DUAL MODALITY IMAGING WITH SPECT-CT

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Dual-Modality Imaging with

SPECT/CT

Seminar Report

SUBMITTED BY


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CONTENTS

1.INTRODUCTION.2.INSTRUMENTATION FOR SPECT/CT.3.MATERIALS AND METHODS.4.SPECT/CT IMAGING.

I. IMAGE ACQUISITION.II. IMAGE REGISTRATION AND IMAGE

FUSION.

5.SPECT/CT DUAL MODALITY IMAGING.6.CHALLENGES FOR SPECT/CT IMAGING.7.ADVANTAGES OF SPECT/CT8.APPLICATIONS OF SPECT/CT

REFERENCES.


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1. IntroductionDual-modality imaging is an in vivo diagnostic technique that

obtains structural and functional information directly from patient studies in a way

that cannot be achieved with separate imaging systems alone. Dual-modality

imaging systems are configured by combining Computed tomography (CT) with

radionuclide imaging (using single-photon emission computed tomography

(SPECT)) on a single gantry which allows both functional and structural imaging

to be performed during a single imaging session without having the patient leave

the imaging system.

SPECT is a tomographic scintigraphic technique in which a computer-

generated image of local radioactive tracer distribution in tissues is produced

through the detection of single-photon emissions from radionuclides introduced

into the body. CT is a tomographic imaging technique that uses an external x-ray

source to produce 3-dimensional anatomic image data. Combined SPECT/CT

devices provide both the functional information from SPECT and the anatomic

information from CT in a single examination. Some studies have demonstrated that

the information obtained by SPECT/CT is more accurate in evaluating patients

than that obtained from either SPECT or CT alone. To facilitate the process of

correlating structural and functional information, investigators at UCSF, have

developed a new class of diagnostic instrumentation that combines x-ray CT and

radionuclide imaging with SPECT.


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2. Instrumentation for SPECT/CTThe dual-modality systems use separate detectors for x-ray and

radionuclide imaging, with the detectors integrated on a common gantry to

simplify patient handling, data acquisition, and coregistration of the CT and

radionuclide image data. A SPECT/CT scanner is an integrated device containing

both a CT scanner and a SPECT g-camera with a single patient table and therefore

capable of obtaining a CT scan, a SPECT scan, or both. If the patient does not

move on the bed between the scans, the reconstructed SPECT and CT images will

be spatially registered. CT and radionuclide scans are acquired by translating the

patient from one detector to the other while the patient remains on the patient table.

This allows the CT and radionuclide images to be acquired with a consistent

scanner geometry and body habitus, and with minimal delay between the two

acquisitions.

SPECT


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CT

SPECT/CT


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3. Materials and MethodsThe radionuclide image is obtained using a GE 600 XR/T scintillation

camera with a large rectangular field of view (approximately 400 mm 500 mm).

SPECT images are acquired as 128 128 image matrices. The SPECT images are

reconstructed with a filtered back projection algorithm. CT images are obtained

with a GE 9800 Quick CT scanner using a standard technique (140 kVp, 120 mA).

Images acquired with a 512 512 matrix with field of view (FOV) of 400450 mm

and are reconstructed using filtered-backprojection. Patients were instructed to

breathe normally and the intention was to acquire the CT during tidal breathing.Total imaging time for most studies was approximately 35 minutes, with the

SPECT acquisition requiring approximately 25 minutes and the CT acquisition,

approximately 10 minutes. CT studies were reconstructed using a slice thickness of

4 mm, which is reduced to 1.52mm for imaging small structures such as

parathyroid and sentinel node imaging. CT and SPECT studies were acquired with

the patients arms above the head in all cases and with the arms at the patients side

in two cases.


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4. SPECT/CT ImagingI. Image Acquisition

The patient to be scanned first in the CT scanner. CT and SPECT

scanning can be performed without moving the patient. The CT bed moves with

ease along a track, which can be locked in either the CT position or SPECT

position. This reduces misregistration artifacts as no change in patient positioning

is required between studies. Then the bed positioned so that the SPECT study

could be done immediately after the CT acquisition. CT and SPECT studies were

acquired with the patients arms above the head. A critical benefit of SPECT/CT

and other dual-modality imaging techniques is that radionuclide and anatomical

images are acquired with minimal delay between the two image data sets. This

occurs because the two scanners are fixed relative to one another and function as

an integrated system with a common patient table.

II. Image Registration and Image FusionThe CT and radionuclide data are acquired with the patient in the

same position to facilitate image correlation between the CT and SPECT. After

both sets of images are acquired and reconstructed, image registration software isused to fuse the X-ray and radionuclide images in a way that accounts for

differences in scanner geometry and image format between the two data sets.

Images are reconstructed using an iterative maximum-likelihood expectation-

maximization (ML-EM) algorithm. SPECT/CT registration is the process of


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aligning SPECT and CT images for the purposes of combined image display

(fusion) and image analysis. For image registration, patients are imaged by placing

fiducial markers on the patient table and on the patients surface to provide a

common set of coordinates for the SPECT and CT systems. The fiducial markers

contain a small volume (0.2 ml) of a solution containing 300 mg/ml of K2HPO4

that can be visualized with CT, and 33 Ci/ml of 99mTc that can be visualized

with SPECT. After the images are acquired, they are reviewed by an observer who

identifies the approximate centers of the fiducial markers on both the CT and

SPECT images. A computer program has been developed to calculate the centroid

of the markers based on the pixel values in both the CT and SPECT images (31).

The measured coordinates of the markers then are used to derive a transformation

matrix that translates, rotates, and magnifies the coordinates of the SPECT image

so that they match corresponding points in the CT image. The transformed SPECT

data are reformatted so that they have the same image matrix size, slice thickness,

and dimensions as the original CT image to account for differences in scanner

geometry and image format (e.g., 128 128 vs. 512 512) between the two data

sets. Image registration technique is accurate to well within one pixel for both the

standard reconstructed x-ray CT images (0.94 mm for a 512 512 large field-of-

view CT image) and reconstructed radionuclide images (4.32 mm for 128 128

images). Once the SPECT and CT coordinate systems are registered as described

above, image fusion then can be performed with in-house software that displays

the radionuclide data in color superimposed on a grayscale CT image.


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Displays the radionuclide data in color superimposed on a grayscale CT

image.


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This shows the coronal and sagittal plane of SPECT/CT images.


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6. Challenges for SPECT/CT Imaging

There are several sources of error in the application of SPECT/CT, depending on

the system configuration. These errors include misregistration, truncation, scatter,

and beam hardening artifacts. A major issue for CT type systems is misregistration

between the emission and transmission data, resulting inincorrect matching of the

attenuation map to the emission data. This may occur for a number of reasons,

including sagging of the emission table, respiratory and cardiac motion, and patient

motion. Patients were instructed to breathe normally. Truncation is also a great

challenge. Current low-dose CT devices have an x-ray field of view of 40 cm andhence are unable to adequately image patients with a chest circumference of

greater than 55 cm. Therefore part of the patient is beyond the field of view.

Artifacts from metal or beam hardening can also affect CT image quality and may

lead to artifactual focal uptake on attenuation-corrected SPECT images, which is

caused by incorrect scaling of the Hounsfield units into the SPECT attenuation

map.


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7. Advantages of SPECT/CT

Anatomic imaging techniques allow accurate detection and localization ofmorphologic abnormalities.

Fusion of SPECT and CT in single examination. More accurate. Improved sensitivity and specificity. Improved spatial resolution compared to SPECT or CT alone.

8.APPLICATIONS of SPECT/CT

Cardiac imaging Tumors Thyroid disorders Parathyroid disorders Skeleton disorders Inflammation or infection Brain disorders


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References

Dual-Modality Imaging with SPECT/ CT : University of California SanFrancisco. Technology in Cancer Research & Treatment .Volume 1, Number

6, December (2002)

SPECT/CT: Basic Instrumentation and Innovations.-Seminars in Nuclear medicine (2006).

Michael K.OConnor and Brad J. Kemp.

SPECT/CT imagingfor anatomical localization: Nuclear medicinecommunications. July 2006, Vol 27 No 12.

SPECT/CT Imaging : Clinical Utility of an Emerging Technology.RadioGraphics.July- August 2008.

www.rsnajnls.org Applications of SPECT/CT in Nuclear Radiology: Nuclear Medicine.

September 2008.

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DUAL MODALITY IMAGING WITH SPECT-CT