beam collimation is considered or the “detector” colli-mation (Fig. 2) (i.e., beam/4 in 4-detector-row MDCT) is used. The selection of pitch for MDCT is affected by several factors such as the required length of coverage, reconstructed section thickness, and image noise. For a given duration of scanning, minimization of collimation (thus, an increase in pitch) will allow a narrow reconstructed section thickness and potentially improve spatial resolution, particularly in nonax-ial planes. However, maximization of pitch may also result in a decrease in contrast resolution. Thus, the most appropriate choice of scanning parameters depends on the imaging prob-lem under consideration. For example, since CT angiography (CTA) allows imaging of structures with very high attenua-tion against a background of much lower attenuation (i.e., high inherent contrast), the loss of contrast resolution caused by maximization of pitch can be disregarded. The smaller the reconstruction interval, the greater the longitudinal (z-axis) resolution, with a resultant loss in z-axis coverage. If multi-planar reconstructions will be required, a small reconstruc-tion interval with overlapping sections is advantageous. With the evolution of CT from single-slice spiral CT to MDCT, the collimation has been reduced from 3–5 mm to 0.5–1 mm of the most recent 64-row MDCT scanner (0.5–0.625 mm). A decrease in pitch from 1–1.5 to 0.75–0.80 has also been achieved. As a consequence, the thickness of the recon-structed images has been decreased from 5 to 0.5 mm. The final result is the “isotropic voxel,” which provides coronal, sagittal, and oblique multiplanar reformats with the same resolution of the original axial CT sections. A further advan-tage of state-of-the-art MDCT is the short scan time which varies from 0.27 to 0.5 s for a single 360° rotation.

MDCT examination can be performed before and after intravenous contrast administration. Breath-hold images that are essentially free from motion artifact and respiratory mis-registration can be quickly obtained during multiple phases of renal enhancement. The preliminary scan without intravenous

a bBeam collimation

Table feed

Fig. 2 (a) Scanning geometry of the fan beam produced by the MDCT scanner (courtesy of Toshiba). (b) The concept of pitch that corre-sponds to the table feed during a 360° rotation of the tube-detector apparatus divided by the collimator width. For single detector computed

tomography (CT) scanners, the pitch corresponds to the table feed/col-limation. For MDCT, there are two distinct versions of pitch, depending on whether the detector collimation (i.e., beam/4 in 4 detector row mul-tislice CT) or the entire beam collimation is considered

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contrast administration should be performed also without administration of hyperdense oral contrast agents (e.g., Gastrografin), because these might impair visualization of the kidneys or the ureters in MIP images. The timing of peak aortic and hepatic contrast enhancement is primarily dependent on the rate of injection. Rapid or low-volume (shorter- duration) injections produce earlier peak enhance-ment, whereas slow or high-volume (longer-duration) injec-tions result in later peak enhancement. These factors must be taken into account before a fixed scanning delay is instituted. A preliminary minibolus (5 mL/s for 4 s), with scanning every 2 s beginning 10 s after the injection is started, can be used. The time to peak aortic enhancement is determined from the resultant time attenuation curve and is used to calcu-late the scanning delay. The scanning delay can also be accu-rately timed with a bolus tracking software program.

1.2.2 ArtifactsIn CT, the term artifact is applied to any systematic discrep-ancy between the CT numbers in the reconstructed image and the true attenuation coefficients of the object.• Beam hardening: an X-ray beam is composed of individ-

ual photons with a range of energies. As the beam passes through an object, its mean energy increases (harder X-ray beam), because the lower-energy photons are absorbed more rapidly than the higher-energy photons (Barrett and Keat 2004). Two types of artifact can result from this effect: the cupping artifact and the appearance of dark bands or streaks between dense objects in the

image. Cupping artifact manifests when the X-rays pass-ing through the middle portion of a uniform cylindrical phantom are hardened more than those passing through the edges because they are passing through more material. As the beam becomes harder, the rate at which it is attenu-ated decreases. So the beam is more intense than expected when it reaches the detectors. Therefore, the resultant attenuation profile differs from the ideal profile that would be obtained without beam hardening. Dark bands or streak artifacts can appear between two dense objects in an image. They occur because the portion of the beam that passes through one of the objects at certain tube posi-tions is hardened less than when it passes through both objects at other tube positions. This type of artifact can occur both in bony regions of the body and in scans where a contrast medium is used (Barrett and Keat 2004).

• Renal cyst pseudoenhancement: renal cyst pseudoen-hancement refers to the artifactual increase in the attenu-ation (>10 HU) of simple intrarenal cysts surrounded by normally enhancing renal parenchyma, following contrast material administration, even when the effects of partial volume averaging have been removed (Prasad et al. 2008). Pseudoenhancement is a consequence of overcorrection for beam-hardening effects of the enhanced renal paren-chyma combined with artifact introduced by the CT image reconstruction algorithm (Abdulla et al. 2002; Coulam et al. 2000). This effect is more pronounced at high levels of renal parenchymal enhancement (Fig. 3) in small intrarenal lesions <2 cm because volume averaging

a b

Fig. 3 (a, b) Pseudoenhancement in a small cyst (arrow) of the upper renal pole of the left kidney. Seventy-year-old woman with incidental renal lesion. (a) Unenhanced CT image (collimation, 0.5 mm) shows a 1-cm lesion isodense partially intraparenchymal lesion measuring

20 HU. (b) Nephrographic phase (collimation 0.5 mm, beam pitch 1) of the same lesion in (a). Enhanced attenuation was 50 HU (pseudoen-hancement 30 HU). Lesion revealed a cystic pattern on US and was stable for more than 3 years on CT follow-up (not shown)

Computed Tomography

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and beam-hardening effects have a higher statistical impact on the measurements within small renal lesions. This problem is magnified by the fact that renal cysts are present in 20–40 % of the population. The differential diagnosis with certain subtypes of renal cell carcinomas, as the papillary one, may be difficult because they both have a weak and homogeneous enhancement. Therefore, pseudoenhancement may lead to mischaracterization of a small renal cyst as an enhancing neoplasm. Lesion char-acterization using US may solve this problem; US pres-ents the highest accuracy in the differentiation of solid vs. cystic renal masses. A recent report suggests that pseu-doenhancement is more common with newer-generation MDCT scanners than with earlier scanners (Wang et al. 2008).

• Partial volume: the partial volume effect represents a sep-arate problem from partial volume averaging, which yields a CT number representative of the average attenuation of the materials within a voxel. There are a number of ways (Figs. 4 and 5) in which the partial vol-ume effect can lead to image artifacts. One type of partial volume artifact occurs when an object (Figs. 4c and 5c) lying off-center protrudes partly into the width of the X-ray beam creating the false impression of contrast

a

c d

b

Fig. 5 (a–d) Partial volume effect on the renal lesion density. (a) Solid renal mass with hyperdense appearance (arrow); (b) multiple cystic renal lesions correctly represented with a liquid attenuation (arrows); (c) cystic renal lesions (arrows) partly crossing the CT

section limits with equivocal density due to partial volume artifact. (d) Small renal cystic lesions (arrows) with a diameter smaller than the CT section thickness with equivocal density due to partial volume artifact

A

B

C

D

A

B

C

D

Fig. 4 Scheme showing the partial volume effect on small renal masses. (A) Solid mass completely filling the CT section thickness appearing hyper- or isodense in comparison to the adjacent renal parenchyma; (B) Cystic renal mass completely filling the CT section thickness correctly represented with liquid attenuation; (C) Partial volume effect. Cystic renal mass partly crossing the CT section limits and incorrectly represented with a supraliquid attenuation. (D) Partial volume effect. Cystic renal mass with a diameter smaller than the CT section thickness and represented with an attenuation between the liquid and renal parenchyma

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enhancement. This type of artifacts can be avoided by using a thin acquisition section width. Small cystic renal lesions with a diameter smaller than 1 cm are more fre-quently affected by partial volume effect. The distinction between solid and cystic renal lesions, seen at CT, is determined primarily on the evaluation of whether the lesion enhances after the intravenous administration of iodinated contrast material. While this approach is gener-ally robust, the assessment of enhancement in small renal cystic lesions may be unreliable. In some cases, the dif-ferential diagnosis between a small renal tumor and a small cystic lesion cannot be established. This is because attenuation measurements for such small lesions may erroneously suggest enhancement. Thin slices (0.5–1 mm) minimize the partial volume effect and allow the mea-surement of the exact density in Hounsfield units (HU) in most cases. Another type of partial volume artifact occurs when a small cystic renal mass has a diameter smaller than the CT section thickness (Figs. 4d and 5d); its attenu-ation usually ranges between the HU values of water and HU values of renal parenchyma.

• Photon starvation: a potential source of serious streaking artifacts is photon starvation, which can occur in highly attenuating areas such as the shoulders or when the patient’s arms are lying along the body. Studies have shown that X-ray is less attenuated in the anteroposterior direction than in the lateral direction in which insufficient photons reach the detectors. Therefore, at these tube angulations, very noisy projections are produced. The reconstruction process has the effect of greatly magnify-ing the noise, resulting in horizontal streaks in the image. On some scanner models, the tube current is automati-cally adjusted during the course of each rotation, in order to maintain a user-specified noise level in the image data. This process is known as milliamperage modulation. Thanks to this technique, sufficient photons pass through the widest parts of the patient without unnecessary dose to the narrower parts.

• Cone beam effect (artifact): with the increasing number of sections acquired per rotation, a wider collimation is required and X-ray beam becomes cone shaped rather than fan shaped. As the tube and detectors rotate around the patient (in a plane perpendicular to the diagram), the data collected by each detector correspond to a volume contained between two cones, instead of the ideal flat plane. This leads to artifacts similar to those caused by partial volume around off-axis objects. The cone beam artifacts are more pronounced for the outer detector rows than for the inner ones where the data collected corre-spond more closely to a plane (Barrett and Keat 2004) and increase with the increase in the number of detector rows. With 4-slice scanners, the cone beam effects were tolera-ble, and conventional filtered back projection reconstruc-tion was still usable because the total X-ray beam width was sufficiently narrow (e.g., 5-mm wide for four 1.25-

mm slices) or else the slices were sufficiently thick (four 5-mm slices). MDCT scanners of later generations required the development of alternate cone beam recon-struction algorithms (Goldman 2008).

1.2.3 Radiation DoseCT scanning provides high-quality diagnostic information, but it is also usually described as a high-dose procedure (Cohnen et al. 2003). Factors that may contribute to the delivery of high radiation dose related to CT evaluation of the urinary tract include replacement of conventional radi-ography by CT, need for multiple follow-up CT studies, and need for multiphase CT protocols (Kalra and Singh 2008).

Absorbed dose is defined as the energy absorbed per unit mass and is measured in gray (Gy) and its subunit, the milligray (mGy). The effective dose is defined, by the International Commission on Radiological Protection (ICRP 2007), as a single dose quantity reflecting the over-all risk from any radiation exposure to a reference person, where the risk is averaged over all ages and both sexes. Effective dose is calculated by summing the absorbed dose in individual organs, weighted by the radiation sen-sitivity. It is useful for comparing the risks to a reference patient from different imaging techniques and procedures, even though it should not be used for individual patients or detailed risk assessment. The equivalent dose, mea-sured in sieverts (Sv), accounts for differences in the sen-sitivity of target organs to radiation damage and is calculated by multiplying the absorbed dose to a specific tissue with the radiation weighting factor, which is equal to 1 for X-rays.

Noise has an important influence on image quality, it is inversely related to the radiation dose, and it represents a limiting factor in the improvement of spatial resolution. CT image noise generally depends on the number of X-ray photons interacting with the detector array (quantum noise), electronic noise of the detectors, and reconstruc-tion kernel (sharper kernels give noisier images). Noise is also dependent on the patient’s individual attenuation, convolution filters, slice thickness, pixel dimension, and radiation dose. The level of noise can be quantified easily from images by positioning a standard region of interest in an anatomical structure of known density and measuring the standard deviation of the HUs values. Reduced dose should always be weighed against diagnostic image quality.

MDCT radiation dose and image quality are also affected by helical pitch (table feed/X-ray beam collimation), and radiation dose is inversely related to the pitch value. The radiation dose decreases proportionally with increasing pitch in CT systems with a single detector row, as the tube voltage and current are kept constant, while for multislice CT scan-ners the relationship between pitch and radiation dose is non-linear in ECG-gated acquisition and linear in noncardiac

Computed Tomography

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mode. MDCT increases dose as a result of thinner collima-tion, overbeaming, and overranging effects. For example, in four-row scanners, effective dose is about 30 % higher with a collimation of 1 mm than with a collimation of 2.5 mm.

In current MDCT scanners with more than four rows, which allow for two different collimations (millimeter and submillimeter), radiation dose increases only by about 10 % when the smaller collimation is chosen. In spiral technique, additional tube rotations have to be performed at the begin-ning and at the end of the scan range because adjacent data from both sides are necessary for image reconstruction (interpolation). Therefore, the scanned volume exceeds the

reconstructed volume. Moreover, the number of additional rotations depends on pitch, cone beam correction, and scan-ner type. The relative effect is high with short scan lengths or when a large collimation and high pitch are combined. The overranging effect (Fig. 6a) may cause considerable increase of dose resulting from the difference between the total table feed (table feed × (scan time/rotation time)) and the scan length as planned. A further problem of MDCT is the width of the fan beam in the z-axis direction. As four or more detector chambers have to be exposed, a broader X-ray beam is used compared with the fan beam in single-slice CT. In order to avoid penumbral effects in the external portions of the detector array, the primary collimation must be made wider than necessary to expose only the detector array.

Overbeaming or “penumbra” effect (Fig. 6b) is a specific characteristic of MDCT, which reduces the portion of the beam that is captured by the detector array (beam efficiency) with a consequent increase in noise and decrease in the contrast- to-noise ratio. Even when the incident X-ray beam is collimated to the targeted detector rows, the beam is always slightly wider than the rows. For example, in MDCT, the incident X-ray beam is about 2 mm wider than the selected detector configuration (Saini 2004). Hence, the wider the incident beam, the smaller the percentage of “wasted” radiation due to overbeaming. Since unused radia-tion is delivered to the patient owing to overbeaming, when the beam collimation is consequently reduced, the number of rotations will need to be increased, contributing “wasted” radiation. Thus, for example, a 16 × 0.625-mm acquisition will require twice as many rotations as a 16 × 1.25-mm acqui-sition, and, therefore, an approximately 3 % higher radiation dose will be delivered (Saini 2004) (Fig. 6b).

Moreover, the bowtie filters are commonly employed in CT scanners to minimize radiation dose by reducing inten-sity variations across detector elements in the presence of patient anatomy. This filtration modifies a number of X-ray beam properties (effective energy, flux, first- and second- order statistics), making them nonuniform across the fan beam field of view.

Tube load parameters are very scanner specific, and CT dose index (CTDI) and dose-length product (DLP) are better usable entities to indicate the absorbed dose for protocol comparison and optimization. The fundamental parameter of radiation dose is the CTDI measured by a dose phantom in grays (Gy) and represents the integrated dose along the z-axis from a single rotation of the X-ray tube. CTDI can be measured in air or in a perspex phantom of 16 cm (repre-senting head) or 32 cm (representing body) diameter. Measurements are commonly made both at the center and the periphery (1 cm from the surface). The weighted sum of central and peripheral CTDI values is known as the weighted CTDI (CTDIw) and represents the mean dose in the x–y

Total exposed scan length

Total image scan length

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Beam width Beam width Nominal beam width1−10 mm 2−5 mm 5−10 mm

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Fig. 6 (a) Overranging effect. In current MDCT scanners with more than four rows, additional tube rotations have to be performed at the beginning and at the end of the scan range because adjacent data from both sides are necessary for image reconstruction (interpolation). Therefore, the scanned volume (red) exceeds the reconstructed volume (blue). The num-ber of additional rotations depends on pitch, cone beam correction, and scanner type. (b) Overbeaming effect. The width of the fan beam in the z-axis direction is broader in MDCT scanner with four or more detector rows, and overbeaming reduces the portion of the beam that is captured by the detector array (beam efficiency). In order to avoid penumbral effects in the outer portions of the detector array (detector area covered by the red area of the fan beam), the primary collimation must be made wider than necessary to expose only the detector array. The wider the incident beam, the smaller the percentage of “wasted” radiation due to overbeaming. Since unused radiation is delivered to the patient owing to overbeaming, when the beam collimation is consequently reduced, for a given z-axis coverage at a constant pitch, the number of rotations will need to be increased, contributing “wasted” radiation

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plane. However, the CTDI does not indicate the precise dose for any individual patient but is rather an index of the dose as measured and calculated in a phantom, and it does not take into account patient-associated parameters such as size, shape, and inhomogeneous composition (Lee et al. 2008). Furthermore, in helical scanning, the pitch must be taken into account to give the mean dose within a scanned volume, and the volume CTDI (CTDIvol) corresponds to the weighted CTDI corrected by the pitch factor (dose index divided by pitch). The CTDIvol can be used to express the average dose delivered to the scan volume for a specific examination. This is the mean dose within a scanned medium. Normalized weighted CTDI (nCTDIw) in air is 0.070 mGy/mAs. CTDIw multiplied to a conversion factor (1/0.38) is converted to

nCTDIw.Another parameter used is the DLP, which is related to the

total radiation exposure (mGy × cm) and gives an approximation of radiation risk, but it cannot give a direct assessment of the patient dose since it does not take specific organs into account. DLP is calculated from CTDIw as

DLP mGy cm CTDIw×( ) = × ×n N h,

where CTDIw is the effective weighted CTDI (mGy); n is tube rotations; N is the number of detectors; and h is the slice thickness (in centimeters). DLP may be calculated also from CTDIvol multiplied by the length of the volume scanned:

DLP mGy cm CTDIvol×( ) = × L

Patient effective dose may be calculated by multiplying DLP for a specific coefficient representing typical adult and pediatric patients. This provides sufficiently precise estima-tion of effective dose for standard protocols and reference patients (Deak et al. 2008; Huda 2007; Shrimpton 1997).

Recent advances in CT technology such as automated tube current modulation, optimizing X-ray tube voltage, and the use of iterative reconstruction have allowed reduced radi-ation dose preserving good image quality.

The aim of automated tube current modulation, ATCM (Fig. 7), is to adjust radiation dose according to the patient’s attenuation. Dose modulation is based on the principle that decrease in body diameter (i.e., the anteroposterior diameter when compared to the lateral diameter) translates into decrease in radiation necessary to obtain a certain contrast-to- noise ratio in the resulting CT image data. Dose manage-ment technology in CT has led to the optimization of CT protocols using in-plane (xy-axes), longitudinal (z-axis), or combined (xyz-axes) modulation of the dose through dedicated automated dose modulation algorithms. It is important to realize that ATCM reduces dose in thinner patients, even though the dose may actually increase in thicker patients to compensate for the increase in image noise (Kaza et al. 2014).

Tube voltage has an exponential effect on radiation dose. At constant milliAmperage (mA), radiation dose decreases by approximately 65 % if the kV are reduced from 120 to 80 kV. The optimal kilovoltage for a CT study should be cho-sen based on imaging task and the habitus of the patient. Reducing kilovoltage implies an increase of the noise and this is especially evident in obese patients. To compensate for the reduced energy of the X-ray beam, the milliamperage needs to be increased. Iodinated structures have higher HU values at lower energies because the effective energy of the X-ray beam is closer to the k-edge of the iodine. Therefore, lesion characterization based on HU measured on CT scans per-formed at 120 kV would not be applicable (Kaza et al. 2014).

Iterative reconstructions are iterative algorithms to recon-struct 2D and 3D images. They are used to recognize the factor which contributes to noise development using a statis-tical model of noise and to remove the noise from the CT images improving the image quality. They provide improved image quality without increasing radiation dose or mainte-nance of image quality at a lower radiation dose (i.e., low kilovoltage) compared to conventional filtered back projec-tion reconstruction (Kaza et al. 2014; Juri et al. 2013; Vardhanabhuti et al. 2013).

1.3 Unenhanced and Contrast-Enhanced CT

A typical CT examination of the kidneys consists of a multi-phasic helical CT protocol. After acquisition of a digital pro-jection radiograph (topogram), an unenhanced CT scan is usually obtained. According to the clinical indications contrast enhanced CT scans can be acquired in different

Tube current

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Fig. 7 Automated tube current modulation (ATCM). Dose modulation is based on the principle that decrease in body diameter (i.e., the antero-posterior diameter when compared to the lateral diameter) translates into decrease in radiation necessary to obtain a certain contrast-to-noise ratio in the resulting CT image data. Tube current is in milliampere; attenuation is expressed as the ratio between the incident intensity over the transmitted radiation intensity through a layer of material

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contrast-enhancement phases: arterial, corticomedullary, nephrographic, and excretory phases. Examinations are usu-ally performed from the superior edge of the kidneys, com-prising the adrenal glands, to the pubic symphysis and the bladder. Tube current (100–330 mA, with a mean of 150 mA) is adjusted for each examination according to patient body habitus by using automated dose modulation algorithm. Breath-hold scanning is possible in less than 10 s, allowing the acquisition of thin-section images of the organ during the corticomedullary, nephrographic, and excretory phases with little or no respiratory or patient-motion artifacts (Prasad et al. 2008).

1.3.1 Patient PreparationSufficient hydration of the patient is essential to guarantee optimal excretion of contrast media during renal CT. This can be achieved by oral or intravenous administration of fluid volume. Nonetheless, concomitant disease, such as car-diac or renal insufficiency, has to be considered. Hydration may also minimize nephrotoxic effects of intravenous con-trast media. Simultaneous oral contrast media administration may be useful; however, the application of positive oral con-trast could produce interfering effects with CT arteriography or CT urography.

1.3.2 Unenhanced CTPrecontrast ScanIn CT protocols, the unenhanced CT scans are mandatory to:• Locate the kidneys and visualize possible morphological

anomalies.• Detect renal or urinary stones (Fig. 8).• Detect calcifications in the renal parenchyma (Fig. 9) or

in a complex renal mass (usually previously detected at US) (Fig. 10).

• Detect hemorrhage in the renal and perirenal collections (Fig. 11a, b) or in solid renal masses (Fig. 11c).

• Detect the macroscopic fat component in a solid renal mass (Fig. 12a).

• Scan patients with acute or chronic renal failure.• Detect air (Fig. 12b) (e.g., in emphysematous

pyelonephritis).Moreover, unenhanced CT scan of the kidneys should be

part of every protocol for the evaluation of a suspected renal mass. It provides a baseline from which to measure the enhancement within the lesion after the administration of intravenous contrast material and provides the baseline attenuation value of renal masses (Sheth et al. 2001).

Renal Colic (Acute Flank Pain)Flank pain is a common complaint of patients presenting to emergency departments. The most common cause of persis-tent flank pain of sudden onset is acute urolithiasis. Acute

urolithiasis is the presence of a stone in the ureter, resulting in partial or complete obstruction. The principal indication for unenhanced CT is the detection of renal stones within the uri-nary tract in patients with renal colic. Unenhanced spiral or multidetector-row CT is now considered the reference imag-ing techniques for urolithiasis. In contrast to standard radiography, excretory urography, ultrasonography, and nephrotomography, CT enables to accurately assess location, size, and chemical composition of urinary calculi and allows the differentiation between urinary calculi and other pathologic processes, such as blood clots and tumors (Boll et al. 2009). Unenhanced CT is more effective than intravenous urography (IVU) – now replaced by MDCT urography – in precisely identifying ureteric stones and is equally effective as IVU in the determination of the presence or absence of ureteric obstruction (Worster et al. 2002). Table 2 shows the value of the technical parameters for unenhanced CT for renal colic. The most important factors for the clinical care of patients with urolithiasis are the location, size, and chemical composi-tion of calculi, especially, if extracorporeal shock wave litho-tripsy is considered. Size and location of the urinary stones and the anatomy of the urinary tract are of great importance to ensure smooth passage of the fragmented calculi, so a low-dose technique may be applied for this indication.

Unenhanced CT shows direct and indirect findings of colic pain due to stone. The direct finding is the visualization of the calculus itself. Regardless of composition, almost all renal stones are detected at unenhanced CT because the attenuation of stones is higher than that of surrounding tissue, even though the attenuation of uric stones is lower than the attenuation of calcium stones (Taourel et al. 2008). Dual- energy MDCT may simultaneously acquire low- and high- energy attenuation profiles, allowing the development and implementation of pixel-by-pixel postprocessing algo-rithms. Renal stone characterization with spectral analysis consisting in the assessment of tissue-specific attenuation values at distinct X-ray voltages is now possible by dual-energy MDCT (Boll et al. 2009). Multiplanar reconstruction is indicated in the study of the entire ureter course to identify the exact site of the calcification. Moreover, it allows the urologists to perform an evaluation similar to that obtained by urography. Indirect signs include hydronephrosis (Fig. 8b) and ureteral dilatation (frequency 65–90 %), perinephric and periureteral stranding (36–82 %), rim sign around the ureter (frequency 50–77 %), renal enlargement (frequency 36–71 %), renal sinus fat blurring, and reduced attenuation (>5 HU) of the renal parenchyma (Niall et al. 1999; Sourtzis et al. 1999; Türkbey et al. 2010). However, the reduced attenuation of the renal parenchyma is not specific for the renal colic since it may be caused also by interstitial edema in acute pyelonephritis and by venous congestion in renal vein thrombosis.

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Another important task of unenhanced CT is the differentiation of renal colic from other causes of acute flank pain including renal hemorrhage (bleeding from an underlying renal mass, vascular abnormality or renal ischemia, anticoagulation or blood disorders – Fig. 11), renal infections, large renal tumors, diverticulitis or appendicitis, pelvic masses, bowel obstruction, and aortic aneurysm. Most of these pathologic entities may be char-acterized by unenhanced CT. The addition of intravenous

contrast agent can improve the diagnostic skills and prove the diagnosis.

Since the radiation exposure from standard radiography is 0.5–0.9 mSv, from IVU is 1.33–3.5 mSv, and from regular dose CT is 4.3–16.1 mSv, the employment of low-dose unenhanced CT is advocated. With the most recent low-dose CT protocols (Table 2), it is possible to achieve a dose of 0.97–1.35 mSv (Knopfle et al. 2003), while with ultralow-dose CT (20 mAs) a dose of 0.5–0.7 mSv can be obtained (Kluner et al. 2006).

a

b

c

Fig. 8 (a–c) Unenhanced CT. Calcifications due to renal or urinary stones. (a) Axial plane. Renal stone (arrow). (b, c) Coronal reformations. (b) Urinary stone (arrow). (c) Staghorn renal calculosis

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Low-dose CT achieves sensitivities and specificities close to those of standard-dose CT in assessing the diagnosis of renal colic, depicting ureteral calculi > 3 mm in patients with a BMI <30, and correctly identifying alternative diagnoses (Poletti et al. 2007). Additional dose reduction may be obtained using iterative reconstruction without affecting image quality and diagnostic confidence (Kulkarni et al. 2012).

1.3.3 Contrast-Enhanced CTTable 3 shows the value of the technical parameters suggested in contrast-enhanced CT for renal parenchyma examination. After contrast material injection, different phases of contrast uptake can be discriminated. Usually, 80–120 mL (350–370 mg/I) of nonionic contrast medium is administered through an 18–20-G catheter in the antecubital vein at 4 mL/s, followed by 30–40 mL of saline solution at the same injection rate.

Contrast-enhanced CT scans are acquired in different contrast-enhancement phases: arterial phase, corticomedul-lary phase, nephrographic phase, and urographic phase. Their relative onset times depend on methods of contrast medium administration and patient characteristics. Not all phases have to be acquired and it is important to avoid unnecessary acquisitions to reduce the radiation dose. Imaging of kidneys in the various phases depends on sequen-tial arrival in different parts of the renal parenchyma of blood carrying contrast media (Fig. 13). With normal renal function, iodinated intravascular contrast medium is excreted by passive glomerular filtration. The amount of a substance undergoing glomerular excretion is determined by the prod-uct of its “freely filterable” serum concentration and glomerular filtration rate (GFR). GFR is straightly related with weight, age, and gender; it decreases in systemic

a b

Fig. 9 (a, b) Unenhanced CT. Calcifications in the renal parenchyma. Left medullary nephrocalcinosis with diffuse calcium deposition within the renal medulla of the left kidney

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Fig. 10 (a, b) Unenhanced CT. Calcific density. (a) Diffuse pericystic calcifications (arrows) in a polycystic adult kidney involving also the liver (arrow). (b) Intratumoral calcifications in a solid clear-cell-type renal carcinoma (arrow)

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hypotension, renal vasoconstriction, or glomerulonephritis (decreased number of functional nephrons). Imaging phases have to be selected carefully in accordance with clinical indication. The comprehensive evaluation of renal masses by CT requires a dedicated renal CT protocol including unenhanced, arterial, corticomedullary, nephrographic, and excretory phases. As previously said, unenhanced CT scans are used to detect calcifications and allow quantification of tumoral enhancement on the postcontrast scans (Prasad et al. 2008).

The arterial CT phase is reached about 15–25 s after intravenous injection of contrast medium (for renal arteri-

ography, a contrast bolus of 3.5 mL/s is desirable). When performing a contrast-enhanced CT of the kidney, it is critical, mainly for the arterial phase, to scan at the peak of contrast enhancement in the renal cortex. Nowadays, this is easily achieved with the “bolus tracking” technique, which has replaced the simple “test bolus” technique. The use of empirical fixed delays (i.e., 25–30 s), with small varia-tions depending upon patient age or body habitus, should be avoided. With state-of-the-art MDCT, the temporal window to perform the acquisition of the examination is very nar-row (Bae et al. 2008; Erturk et al. 2008; Kock et al. 2007; Schoellnast et al. 2005; Stacul et al. 2008). Alternatively,

Fig. 11 (a–c) Unenhanced CT. Hyperdensity from hemorrhage. (a) Perirenal hyperdense hemorrhagic collection (arrow) due to renal parenchyma laceration following motorbike accident. (b) Perirenal and

renal hematoma (arrow) in a patient who underwent renal biopsy. (c) Perirenal and renal hematoma (arrow) from a hemorrhagic renal cyst (arrowhead)

b

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a test bolus scan or a bolus tracking program can be applied for optimal start of the arterial scan.

In the arterial phase, renal arterial stenosis or evaluation of stent implantation can be judged, and acute bleeding can be detected or ruled out.

The corticomedullary CT phase (about 25–70 s after i.v. contrast) shows high contrast in the renal cortex (Fig. 13a, b). During the corticomedullary phase, the contrast is located in the capillaries of the renal cortex and peritubular cells, prox-imal convoluted tubules, and columns of Bertin (Schreyer et al. 2002). The renal cortex becomes clearly distinguish-able from the medulla because contrast material fills only the more proximal tubular structures due to greater cortical vascularity. The cortical phase gives a perfect visualization of the cortex (hyperdense) and medulla (hypodense) and, therefore, of the morphology of the kidney. Peak enhance-ment of the renal vessels during the early corticomedullary

a bFig. 12 (a, b) Unenhanced CT, fat and air component. (a) Macroscopic fat component in a renal angiomyolipoma (arrow). (b) Intrarenal air component (arrow) due to emphysematous pyelonephritis in a diabetic patient

Table 2 Low- and ultralow-dose unenhanced CT protocol

CT systems 1 slice 4 slices 16 slices 64 slices

Peak voltage (kVp) 120 120 120 120Exposurea 160 70 70 80

30–40b 30–40b 30–40b

Collimationc 3 4 × 1 16 × 0.75 64 × 0.532 × 0.1

Table feed per rotation (mm/rotation)

5 5 24 32

Gantry rotation time (s/rotation)

0.8 0.5 0.5 0.3

Helical pitchd, e 1.4 1.25 1–1.4 0.9Reconstruction interval (mm)

3 1 1 0.5–1

Reconstruction section thicknessf (mm)

3 3 2

Note: The principal parameters employed in the unenhanced CT for urinary stone detection. Scan is performed from the superior aspect of the kidneys to the pubic symphysisaData are in milliampere-secondsbUltralow-dose CTcDetector configuration. Number of detector rows (channels) × mmdEqual to table feed per 360° rotation/single-section collimation for single-slice CT scannerseEqual to table increment per 360° rotation/total beam width for mul-tislice CT scannersfCorresponding to increment

Table 3 Contrast-enhanced CT protocol for the renal parenchyma

CT systems 1 slice 4 slices 16 slices 64 slices

Peak voltage (kVp) 120 120 120 120Exposurea 160 180 200 200–250Collimationb 5 4 × 1 16 × 0.75 64 × 0.5

32 × 1Table feed per rotation (mm/rotation)

5 5 24 32

Gantry rotation time (s/rotation)

0.8 0.5 0.5 0.3

Helical pitchc, d 1.4 1.25 1–1.4 0.9Reconstruction interval (mm)

5 5 3 0.5–1

aData are in milliampere-secondsbDetector configuration. Number of detector rows (channels) × mmcEqual to table feed per 360° rotation/single-section collimation for single-slice CT scannersdEqual to table increment per 360° rotation/total beam width for mul-tislice CT scanners

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phase also provides information on vascular anatomy and patency. Moreover, early opacification of renal veins is also detectable. The optimal delay time for the corticomedullary phase depends on the rate of contrast injection, the amount of contrast material administered, and the patient’s cardiac output (Prasad et al. 2008).

This is the best phase to evaluate the vascularity of a mass. As most renal tumors are hypervascular, this phase is useful in the characterization of renal masses. The corticomedullary phase is also fundamental when evaluating renal arteries (e.g., in patients with hypertension of suspected renovascular origin) and suitable for the depiction of vascular renal anat-omy (e.g., aneurysm, arteriovenous malformation, fistula) and the assessment of vessel patency.

The nephrographic (parenchymal) CT phase (80–180 s delay) shows the renal cortex and medulla with equal enhance-ment due to contrast medium equilibrium between vascular and interstitial compartments entering loops of Henle and col-lecting tubules. This particular phase may be divided into an early phase and a late phase (Fig. 13c, d), with the latter over-lapping the excretory phase. During the nephrographic phase, the renal parenchyma enhances, homogeneously allowing the best opportunity for discrimination between the normal renal medulla and masses. This is the best phase to detect renal

masses since all lesions appear hypodense compared to the renal parenchyma. It is also useful for the evaluation of the renal veins and inferior vena cava.

CT examination usually a noncontrast phase followed by the arterial and nephrographic phases. This technique is sug-gested for the diagnosis of renal tumors, complex renal masses, trauma, and infections. An additional delayed scan in the urographic phase or 3 h after the injection of contrast material may be added to evaluate patients with trauma or infections.

The excretory CT phase (Fig. 13e, f) begins 3 min after intravenous contrast media administration and shows the opacified renal pelvis, ureter, and urinary bladder (complete filling of the urinary bladder with contrast media usually takes about 20 min). The excretory phase may be part of a CT examination of the kidney, added to the previous phases at 5–10 min from the injection, or performed as a separate protocol known as MDCT urography. During this phase, the nephrogram remains homogeneous, but its attenuation is diminished. According to patient history and clinical pre-sentation, different CT protocols can be employed. In benign indications (variants of urinary tract anatomy, con-genital anomalies of the urinary tract, ureteral pseudodiver-ticulosis, and iatrogenic ureter trauma), single-phase CT

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Fig. 13 (a–f) Transverse images and coronal reformations of the arte-rial corticomedullary phase (a, b), the late venous nephrographic phase (c, d), and the delayed excretory phase (e, f), after a split-bolus contrast

administration protocol (From Professor Roberto Pozzi Mucelli, University of Verona, Italy)

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urography may be enough. For lower-risk groups, CT urog-raphy can be used as a problem-solving test if traditional work-up remains negative and significant undiagnosed symptoms persist. Patients with more complex benign dis-eases and those with chronic symptomatic urolithiasis (complex infections, percutaneous nephrolithotomy plan-ning) may benefit from adding an unenhanced phase to the excretory phase. In the setting of urinary tract trauma, sim-ply using nephrographic- and excretory-phase CT scans are sufficient in most patients.

CT Examination of Renal MassesThe great majority of renal masses are found incidentally as a result of the use of CT, US, and MRI imaging. Most of these are simple renal cysts that can be easily diagnosed and do not require treatment. However, solid and complex cystic renal masses are also discovered, many of which are clearly malignant and need to be surgically removed (Israel and Bosniak 2005). The accurate diagnosis of a renal mass is dependent on many factors, including the clinical history, the nature of the imaging findings, the experience of the radiologist, and the quality of the examination (Israel and Bosniak 2005).

Contrast-enhanced CT is considered the gold standard in the detection and characterization of renal lesions, even though there are still some limitations (e.g., nephrotoxicity of contrast media and the obvious lack of clear discrimination between benign and malignant tumors). Comprehensive evaluation of a renal mass requires a dedi-cated renal CT protocol including unenhanced scan and contrast-enhanced scans during corticomedullary, nephro-graphic, and excretory phases. The preliminary unen-hanced CT scan is used to detect intratumoral calcification, fat, or hemorrhage or also to quantify contrast enhance-ment on the postcontrast scan. The corticomedullary-phase scan allows the differentiation of normal variants of renal parenchyma from renal masses and the depiction of tumoral lesions. Moreover, the maximal opacification of the renal arteries and veins, during this phase, permits confident diagnosis of venous extension of tumoral tissue (Sheth et al. 2001). The nephrographic phase is considered the optimal phase for the detection and characterization of small renal masses (Szolar et al. 1997; Prasad et al. 2008), even though it may miss transient lesion enhancement. The excretory phase defines the relationship of the renal mass with the collecting system, especially for planning neph-ron-sparing surgery.

Detection of renal masses: small renal tumors or com-plex cysts detected by US need confirmation and character-ization by CT. CT is more sensitive than US in the detection of small renal tumors especially when <1.5 cm. Contrast material- enhanced CT is considered a reliable technique in the detection of renal tumors (Jamis-Dow et al. 1996;

Szolar et al. 1997; Zagoria 2000). Approximately 15 % of all renal masses detected on CT are benign, while the remaining 85 % are malignant renal tumors (Zagoria 2000). Since the only curative treatment for renal cell carcinoma remains complete surgical excision, the goals of preop-erative CT are the identification of the primary tumor and eventually the venous spread and the detection of local or distant metastases (Sheth et al. 2001). The corticomedul-lary phase is not mandatory for tumor evaluation, since the nephrographic phase is more sensitive for tumor detection (Cohan et al. 1995) (Fig. 14). The two main disadvantages of the corticomedullary phase are the difficulty in detect-ing small hypovascular lesions of the renal medulla, which appears hypoattenuating during this phase, and in detecting small hypervascular tumors of the renal cortex, which may enhance to the same degree of the renal cortex.

Some diagnostic pitfalls have been described for contrast material-enhanced CT (Kopka et al. 1997; Szolar et al. 1997). The principal diagnostic pitfalls are small hypervas-cular renal cell carcinomas with similar contrast enhancement of the renal cortex, which may be mistaken for normal parenchyma at the corticomedullary phase, and centrally located tumors, which are mistaken for the normal hypoat-tenuating renal medulla (Yuh and Cohan 1999). In order to characterize a solid renal mass, the CT examination should be biphasic including the corticomedullary phase to recog-nize the vascularization of the tumor and the tubular phase to recognize the enhancement level and all the outline of the lesion.

Characterization of renal masses: any mass detected ini-tially on US which does not observe the strict US criteria of a cystic lesion should be further evaluated with CT or MR imaging (Zagoria 2000). Based on imaging findings, renal masses may be classified into predominant soft tissue, adi-pose tissue, or cystic masses (Prasad et al. 2008). Renal cell carcinoma is the most common soft tissue mass in the kid-ney. Renal cell carcinomas may demonstrate different intra-tumoral components and may appear entirely cystic or show a small proportion of macroscopic fat. Benign renal tumors, including oncocytoma, metanephric adenomas, and mesen-chymal neoplasms, usually manifest as a soft tissue renal mass. Angiomyolipomas, usually, contain macroscopic fat. Cystic renal lesions include simple, hemorrhagic, or protein-aceous cysts, abscesses, and cystic neoplasms including mul-tilocular cystic renal cell carcinoma, cystic nephroma, and mixed epithelial and stromal tumors.

Hyperattenuating renal masses consist entirely or pre-dominantly of lesions with CT attenuation that is higher (40–90 HU) than that of the surrounding renal parenchyma (30–40 HU); therefore, they appear dense on unenhanced CT images. Typical hyperattenuation lesions on unenhanced scan are hemorrhagic or high protein content cyst. Other possible causes include an elevation of iron content, colloid

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formation, and infection. Masses that are composed either entirely or partially of solid elements may show hyperattenu-ation. Attenuation at CT is affected primarily by Compton interactions and scattering, which are related to electron den-sity. Since electron density is related to physical density, densely packed cells may be one reason why the CT attenuation of some solid tumors is higher than other; diffuse microcalcifications are another possible explanation (Silverman et al. 2007).

The unenhanced CT scan provides a baseline from which to measure the enhancement within the lesion after the administration of intravenous contrast material. The demon-stration of enhancement is considered a reliable sign of vas-cularization, even though it is not considered a sign of malignancy. Renal mass enhancement is dependent on mul-tiple factors: the amount and rate of the contrast material injection, the imaging delay, and the nature of the tissue within the mass (Israel and Bosniak 2005, 2008).

The enhancement characteristic is important in distin-guishing hyperdense cysts from solid tumors. A cystic renal mass can be considered benign when it entirely shows a fluid attenuation (0–20 HU) surrounded by a hair-line-thin smooth wall which does not enhance. If a cystic renal mass contains fluid that is of higher attenuation than simple fluid, has a thickened wall or septa in which calcifi-cation could be present, or contains an enhancing soft tis-sue component, the mass may be benign or malignant according to the Bosniak classification (1986).

Most renal cell carcinomas are solid lesions with attenua-tion values of 20 HU or greater at unenhanced CT (Silverman et al. 1994). Nowadays there is no universally agreed-upon specific HU value that can be used as definitive and unequiv-ocal evidence of enhancement within a renal mass. However, an increase of 20-HU attenuation (Maki et al. 1999; Israel and Bosniak 2005; Silverman et al. 2008) within a renal lesion after the intravenous administration of contrast agent has been generally accepted as a threshold for contrast enhancement. A solid lesion, measuring at least 2 cm, which appears hyperdense with a change in lesion density >20 HU compared to baseline, has to be considered a solid enhancing malignant mass as a first hypothesis because renal cell carci-noma is often hypervascularized. Lesions with an increase in density between 10 and 20 HU (equivocal enhancement) or <10 HU (absent enhancement) may be solid, benign or malignant (typically papillary-cell carcinoma), or cystic (Siegel et al. 1999; Israel and Bosniak 2005). In this case the renal lesion should be considered indeterminate, and further evaluation, with contrast-enhanced US or MR imaging with image subtraction technique (Hecht et al. 2004), should be performed.

The enhancement patterns of neoplastic renal masses are variable and mainly depend on tumoral vascularity and histo-type. In many cases, the enhancement degree within a renal neoplasm is dense and heterogeneous (Fig. 15). In case of a low enhancement mass (at visual analysis) with irregular or nodular shape (Fig. 16), the mass is most likely neoplastic.

a b

Fig. 14 (a) Difficulty in detecting low-attenuation medullary lesions in the corticomedullary phase. (b) A small simple cyst (arrow) is easily identified in the medullary region during the nephrographic phase

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Multiplanar and 3D reformations may allow a better assess-ment of contrast enhancement within a solid renal mass and its relationships with the adjacent anatomical structures (Fig. 17). However, some hypovascular renal masses may present more subtle and uniform enhancement (Figs. 18, 19, and 20). For this reason, it is difficult to confidently character-ize a mass as enhancing, especially in the case of minimally enhancing renal cell carcinoma or hyperattenuating cysts. So, the current standard practice in genitourinary radiology is to compare attenuation measurements between the unenhanced CT scan and each of the postcontrast phases. Moreover, cystic areas may, also, be present, so that tumor presents a frank cystic appearance, and distinct areas of fat may be present too.

In renal cell carcinoma, contrast enhancement at CT is usually diffuse and intense sparing necrotic intratumoral areas (Soyer et al. 1997), except for papillary-cell subtype which appears hypovascular on contrast material-enhanced CT (Figs. 18 and 19) or MR imaging (Choyke et al. 1997, 2003). Papillary renal cell carcinomas are more often multi-centric than clear cell renal cell carcinomas and commonly appear as homogeneous soft tissue cortical masses with foci of calcifications, hemorrhage, or necrosis. Chromophobe renal cell carcinomas typically appear as a homogeneous renal mass with a cortical epicenter and show a uniform, or slightly heterogeneous, and low contrast enhancement (Fig. 20). Renal oncocytomas are typically hypervascular at contrast-enhanced CT.

The contrast washout, during the nephrographic phase, can be as useful as the initial enhancement, in renal mass

characterization. It may be helpful, especially, if only a single- phase contrast-enhanced CT scan of the abdomen has been performed (e.g., 25–35 s after contrast administra-tion in CTA or 70–90 s after contrast administration in abdominal tumor staging or follow-up) and the renal mass is incidentally detected before the patient left the imaging suite (Prasad et al. 2008). In this case, rescanning the kid-neys allows the assessment of “deenhancement” (Macari and Bosniak 1999); if the area decreases visually or quanti-tatively (at least of 10 HU), neoplasm is suspected. Recently, dual-energy CT (DECT) has been shown to be highly effec-tive in the differentiation of enhancing renal masses from hyperdense cysts in a phantom model (Brown et al. 2009) by taking advantage of the unique capability of DECT to classify iodine-containing voxels in the area of interest and, therefore, characterizing the lesion as an enhancing tumor.

Renal angiomyolipoma represents the most frequent renal mass with predominant macroscopic fat (Fig. 21), which is readily identified on unenhanced CT. However, the intratu-moral fat content may be variable, and angiomyolipomas with minimal fat cannot be distinguished by other renal tumors by imaging criteria (see chapter “Benign solid renal tumors”).

Most cystic renal lesions are benign (Fig. 22). Renal cystic lesions that require interventional or surgical management include abscesses and cystic neoplasms. CT scans of kidneys, in patients with renal abscesses, obtained approximately 3 h after contrast administration, often unfold useful information (Dalla Palma et al. 1997), by revealing a focal staining or

a b c

Fig. 15 (a–c) Soft tissue density renal mass. Clear cell renal cell carci-noma. Hyperdensity after contrast administration. (a) Unenhanced CT. The solid renal tumor (arrow) appears isodense to the adjacent renal parenchyma. (b, c) Contrast-enhanced CT. The solid renal tumor

(arrow) appears hyperdense in comparison to the adjacent renal paren-chyma during the corticomedullary phase (b) and hypodense due to contrast washout during the nephrographic phase (c)

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a hyperdense rim surrounding micro- and macroabscesses (Fig. 23). In cystic neoplasms, contrast-enhanced CT shows intratumoral septations (Fig. 24) and allows a correct assess-ment of the cyst wall with the possibility to indentify mural nodules (Fig. 25). Hemorrhagic (Fig. 26) or proteinaceous (Fig. 27) hyperdense renal cysts with nonsignificant contrast enhancement after iodinated contrast agent injection are dif-ficult to differentiate from solid renal tumor by CT. In these

cases, US may help the differential diagnosis by depicting the cystic or solid nature of the lesion.

CT Examination of Renal TraumasRenal trauma, blunt or penetrating, usually requires a multiphase protocol including a preliminary unenhanced CT scan to identify renal parenchyma hemorrhage (Fig. 11), a nephrographic phase (Fig. 28), and an excre-

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Fig. 16 (a–c) Clear cell renal cell carcinoma (arrow). Transversale plane. (a) Isodense renal lesion in unenhanced CT scan (arrow). (b) Corticomedullary phase. Nodular appearance of tumoral contrast

enhancement (arrow) after iodinated contrast agent administration. (c) Nephrographic phase. Tumoral contrast washout (arrow)

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Fig. 17 (a–f) Soft tissue density renal mass. Hyperdensity after con-trast administration. Large renal cell carcinoma at the lower pole of the right kidney: transverse images in the arterial (a) and nephrographic phase (b) after bolus contrast medium administration. Curved maxi-mum intensity projection (MIP) images showing the renal arteries (c)

and the polar artery for the lower pole of the right kidney (d). Volume rendering image depicting the renal arteries and the right inferior polar artery (e). Coronal 5-mm multiplanar reformation (MPR) shows the relationship between the tumor and the collecting system (f) (From Professor Roberto Pozzi Mucelli, University of Verona, Italy)

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tory phase to detect contrast extravasation due to renal pelvis rupture. If active bleeding is suspected, arterial or corticomedullary phase should be added. The same pro-tocol is used in iatrogenic renal traumas (e.g., biopsy, percutaneous nephrostomy, or postextracorporeal shock wave lithotripsy – Fig. 29) (see chapter “Imaging of renal trauma”).

CT Examination of the Postoperative KidneyThe assessment of the kidney immediately after surgery or interventional procedures is usually performed by a multiphase protocol including the corticomedullary phase, the nephrotomographic phase, and the excretory phase to assess lesions of the urinary tract (see chapter “Imaging of the postoperative kidney”).

a b

Fig. 18 (a, b) Soft tissue density renal mass. Hypodensity after con-trast administration. (a) Hypodense solid renal tumor (arrow) corre-sponding to a papillary-cell-type renal carcinoma. (b) The tumor shows

no significant contrast enhancement after iodinated contrast agent injection (50 vs. 57 HU) (arrow)

a b

Fig. 19 (a, b) Soft tissue density renal mass. Solid papillary-cell-type renal carcinoma (arrows) in the left kidney. (a) Unenhanced CT. Hyperdense solid renal mass (arrows) with 55-HU density. (b) Contrast-

enhanced CT after iodinated contrast agent injection. Corticomedullary phase. The renal mass (arrows) shows 60-HU density with a nonsignifi-cant contrast enhancement if compared to unenhanced CT scan

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1.4 CT Nephrogram Alterations

The CT nephrogram alterations are expressions of impaired contrast medium delivery, transit, and excretion by the kid-ney. This is strictly related to the inner pathophysiological mechanisms underlying each condition associated with alter-ation of the renal function.

1.4.1 Delayed CT NephrogramThe CT nephrogram is delayed (Figs. 30 and 31) when it appears more than 120 s after contrast medium injection. The principal causes are:

• Prerenal vascular causes (i.e., renal artery stenosis). Reduced renal perfusion causes delayed contrast material delivery to the kidney; GFR is also decreased, determin-ing a low-density nephrogram.

• Hypotension or abnormality in renal function (always bilateral) resulting in reduced GFR. Furthermore, concur-rent tubular stasis causes increased water reabsorption and tubular concentration of the contrast medium.

• Venous obstruction.• Urinary tract obstruction.

The delayed parenchymogram is caused by increased intrarenal resistance against renal artery flow. In renal vein or urinary tract obstruction, CT nephrogram is delayed; more-over, its density also appears persistent due to increased intrarenal resistance (often unilateral) which can reduce tubular transit rate, resulting in contrast material persisting in the kidney.

1.4.2 Increased Density of CT NephrogramThe CT nephrogram density increases in:• High dose of iodinated contrast medium or high infusion

rate (always bilateral) due to increased contrast medium volume and iodinate concentration

• High transit time (also unilateral) with increased sodium and water absorption and consequently increased preu-rine concentration due to tubular stasis

Increased pyelogram density is due to:

a b

Fig. 20 (a, b) Soft tissue density renal mass. Solid chromophobe-cell-type renal carcinoma (arrow) in the left kidney. (a) Unenhanced CT. Heterogeneous renal mass (arrow). (b) Contrast-enhanced CT after

iodinated contrast agent injection. Corticomedullary phase. The renal mass (arrow) shows slightly heterogeneous contrast enhancement dis-tributed throughout the lesion

Fig. 21 Fat tissue density renal mass. Giant angiomyolipoma with pre-dominant fat component

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b

Fig. 22 (a–c) Simple renal cyst (arrow). Coronal reformations. (a) Unenhanced CT. Hypodense homogeneous renal lesion (arrow) with the typical cyst density. (b–c) Contrast-enhanced CT after iodinated

contrast agent injection. (b) Corticomedullary and (c) nephrographic phase. Absence of contrast enhancement

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b

Fig. 23 (a–c) Renal abscess. a) Unenhanced CT. Hypodense renal lesion (arrow) is identified on the left kidney. b) Contrast-enhanced CT. Nephrographic phase. The renal lesion presents a cystic density (arrow)

with a peripheral dense rim corresponding to the pyogenic wall. (c) CT scans of the kidneys obtained approximately 3 h after contrast adminis-tration reveal a hyperdense rim (arrow) surrounding the abscess

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• Low osmolarity contrast medium (always bilateral) which reduced osmolar effect with reduced urine volume and dehydration (always bilateral)

• Urinary tract obstruction (see chapter “Obstructive uropathy, pyonephrosis, and reflux nephropathy in adults”)

Pyelogram density depends on the urine iodinate concentra-tion and volume.

1.4.3 Decreased Density or Absence of CT Nephrogram

The CT nephrogram density decreases homogeneously in:• Renal artery occlusion resulting in reduced perfusion in

the territory of the occluded artery.• Renal diseases reducing GFR (always bilateral) such as

glomerulonephritis which reduces the number of func-tioning nephrons or renal perfusion.

• Renal obstructive disease (more often unilateral) due to reduced GFR and renal blood flow following vasocon-strictors (acute obstruction) or increased intrarenal resis-tance due to vessel compression by a dilated excretory system (chronic obstruction). In the renal obstructive disease, the nephrogram may also be delayed (more than 120 s).The CT nephrogram density decrease is more often

expressed by wedge-shaped areas of decreased attenuation radiating from the papilla in the medulla to the cortical sur-face alternating linear bands of high and low attenuation

a b

Fig. 24 (a, b) Complex renal cyst, Bosniak IV. Coronal reformation. (a) Unenhanced CT scan showing a big cystic lesion of the right kidney (b) with thick and enhancing septa and vegetation after the injection of contrast material (arrow)

Fig. 25 Cystic clear cell renal carcinoma. Contrast-enhanced CT, cor-ticomedullary phase. Multiple enhancing (as shown by the region of interest 2, if compared to the region of interest 1 placed in the liquid component) mural nodules (arrows) are visualized

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oriented parallel to the axis of the tubules and collecting ducts. This pattern can be sustained by:• Acute pyelonephritis in which striations are due to alter-

nating normal tubules and obstructed ones by inflamma-tory cell and debris (Figs. 32 and 33).

• Urinary tract obstruction with contrast medium hypercon-centration within dilated medullary ray.

• Renal contusion.• Renal vein thrombosis.• Renal lymphoma: striations are due to the intrarenal dif-

fusion of the disease.In arterial occlusion, the nephrogram appears segmen-

tally decreased with segmental disappearance of the nephrogram in the territory of the occluded artery(ies) (Fig. 31). In patients with renal infections, CT scans of the kidneys, obtained approximately 3 h after contrast administration, often reveal useful information (Dalla

Palma et al. 1997). A delayed nephrogram replaces a vari-able portion of low- density areas depicted in the early enhanced phase (Dalla Palma et al. 1997). In renal vein thrombosis, the nephrogram may be absent or heteroge-neous (Fig. 34).

1.4.4 Cortical (Rim) NephrogramCortical or “rim” nephrogram consists in the opacification only of a peripheral rim of the renal cortex with prolonged persistence of corticomedullary differentiation. The findings have been described in several situations:• Renal artery occlusion: the cortical rim sign (Fig. 35) rep-

resents the peripheral cortex spared from the ischemic damage through collateral capsular, peripelvic, and peri-ureteric arteries. It may be considered the best differential CT feature in distinguishing renal infarction from pyelo-nephritis (Wong et al. 1984).

a b

Fig. 26 (a–b) Hemorrhagic renal cyst (arrow) in the left kidney. Coronal reformation. (a) Unenhanced CT. Hyperdense renal mass (arrow) with 95 HU density in the left kidney. (b) Contrast-enhanced

CT after iodinated contrast agent injection. Nephrographic phase. The hyperdense renal mass (arrow) shows 97 HU density with a nonsignifi-cant contrast enhancement if compared to unenhanced CT scan

a b

Fig. 27 (a, b) Infected renal cyst with proteinaceous content (40–45-HU density measured by the regions of interest 1 and 2). (a) Unenhanced CT reveals a large homogeneous lesion on the left kidney. (b) Contrast-

enhanced CT, nephrographic phase. The lesion reveals its cystic nature with peripheral wall thickening

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a b

Fig. 28 (a, b) Renal fracture (arrow) due to a motorbike accident. Contrast-enhanced CT. Nephrographic phase

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Fig. 29 (a–c) Contrast-enhanced CT. Corticomedullary phase. (a, b) Transverse plane; (c) sagittal reformation. Perirenal hematoma after extra-corporeal shock wave lithotripsy. Contrast extravasation (arrow) is visible on the right perinephric space

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• Renal vein thrombosis: the cortical rim is a consequence of altered perfusion gradient due to the interstitial pressure increased by venous occlusion, with preserved renal cortex.

• Acute ureteral obstruction: increased tubular pressure transmitted in a retrograde manner to the nephron results in impaired arterial perfusion gradient. The outer cortex is spared probably by contribution of collateral vessels. Furthermore, increased tubular pressure due to stasis causes reduced or absent progression of contrast medium into the tubules.Anyway, during acute obstruction, the presence of con-

trast medium in the renal pelvis without opacification of

the renal medulla is noted. This is probably due to the sub-stitution of glomerular filtration by tubular active secre-tion, thereby explaining contrast medium accumulation in the collecting system with no significant medullary opacification.

2 Multidetector CT Urography and CT Angiography

R. Pozzi-Mucelli, G. Zamboni, L. Bernardin, and A. Contro

2.1 Multidetector CT Urography

MDCT scanners allow a high spatial resolution due to the narrow collimation, which results in isotropic voxels in 64-channel MDCT scanners. Small slice thickness improves the detection of small structures and minimizes partial volume effects, while slice fusion options improve contrast and contrast-to-noise ratio. These features make MDCT the ideal diagnostic tool to evaluate the urinary tract. Intravenous excretory urography was used in the past for a large variety of clinical indications in the diagnostic assessment of the urinary tract, and its utilization decreased when cross-sec-tional imaging techniques have since proved superior to excretory urography for most, if not all, remaining indica-tions (Silverman et al. 2009). When unenhanced CT was shown to reliably detect urolithiasis, excretory urography was pronounced dead in 1999 with the caveat that there might remain rare instances in which it is an appropriate examination (Amis 1999). MDCT excretory-phase imaging of the urinary tract was initially described by several groups (Perlman et al. 1996; McNicholas et al. 1998). MDCT provided the ability to obtain thin (submillimeter) collimated data of the entire urinary tract during a short, single breath-hold.

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Fig. 30 (a, b) Contrast-enhanced CT. Nephrographic phase. Transverse and coronal reformations. Delayed nephrogram on the right kidney in a patient with right pyelonephritis

a b

Fig. 31 (a, b) CT nephrogram alteration. Contrast-enhanced CT. Nephrographic phase. Segmentally altered nephrogram in the right kidney due to multiple renal perfusion defects (arrows) due to embolic parenchymal infarctions

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Nowadays, the development of MDCT urography has completely covered all the clinical indications previously addressed by intravenous excretory urography. As suggested by the CT Urography Working Group of the European Society of Urogenital Radiology (Stacul et al. 2008), MDCT urography is a dedicated multiphasic CT scanning technique optimized for imaging the urinary tract in which intravascu-

lar contrast medium is used and in which high-resolution images of the renal parenchyma and urinary tract (including the bladder) are obtained. The use of this definition means that CT exams performed for many of the current indications for CT of the urinary tract, such as characterization of complex renal masses, staging of renal cell carcinoma, acute flank pain, renal infection, and evaluation of renal arteries for

a c

db

Fig. 32 (a–d) CT nephrogram alteration. Contrast-enhanced CT. Nephrographic (a, b) and delayed phase scan at 3 h (c, d). (a, b) Altered nephrogram in the left kidney (arrows) due to parenchymal perfusion

defects due to acute pyelonephritis. (c, d) Delayed scan reveals contrast enhancement (arrows) replacing the low-density areas evident on the nephrographic phase

a bFig. 33 (a, b) CT nephrogram alteration. Contrast-enhanced CT. Transverse and coronal reformations. Altered nephrogram in the left kidney (arrow) due to focal hypodensity corresponding to focal pyelonephritis

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possible renovascular hypertension, should not be labeled MDCT urography (Stacul et al. 2008). In fact, this procedure is optimized for evaluating both the renal parenchyma and the entire excretory pathway.

Dalla Palma (2001) summarized the residual roles of IVU, which then was indicated for asymptomatic hematuria (where it complemented ultrasound, plain film of the abdomen and cystoscopy), congenital anomalies of the urinary tract, prior to endourological procedures, possible fistulas, renal trans-plantation, urinary tuberculosis, and suspected ureteral pathology. Five years later, Nolte-Ernsting and Cowan (2006) reported the most frequent indications for multidetector com-puted tomography urography (MDCTU): suspected urothe-lial tumor (hematuria or unexplained hydronephrosis), urinary

tract trauma, pre- and postoperative urinary tract assessment, 3D planning for difficult cases of percutaneous nephrolithot-omy (PCNL), and complex cases of urinary tract infection, including tuberculosis. There is considerable overlap between these indications and those listed in 2001 as residual roles of IVU. Morcos (2007) was even more determined, stating that MDCTU has the same applications as IVU. Nolte-Ernsting and Cowan (2006), on the other hand, observed that no con-sensus had yet been reached on the indications for MDCTU (Morcos 2007). An analysis of the results of comparative studies of MDCTU and IVU is required in order to compare the diagnostic accuracy of the two techniques.

2.1.1 TechniqueThe radiological literature describes several technical vari-ants for performing MDCTU (Caoili et al. 2005b; Chow and Sommer 2001; Heneghan et al. 2001; Huang et al. 2006; Kemper et al. 2005); the differences mainly concern contrast agent administration, number of phases, examination tech-nique (supine or prone patient position, use of ureteral com-pression, saline infusion, oral hydration, or use of diuretics such as furosemide), and acquisition timing.

The MDCTU study does not require any special preparation. No bowel preparation is needed. The use of positive contrast medium for bowel should be avoided as it interferes with subsequent evaluation of 3D images. Oral hydration (900 mL) with water, instead of contrast, is with-out any additional cost. It avoids dehydration, promotes diuresis, and acts as a negative contrast medium for the gas-trointestinal tract. Up to 1,000 mL of water in 20–60 min before CT has been used. It can improve delineation of ure-teral segments and may facilitate the diagnosis of incidental findings. If patients cannot tolerate the oral intake of water, diuresis may be promoted somewhat by a slow intravenous drip infusion of 0.9 % saline (to a maximum of 500 mL) before and during the CT urography. Patients are asked to void immediately prior to the examination, and bladder cath-eters must be clamped before CT urography.

Supine positioning is the standard practice for MDCTU. Prone imaging may be advantageous in the unenhanced phase to discriminate ureterovesical junction stones from stones passed into the bladder. Prone positioning for improved depiction of the upper urinary tract during excre-tory phase has shown mixed results. An early study showed better opacification of the collecting system to the midureter by prone positioning, but more recent studies could not show any benefit and other factors than positioning play a greater role. Given the more cumbersome position for the patients, prone imaging is not advocated for routine use. However, turning the patient several times before excretory-phase imaging can avoid layering effects of the contrast medium, especially when the renal collecting system is dilated.

Fig. 34 CT nephrogram alteration. Contrast-enhanced CT. Nephro-graphic phase. Reduced and heterogeneous nephrogram in the right kidney (arrow) due to right renal vein thrombosis

Fig. 35 CT nephrogram alteration. Contrast-enhanced CT. Nephro-graphic phase. Complete renal infarction with evidence of cortical rim sign (arrow) due to right renal artery occlusion determining peripheral cortical enhancement due to peripheral revascularization

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Typically, CT urography consists of a multiphasic helical CT protocol (Browne et al. 2005). A preliminary unenhanced CT scan is obtained from the upper pole of the kidney to the lower edge of the symphysis pubis to detect calculi, reveal the unenhanced appearance of masses (throughout the uri-nary tract), and provide a baseline attenuation value to calcu-late enhancement of masses and other abnormalities. Unenhanced images are also useful for evaluating masses for fat or calcium (Silverman et al. 2009). The acquisition proto-cols for the most commonly acquired phases in MDCTU are detailed in Table 4. Traditionally, CTU studies have been examined in the transverse plane. Thicker sections are easier to review than thinner ones, because there are fewer images (Dillman et al. 2007a). Thinner slices can be used for better characterization of lesions, although these images can be grainy due to a lower signal-to-noise ratio.

The most frequently used contrast medium concentra-tions contain 300–350 mg of iodine per milliliter (mgI/mL). However, ideally the volume of contrast medium should be adapted to the contrast medium concentration and the patient’s weight (e.g., 1.7–2.0 mL/kg of 300 mgI/mL CM or 1.4–1.6 mL/kg of 370 mgI/mL contrast medium), while adaptation of the injection rate to the patient’s weight (e.g., 0.04 mL/s/kg) ensures a constant injection duration, which is optimal for MDCT. Two major approaches have been followed: (1) a single bolus of contrast medium, com-bined with a three- to four-phase study using unenhanced-, nephrographic-, and excretory-phase series, vs. (2) a split-bolus contrast medium injection, combined with a two- to three- phase study using unenhanced- and a combined nephrographic–excretory-phase series.

The number of CT urography phases in the single-bolus technique generally varies between two and four: (1) an unenhanced phase of the abdomen and pelvis, (2) a nephro-graphic phase of the kidney, and (3) an excretory phase of the abdomen and pelvis. Only a few groups employ the use of a corticomedullary phase since the arterial phase information

is not needed routinely. Some extend the nephrographic phase to the pelvis, especially when the patient is at increased risk of malignancy. Such a nephrographic phase facilitates complete tumor staging or evaluation of associated findings. Because of the high radiation dose of CT urography, the number of phases should be kept to a minimum.

The so-called “split-bolus” technique is the one which has probably the largest consensus, also because it limits the radiation exposure to the patient. Instead of obtaining two (nephrographic and excretory phase) separate scans after intravenous contrast material administration, a “split-bolus” technique has been described to reduce the total number of scans from three to two and, therefore, decrease radiation exposure (Chow et al. 2007). A split-bolus technique allows a two-phase CT urography with a synchronous nephro-graphic and pyelographic excretory phase (Chow et al. 2007; Dillman et al. 2007b) (Fig. 36). Once the patient is posi-tioned on the table, furosemide is administered i.v. (0.1 mg/kg), and an unenhanced scan is acquired from the upper pole of the kidneys to the bladder base. The contrast agent is then injected in two boluses, for a total patient dose of 600 mgI/kg. A first bolus of 400 mgI/kg is injected, followed by a second bolus of 200 mgI/kg after a 6-min delay and a saline flush. Two minutes after the second bolus, a second scan is acquired of the entire abdomen and pelvis, which affords

Table 4 CT urography protocol

MDCT parameters 4-row/16-row/64-row

Unenhanced 4-row/16-row/64-row

Nephrographic 4-row/16-row/64-row Excretory 4-row/16-row/64-row

Scan delay – 120 8 minScout A-P abdomen and pelvisPatient position Supine Supine SupineScan interval Kidneys–bladder Diaphragm–kidneys Kidneys–bladderTube voltage (kVp) 120 120 120Tube current (mA) 80–180 80–180 80–180Collimation 4 × 2.5/16 × 1.5/64 × 0.6 4 × 2.5/16 × 1.5/64 × 0.6 4 × 2.5/16 × 1.5/64 × 0.6Pitch 1 1 1Table feed 10 10 10Slice thickness (mm) 3 3 3Reconstruction interval (mm) 3 3 3

Note: The principal parameters employed in the CT urography according to the CT equipment. A-P anteroposterior

Unenhanced scan

Furosemide 1st bolus400 mgl/kg at 2 mL/s

2nd bolus200 mgl/kg at 2 mL/s

+ saline 30 mL

Nephropyelographic scan

6 min 2 min

Fig. 36 CT urography. Scheme of the split-bolus technique (see text)

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both an effective nephrographic-phase image (thanks to the second bolus of contrast material) and good opacification and distension of the urinary tract (thanks to the time delay after the first bolus) (Fig. 37).

The advantage of split-bolus administration is to obtain a combined nephro-pyelographic phase, instead of two sepa-rate phases, with a relatively reduced radiation exposure. Anyway, radiation exposure is a critical issue, and further optimization is achieved by using low-dose protocols and tube current modulation. The main drawbacks of split-bolus technique is the reduced contrast effect due to contrast frac-tionation with ureters and bladder not imaged appropriately, the evidence of beam-hardening artifacts in the kidney if furosemide is not provided, and limited reduction of dose (about 15 %) if compared with the standard single-bolus technique with three-phase CT urography.

2.1.2 Optimizing Excretory PhaseThe goal in MDCTU, derived from the experience with IVU, is to obtain the maximum distension and opacification of the urinary tract. For optimal timing of the contrast enhancement during the excretory phase, the individual delay time in each patient may be determined exactly using single-slice test images obtained at different intervals after injection of the contrast agent bolus (Silverman et al. 2009).

A number of ancillary maneuvers have been pro-posed to improve the quality of the MDCTU examination. Compression devices, also used during the IVU era, can be applied during CT urography to distend the upper tracts, and when removed, the distal ureters frequently fill due to flush-ing from above (Heneghan et al. 2001). External compres-sion was first proposed by McNicholas et al. (1998) based on the experience with IVU: the authors concluded that it improved opacification of the renal collecting system and the ureters. Caoili et al. (2005b), however, observed that external compression did not significantly improve urinary tract dis-tention or opacification. However, compression devices are cumbersome, necessitate imaging the upper and lower tracts separately, and are often ineffective (Caoili et al. 2005b), par-ticularly in large body habitus patients. Furthermore, these devices, as in the IVU era, are relatively contraindicated in patients with abdominal aortic aneurysms, acute obstruction, or abdominal stomas and in patients who have undergone recent abdominal surgery.

Several authors have attempted intravenous hydration prior to excretory CTU imaging, based on the assumption that an increased intravascular volume would result in increased renal excretion and thus in improved upper urinary tract distention and opacification; moreover, dilution of the hyperdense urine could improve disease identification reduc-ing the risk of artifacts. McTavish et al. (2002) noticed improved distal ureter opacification after i.v. hydration with saline prior to excretory imaging. Caoili et al. (2005b)

observed that i.v. administration of 250 mL of saline solution prior to excretory-phase imaging improves dilatation of the intrarenal collecting system and the proximal ureter.

Other authors, however, observed different results. Sudakoff et al. (2006) did not observe any significant improvement in collecting system opacification after admin-istration of a saline bolus prior to excretory imaging. Similarly, Silverman et al. (2006) observed that the adminis-tration of i.v. saline prior to excretory imaging does not add additional benefit in the setting of furosemide administra-tion. Kawamoto et al. (2006) have proposed oral hydration before performing CTU: if an adequate volume of water is ingested enough time before the exam, the effect is similar to that of i.v. hydration.

Furosemide (5–10 mg i.v.) use provides improved opaci-fication and distension of the middle and distal ureters com-pared with use of i.v. saline alone (Nolte- Ernsting et al. 2001) and improves the enhancement of the entire urinary tract. Furosemide belongs to the class of loop diuretics. Furosemide is secreted by the organic acid transport route into the proximal tubule, and its main site of action is the thick ascending limb of the loop of Henle by inhibiting the Na+–K+–2Cl− cotransport mechanism, thereby enhancing the sodium and chloride secretion. This determines an over-whelming solute and fluid load to the distal tubule and col-lecting system. The use of intravenous furosemide is now standard in most institutions because it increases the urinary flow rate: its diuresis is predominantly due to NaCl reabsorp-tion in the thick ascending limb of Henle. Furosemide has been demonstrated to have a greater effect on ureteral opaci-fication than saline (Silverman et al. 2006). Sanyal et al. (2007) observed that the use of furosemide yielded a 93 % of completely opacified ureters. Contraindications to furose-mide include allergy to sulfa, hypokalemia, exposure to cyto-toxic medications, use of cardiac aminoglycosides, and systolic blood pressure <90 mmHg. Furosemide maximizes the urinary tract and ureteral opacification and distension (Silverman et al. 2006) with a greater effect than saline by increasing the urinary flow rate and the delivery of contrast medium to the middle and distal ureter. Another advantage provided by furosemide is the elimination of beam- hardening artifacts in the kidney due to contrast medium dilution. An additional logistic benefit is that the delay of the excretory phase can be shortened when furosemide is used. In patients with normal renal function, a median delay of the excretory phase of 420 s (mean 450 ± 120 s) was seen. Furosemide administration allows a good bladder distension during the excretory phase necessary to assess masses and wall thicken-ing. The urine opacification maximizes the contrast-to-noise ratio and helps to detect bladder masses. Good mixing of contrast media (no contrast media–urine level) and urine provides homogeneous background from which bladder lesion can be detected. Furosemide injection also provides

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Fig. 37 (a) Transverse unenhanced image shows a large hyperattenuating stone in the distal left ureter. (b) Transverse image in the contrast-enhanced phase after split-bolus contrast medium administration shows reduced parenchymal enhancement in the left kidney, which shows dilated calyces and pelvis, as compared to the right normal kidney. (c, d) Coronal MIP images in the contrast-enhanced phase depict the right ureter, while the left ureter is nonopacified. (e) Coronal 5-mm MPR shows the asymmetric enhancement of the kidneys and the nonopacified left collecting system. (f) Coronal oblique 5-mm MPR shows the nonopacified left ureter

some drawbacks. First, sometimes contrast medium in the urine is too dilute reducing the contrast-to-noise ratio and potentially masking some transitional cell carcinomas (TCCs). Second, some type of stone, for example, urate cal-culi (400 HU), may be isodense to the urine and become not detectable.

2.1.3 Reconstruction AlgorithmsMPR can be used to characterize lesions, especially in the longitudinal axis, and to assess more easily the anatomy. McTavish et al. (2002) used 3-mm-thick coronal slice, with a 3-mm increment. Thinner slices can also be used, as sug-gested by Dillman et al. (2007a) (Fig. 38a).

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Metser et al. (2009) demonstrated that, with 64-MDCT, the use of a coronal plane improves conspicuity and detec-tion rate of stones smaller than 5 mm; for all stones, stone size estimation was improved in the coronal plane, especially for vertically oriented stones, which can be underestimated in the axial plane by more than 20 %. Evaluation of the coro-nal plane may be clinically valuable if the axial plane is neg-ative for stones or if a stone is not confidently diagnosed on the axial plane.

The creation of coronal and curved-planar reformatted images (Figs. 38b, c and 39a–d) comes at no additional radiation cost and provides an increase in the depiction of renal pathologies. The use of curved-planar reformatted images has been proposed, because these images can project the entire length of the upper urinary tract in one single image. The disadvantages include the increased effort and time required to create these images and the distortion of the anatomy and pathology.

Dillman et al. (2008) analyzed the sensitivity of 16-row MDCTU axial, coronal reformatted, and curved-planar reformatted images in the detection of urothelial neoplasms of the upper tract and observed that axial, coronal reformat-ted, and curved-planar reformatted images have similar sen-sitivities; the authors conclude that reviewing multiple image types increases sensitivity.

3D images are created interactively by radiologists on a 3D workstation. Thin-slab volume rendering (VR) (Fig. 38d, e), average intensity projections (AIP) (Fig. 38f), or maxi-mum intensity projection (MIP) 3D images (Figs. 38g–i and 39e) can be used for interactive image evaluation. McNicholas et al. (1998) first described the use of MIP and AIP images in CTU: these techniques allow to display a 3D dataset in a single coronal plane (Fig. 38g–i).

2.1.4 Radiation DoseHigher radiation doses are associated with MDCT urography than with stone unenhanced CT protocol. The relatively high radiation dose of multiphase CT urography is a significant limitation of the widespread acceptance of this technique. Radiation dose with MDCT urography significantly exceeds excretory urography (Nolte-Ernsting and Cowan 2006). Current multiphase CT urography protocols can be associ-ated with effective doses as high as 25–35 mSv, depending upon the number of phases included. Substantial dose reduc-tion currently cannot be recommended in cases of intrinsic urinary tract lesions. If the excretory CT phase is performed additionally in the tumor-staging protocol when obstruction of the urinary tract from the outside is suspected, substantial dose reduction may be reasonable. In a population with a high suspicion for or with known malignant disease, radia-tion dose plays only a relatively small role. However, for patients with benign diseases or for susceptible patient popu-lations, such as children and young or pregnant women, dose

is an issue. Strict indications for multiphase CTU and reduc-tion of the number of phases are important tools to manage this relatively high-dose examination.

In consideration of five possible phases of renal CT (unenhanced, arterial, corticomedullary, nephrographic, and excretory phases), a selection has to be made for clini-cal indications to avoid accumulation of dose. Effective dose was measured about 15 mSv for CT urography. Effective dose can vary depending on the number of abdominal scans. In split-bolus technique, intravenous con-trast media is applied at two different time points (e.g., with an interval of 10 min) before a single CT scan of the kid-neys and urinary tract is obtained. The urinary tract is opacified with excreted contrast media from the first bolus when the scan commences after the second bolus of con-trast media. If the scan is started such that there is a delay of 80–120 s after commencement of the second bolus, the kidneys are depicted in the nephrographic phase, while the renal collecting systems and the ureters are depicted in the excretory phase. Therefore, only one scan is necessary for investigating two contrast media phases. For investigation of children, MRI appears more suitable in most cases. However, there are rare indications that require rapid action, such as polytrauma. In this case, dose reduction can be adjusted to noise or body weight. Tumor staging of nephroblastoma by means of CT has been proven to be rea-sonable in children.

In some modern systems, this can also be combined with 3D adaptive noise filtration. Data on radiation dose optimi-zation or optimal levels of noise indices using these 3D dose modulation or 3D noise filtering techniques for CTU applications are currently lacking, but these techniques may lead to dose reductions of 20–30 %, compared with currently published fixed milliampere-second protocols, without sig-nificant loss in diagnostic image quality.

Radiation dose has been one of the most important driv-ing factors in the optimization of CT urography techniques and in the selection of justified indications. The increased patient dose of CT urography is justified by its increased performance, especially in populations at increased risk for GU malignancy. Few data on CT urography radiation doses have been presented. Four- and 16-slice scanners employing three- to four-phase protocols are usually associated with effective dose levels of 23–35 mSv (McTavish et al. 2002).

For the unenhanced scan, the abdomen and pelvis are imaged by using a maximum collimation of 2.5 mm. Given these preliminary data, we suggest that in average-sized patients (60–80 kg) excretory-phase imaging with thin collimation may well be performed at a CTDIvol of 5–6 mGy. The nephrographic phase may require a slightly higher dose for more optimal diagnosis in the liver (CTDIvol in the order of 7–8 mGy). Even lower doses for unenhanced CT (CTDIvol in the order of 2–3 mGy) are acceptable, and even

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Fig. 38 (a–h) Image visualization modalities: (a) five-millimeter-thick coronal MPRs; (b, c) curved reformations; (d, e) coronal volume rendering (VR) images with different presets; (f) average intensity projection (AIP); (g, h) coronal and (i) coronal oblique thick MIP images

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doses comparable with one abdominal plain film (CTDIvol 0.9 mGy) may suffice. With such optimization, even three-phase protocols with effective doses below 7 mSv can be realized with the limitation of the scan coverage (summed over all scan phases) and efficient selection of scanning acquisition parameters (detector configuration, pitch, tube current–time product).

The use of a low-dose unenhanced scan can further reduce the minimum dose. Furthermore, using the lowest- effective-dose CT urography protocol, filling defects of 0.25 mm could be reliably detected using the newest level of scanner technology. The heterogeneous 4-MDCT protocols represent the oldest level of CT urography technique, which is being rapidly replaced by 16- and 64-MDCT techniques. The trend toward increased dose efficiency is consistent with increased attention to dose reduction by the radiology community.

The mean effective dose for patients undergoing three- phase MDCT urography with the four-section MDCT scanner and a three-scan protocol has been reported to be 14.8 mSv ± 3.1 (Nawfel et al. 2004). These data, however, were obtained without dose modulation at a single institu-tion and cannot be generalized (Silverman et al. 2009). A comprehensive evaluation of hematuria and suspected transitional cell carcinoma can be performed at an effective dose of approximately 20.1 mSv, which is 8.2 mSv (≈70 %) higher than that of a traditional excretory urography exami-nation (11.9 mSv) consisting of five KUBs (anteroposterior), one KUB (prone), three kidney radiographs, and eight kid-ney tomograms (Vrtiska et al. 2009).

A very low-dose protocol using a CTDIvol of 2.1 mGy is unlikely to be sufficient to depict intraluminal lower ureter lesions but may allow biphasic excretory imaging with

g i

h

Fig. 38 (continued)

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improved distal ureteral opacification for selected indica-tions. Other studies indicated that the image quality of excretory- phase CTU is adequate at a CTDIvol of 5.0–7.1 mGy, which is currently used in clinical practice. These data were further substantiated by low-dose simulation experiments in which image quality was negligibly degraded with a dose reduction to a CTDIvol of 6.1–6.7 mGy. Furthermore, lower tube voltages of 90–100 kV have favorable effects on image quality (contrast-to-noise ratio) in the low-dose range.

2.1.5 IndicationsMDCT urography is currently considered the most sensitive and comprehensive imaging modality for the evaluation of the entire urinary tract (Caoili et al. 2002) by providing a com-prehensive examination of the kidneys, collecting systems, ureters, and bladder. CT urography is now accepted as the primary diagnostic investigation for the detailed anatomy of the pelvicaliceal system and ureters. Most frequent present- day indications for CT urography include the i nvestigation of hematuria, patients at increased risk for having upper or lower tract urothelial neoplasms, urinary diversion proce-dures following cystectomy, hydronephrosis, chronic symp-tomatic urolithiasis including planning of percutaneous nephrolithotomy, traumatic and iatrogenic ureteral injury, and complex urinary tract infections. In renal urolithiasis, CT urography is indicated after unenhanced CT when the urologist requests a map of the urinary tract for percutaneous or endoureteral or also surgical procedures, when a urothe-lial tumor is suspected given the need to evaluate the entire urinary tract, and when the renal colic arises in a diabetic patient in whom no stone is detected and papillary necrosis is suspected. The use of CT urography for other indications is largely anecdotal. CT urography may be useful in guiding PCNL procedures, to depict abnormal postoperative findings in patients after urinary diversions, or to effectively diag-nose medullary and papillary necrosis in an early stage when treatment can reverse the ischemic process. Also, during CT urography, significant extraurinary findings may be found. However, additional imaging was needed in few patients and the per-patient incremental costs were minimal.

The first indication for MDCT urography is hematuria (Joffe et al. 2003; Silverman et al. 2009). Hematuria is a common urologic problem that accounts for 4–20 % of all urologic visits. Microscopic hematuria corresponds to >3 red blood cells/high-power microscopic field, and it is present in 9–18 % of normal patients. The prevalence of asymptomatic microscopic hematuria (AMH) varies 0.6–21 % and is among the most important clinical signs of a urologic malignancy. A cause for asymptomatic micro-scopic hematuria can be found in 32–100 % of patients undergoing a full urologic evaluation, with 3.4–56 % of these patients having either moderately or highly clinically significant diagnoses. On the other hand, hematuria (par-

ticularly when microscopic) may not always signal the presence of serious disease and, therefore, it has long been debated how aggressively these patients can be evaluated. It must be underlined that hematuria may be determined by plenty of causes, many of which are insignificant (renal cyst, exercise, polyps, urethritis, urethrotrigonitis), others are significant and require observation (benign prostatic hyperplasia, papillary necrosis, trauma, arteriovenous fis-tula), some others are significant and require treatment (urolithiasis, vesicoureteral reflux, ureteropelvic junction obstruction, renal artery stenosis, renal vein thrombosis, renal infections), while the rest are life-threatening (malig-nancies, abdominal aortic aneurysm).

The American Urological Association Best Practice Policy guidelines for asymptomatic microscopic hematuria pub-lished in 2012 indicates CT urography as the initial test for imaging of the upper tracts thanks to its highest sensitivity and specificity. For patients with relative or absolute contrain-dications to CTU, magnetic resonance urography is an accept-able alternative approach. For the urological evaluation of AMH, a cystoscopy should be performed on all patients aged 35 years and older. In patients younger than age 35, cystos-copy may be performed at the physician’s discretion. A cys-toscopy should be performed on all patients who present with risk factors for urinary tract malignancy (irritative voiding symptoms, current or past tobacco use, chemical exposure) regardless of age. Citology is not recommended as a part of the routine evaluation; it should be performed only in patients with persistent microhematuria following a negative work-up or those with other risk factors for carcinoma in situ. If a patient with persistemt AMH has two consecutive negative annual urinalyses, then no further urinalyses are necessary. For persistent AMH after negative urological work-up, yearly urinalyses should be conducted (Davis et al. 2012).

Studies focusing on CT urography in patients with micro-scopic hematuria show that causes for hematuria are identi-fied in 33.0–42.6 % with overall CT urography sensitivity for identification of the cause of hematuria of 92.4–100 % and specificity of 89.0–97.4 % (Van Der Molen et al. 2008). In studies on microscopic or unselected hematuria, upper tract TCC was present in 0.9–7.3 %. In these populations, CT urography detection of upper tract TCC is high and signifi-cantly better than IVU. When applied to selected high-risk groups of macroscopic hematuria, TCC tumor prevalence may increase to 25–30 %, and it has been shown that CT urography of the upper tract is equivalent to retrograde pyelography (Van Der Molen et al. 2008). CT may still have problems of correctly staging advanced tumors. Thin-section (≤3 mm) MDCT images can be used to depict urothelial abnormalities just as well, if not better than, contrast material- enhanced radiographs (Caoili et al. 2005a; Silverman et al. 2009). Although radiographs have higher spatial resolution (Kawashima et al. 2004), the higher

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contrast resolution and other inherent advantages of cross- sectional imaging outweigh the advantages of conventional radiography in IVU. Radiography is a projectional imaging technique, and, therefore, overlapping structures can obscure important findings (Silverman et al. 2009). Hence, there is a sound rationale for believing that thin-section axial images obtained in the excretory phase are more sensitive than contrast- enhanced radiographs for the detection of urothelial abnormalities. As a result, it is now generally accepted that CT urography is best performed with MDCT alone (Silverman et al. 2009). CT urography can also be powerful in the diagnosis of bladder tumors, but results differ depend-ing on the specific population studied. In a population of

patients with microscopic hematuria, CT urography sensitiv-ity in comparison with cystoscopy was only 40 %, while in a high-risk group with macroscopic hematuria, unequivocal CT urography results were 93 % sensitive and 99 % specific for the detection of bladder cancer, which may obviate the need for many flexible (diagnostic) cystoscopies.

In addition to the evaluation of hematuria, CT urography can be useful in the surveillance of patients with suspected urothelial cancer (positive urine cytology), follow-up of uro-thelial cancers (Silverman et al. 2009), and patients with obstructive uropathy (e.g., hydronephrosis, hydroureter of unknown etiology), or anytime, a comprehensive evaluation of the urinary tract is warranted. Given the relatively high

a

c d

b eFig. 39 (a–e) CT urography in a patient with orthotopic neobladder. Coronal and sagittal reformatted images of the right (a, b) and the left kidney (c, d). (e) MIP 3D image (Courtesy of Prof. Quaia)

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radiation doses associated with multiphase technique, pretest probabilities for cancer should be taken into consideration. CT urography can be justified as a first-line test for the upper and lower urinary tract in hematuria patients with a high pre-test probability for TCC. Important risk factors include age >40 years, macroscopic hematuria, smoking, history of GU malignancy, and occupational exposure. The risk from the use of radiation is relatively less important in such high-risk groups, and any comprehensive two- or three-phase CT urography protocol can be performed.

2.2 CT Angiography

Multidetector CTA is replacing conventional angiography in the evaluation of renal vascular anatomy (Fig. 40) and pathology. The indications for renal CTA include renovascu-lar hypertension work-up and posttreatment assessment, renal donor transplant evaluation, oncologic preoperative staging, and renal anomaly and variant work-up (Beregi et al. 1999; Fraioli et al. 2006; Kawamoto et al. 2003; Namasivayam et al. 2006; Turkvatan et al. 2009). MDCT has a high accuracy for the identification of accessory renal arteries (Kawamoto et al. 2003; Namasivayam et al. 2006) and for the assessment of renal artery stenosis (Beregi et al. 1999).

MDCT angiography of the renal arteries can be performed with any MDCT scanner with submillimeter collimation; scan parameters for 64-row MDCT scanners are reported in Table 5. The scan region should include from the diaphragm to the iliac arteries (Fraioli et al. 2006; Turkvatan et al. 2009) to cover all the potential origins of the accessory renal arteries.

The patient is instructed before the scan in the breath-hold technique. The CTA protocol requests a double scout view. The arterial CT phase is reached about 15–25 s after the commencement of intravenous contrast media injection (for renal arteriography, a contrast bolus of 3.5 mL/s is desir-able). Alternatively, a test bolus scan or a bolus tracking program after iodinated contrast agent administration (350–400 mgI/mL; 100–120 mL of contrast volume; 4 mL/s) can be applied for optimal start of the arterial scan. The extent covered includes the whole abdomen. A delayed phase CT of the abdomen may be performed to assess parenchymal or urinary tract abnormalities. The number, size, course, and relationship of the renal vasculature are eas-ily appreciated utilizing real-time interactive editing. Multiplanar or curvilinear reformations and different post-processing 3D reconstruction algorithms with multiple 3D images rotated in 10° steps are employed. VR has emerged as the rendering technique of choice. With VR, the user can actively interact with the dataset, editing and modifying the position, orientation, opacity, and brightness of the image in real time. For CTA, VR is commonly performed with a win-dow/level transfer function that results in high-density mate-

rial (e.g., enhanced vessels or vascular calcifications) to appear bright and opaque, while less dense structures appear dim and translucent. Overlying structures are easily removed with an interactive clip plane, and the vessels of interest are easily rotated into the best orientation for the depiction of the region of interest. For evaluation of the renal hilum, axial, coronal, and sagittal views are often used in conjunction for optimal evaluation of the number, caliber, and course of the renal arteries and veins. MIP represents the other common reconstruction algorithm commonly employed when evalu-ating the renal vasculature. The MIP technique evaluates each voxel from the viewer’s eye through the dataset and selects the maximal voxel value as the value of the corre-sponding displayed pixel. The image produced lacks depth orientation, but a 3D effect can be produced with rotational viewing of multiple projections. MIP images can provide useful information regarding atherosclerotic burden, vascu-lar stents, and vascular stenoses and are often reconstructed and interpreted in conjunction with volume-rendered images.

CTA has many documented applications in the renal ped-icle, including the evaluation of renal artery stenosis and fibromuscular hyperplasia and renal arterial disease related to aortic diseases, preoperative evaluation of renal donors, and preoperative evaluation of renal anatomy prior to sur-gery. CTA provides an accurate assessment of the renal vas-culature in a fast and efficient manner without the risks of conventional angiography.

2.3 Dual-Energy CT

Two different materials with different elemental composi-tion, such as iodine and calcium, may have the same CT attenuation in HU on images acquired with single-source CT due to their similar linear attenuation coefficient. Each type of material shows a specific change in attenuation between the low- and high-energy spectra. Materials with high atomic numbers, as iodine, show a greater change in attenuation compared to materials with lower atomic number, as calcium and water, due to the photoelectric effect. The analysis of energy-dependent changes in the attenuation between two different materials is possible using dual-energy CT (DECT) which allows the acquisition of two image datasets in the same anatomic location (Kaza et al. 2012).

In the past, different dual-energy approaches have been tested for renal stones characterization, with promising results in vitro using data from consecutive CT examina-tions performed with different X-ray energies (Deveci et al. 2004; Mitcheson et al. 1983; Sheir et al. 2005). However, this approach is not feasible clinically because of the exposure to higher radiation doses and the possible motion artifacts complicating the alignment of the two CT scans. Nowadays, three types of dual-energy CT techniques are

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available: a dual-source–dual-energy CT scanner, fast kilo-voltage switching DECT scanner, and dual-layer detector DECT scanner. On dual- source CT scanner (Somatom Flash; Siemens AG, Forchheim, Germany), two X-ray sources running simultaneously at different voltages acquire two datasets showing different attenuation levels. High-energy scans are obtained at 120 or 140 kVp and low-energy scans are obtained at 80 or 100 kVp. With a fast kilovoltage switching DECT scanner (Discovery CT 750HD; GE Healthcare, Milwaukee, USA), a single X-ray tube switches between 2 kVp settings, 80 and 140 kVp, at intervals of

0.5 ms during a single gantry rotation. The dual-layer detec-tor DECT scanner is a single- source CT, and it uses a modi-fied detector with two scintillation layers to receive separate high and low image data. The top portion of the detector captures the low-energy data while the bottom layer the high-energy data, and two separate datasets are recon-structed (Kaza et al. 2012).

DECT have shown several promising clinical applica-tions in genitourinary imaging providing excellent morpho-logical details and supplying material-specific and quantitative information. DECT has been tested in vitro and

a

c d

b

Fig. 40 (a–d) Coronal VR (a) and MIP (b) images and transverse thick MIP images (c, d) show the bilateral single renal arteries and depict clearly the right renal artery’s early branching to the upper pole

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in vivo, showing high accuracy for the discrimination between uric acid calculi and other calculus types (Graser et al. 2008; Grosjean et al. 2008; Primak et al. 2007; Stolzmann et al. 2008) that is an important information to guide patient’s therapy. Similarly, Thomas et al. (2009) eval-

uated a low-dose dual-energy protocol and observed that dual-energy analysis was able to distinguish between calci-fied and noncalcified calculi in all cases (Fig. 41). Graser et al. (2009) have demonstrated that virtual unenhanced datasets obtained by DECT are quantitatively and qualita-

Table 5 CT angiography (CTA) protocol

64-row MDCT Unenhanced Arterial/corticomedullary Nephrographic

Scan delay – Bolus tracking 90–100Scout A-P abdomen and pelvisPatient position Supine Supine SupineScan interval Kidneys–bladder Kidneys to iliac arteries Diaphragm–kidneysTube voltage (kVp) 120 120 120Tube current (mA) (180–250) modulated (180–250) modulated (180–250) modulatedCollimation 64 × 0.6 64 × 0.6 64 × 0.6Pitch 0.9 0.9 0.9Table feed (mm/rotation) 17.28 17.28 17.28Slice thickness (mm) 3 3 3Reconstruction interval (mm) 3 3 3

Note: The principal parameters employed in the CTA according to the CT equipment

a c

b

Fig. 41 (a–c) DECT for urinary stones characterization. (a) In the dis-tal right ureter of a 70-year-old man, a urinary stone is identified. The stone is coded in orange suggesting a uric acid containing stone. (b) Color coded axial DECT showing a calcified stone in the left kidney of a 40-year-old woman. All calcified structures including the bone of

adjacent vertebral body are color coded in blue. (c) The threshold for color-coding can be varied by adjusting the slope of the dual-energy ratio line. (Courtesy by Marcel Dijkshoorn, Erasmus MC, Rotterdam, The Netherlands)

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tively comparable to conventional unenhanced images. Therefore, the integration of dual-energy scanning into a renal mass protocol could lower radiation exposure by 35 %. Virtual unenhanced images reconstructed from dual-energy datasets can be used also to help the detection of urinary stones within the urinary collecting system filled with con-trast material, potentially reducing the total radiation dose to the patient. Iodine specific dual energy images (colour coded iodine overlay or iodine density images) allows a colour coded display of the iodine distribution within the explored volume. With this method, direct visualization of iodine sig-nal within the lesion allows the differentiation between a non enhancing cyst from a solid enhancing lesion (Brown et al. 2009). More recently, DECT also offers the possibility to obtain a direct quantification of iodine concentration (mg/ml) in a lesion with a single region of interest (ROI) on colour coded iodine images (Kaza et al. 2011). This may rep-resent an alternative approach to conventional attenuation measurements for lesion enhancement determination (Mileto et al. 2014). Recently it has been show that a lesion iodine concentration greater than 0.5 ml/mg is an index of renal lesion enhancement (Ascenti et al. 2013).

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