Overview of Coronary CT Angiography

Normal Variations and Imaging Artifacts That May Mimic Disease at Coronary CT Angiography

Eun Young Kim, MD, Jong Heon Park, MD, Dae Wook Yeh, MD, Yeon Hyeon Choe, MDDepartment of Radiology, Samsung Medical Center,

Sungkyunkwan University School of Medicine, Seoul, Korea

• MDCT enables noninvasive imaging of coronary arteries as well as other cardiac structures

• Coronary CT Angiography (CCTA) : Patient Preparation

Use of beta-blocker : For patients with a prescanning heart rate of 65 beats per minute or higher

To decrease heart rate, heart rate variation during scanning

Short-acting nitroglycerin : Just before the injection of contrast media, 0.6 mg of nitroglycerin is administered sublingually for vessel dilation

• CCTA : Potential Application

Anatomic abnormalities – dominance / origin & course / fistula /stenosis

Characterization of coronary plaques – calcified / noncalcified / lipid rich / mixed

Degree of Stenosis – cross sectional area / luminal diameter

• Recent ImprovementsThere have been stunning improvements in detectors and X-ray tube systems of MDCT, recently. The highest temporal resolution (75 ms) has been achieved by the use of a dual-source CT. The shortest tube rotation time is currently 0.27 s and it is expected to be shortened to 0.20 s. Wide-coverage detector design with or without dual-source system remarkably decreased radiation dose to the patients.

Objective

The aim of this study is to illustrate normal variations and imaging artifacts that may mimic diseases at coronary CT Angiography.

Materials and Methods

Over the past 10 years, we collected interesting cases from the CCTA database that show typical findings of normal variations and imaging artifacts.

Anatomic Variations

Right Atrium _ Prominent Crista Terminalis

Right Ventricle _ Nonspecific Fat Infiltration

Left Atrium _ Accessory Appendages

Left Atrium _ Slow Flow that May Mimic Thrombosis

The line of union between the right atrium and the right auricle is present on the interior of the atrium in the form of a vertical crest, known as the crista terminalis. Regression of the crista terminalis is known to occur to variable degrees; thus, widely variable prominence exhibited by this structure is seen. A prominent crista terminalis can be misinterpreted as a right atrial mass, especially when echocardiography is used as an isolated imaging technique for the heart. CCTA will prevent misinterpretation of the presence of normal intracardiac structures by the accurate identification of the exact position and extension of fibromuscular prominent structures to distinguish the structures from a neoplasm or thrombus.

Figure 1. A 65-year-old female.

(A-D) CCTA at the level of the fossa ovalis (arrow) demonstrates the crista terminalis (arrowheads) as a ridge along the right side of the right atrium. (E,F) Maximum intensity projection (MIP) through the superior right atrium and atrial appendage demonstrates continuation of the crista terminalis (arrow) to the level of the appendage as well as pectinate muscle bundles (arrowhead) within the appendage.

Right ventricular fat infiltration occurred in about 17% of asymptomatic subjects at CT. It most frequently involved the superior wall of the base and middle segments, and the right ventricular outflow tract. It was more severe in elderly subjects. Asymptomatic subjects with right ventricular fat showed no RV dysfunction or significant ECG abnormalities consistent with the diagnosis of arrythmogenic right ventricular dysplasia. MDCT enabled the detection of focal right ventricular fat infiltration in asymptomatic subjects. However, the prognostic implications of right ventricular fat infiltration in these subjects are unclear and require further study.

Figure 2. A 80-year-old female.

(A-D) CCTA shows fatty deposition (arrows) in the right ventricular outflow tract, free wall, and septum of right ventricle without evidence of wall thinnig or irregularity. (E) Right coronary artery shows no significant stenosis in 45% reconstruction image.

Right Ventricle _ Prominent Moderator Band The normal right ventricle has a large infundibular component making up its outflow tract. The infundibulum, is incorporated mainly into the right ventricle, where it forms a conal ring consisting of three components (1) the distal conal septum, which extends on to the parietal or free wall, forming the parietal band (2) the septal band, or proximal conal septum and (3) the moderator band. The infundibulum consists not only of the parietal band, but also of the septal and moderator bands, the latter two which are also known as the trabecula septomarginalis. Prominent moderate band can mimic double-chambered right ventricle (DCRV). DCRV, the partitioning of the right ventricle into 2 chambers, has been recognized since at least 1867. Recently suggested underlying anatomic substrate was the presence of a displaced insertion of moderator band into the septal band. and is produced by prominent anomalous muscle bundles within the right ventricular cavity.

During CCTA using a 64 slice ECG-gated CT scanner, we have occasionally observed additional variations in the anatomy of the left atrium which have been reported before as case reports or in small studies, such as atrial diverticuli, atrial aneurysm, and small accessory veins or fistulas originating from the left atrium which might look like diverticuli.The finding of accessory appendages of the left atria is interesting and could have implications when evaluating patients with atrial fibrillation, or prior to radio frequency catheter ablation procedures, or when evaluating patients for possible sources of emboli from cardiac chamber. Recent report shows left atrial accessory appendages in 10% of the patients.

Figure 4. A 42-year-old male and Figure 5. A 39-year old female

CCTA shows small out-pouching structures (arrows) in the left atrium, which is left atrial accessory appendage.

The mechanism of thrombogenesis is complex, including lack of effective atrial contractility and slow-flow states that may be visualized as smoke-like echoes within the left atrium or left atrial appendage (LAA) by echocardiography. Transesophageal echocardiography (TEE) is the criterion standard for evaluating the LAA and is routinely used for the assessment of atrial thrombus precardioversion in patients with atrial fibrillation and in the workup of embolic stroke. However, contrast-enhanced MDCT also can demonstrate filling defects in the left atrium or LAA, which represent incomplete mixing of CT contrast and blood. These filling defects represent slow-flow states, or thrombus as visualized by TEE. Recent reports reveals two-phase 64-section cardiac CT angiography is a noninvasive sensitive modality for detecting LAA thrombi and differentiating thrombus from circulatory stasis in stroke patients.

Figure 6. A 66-year-old male

(A, B) CCTA shows inhomogeneous, “smoke-like” low density lesions in the left atrial appendage, which represent incomplete mixing of CT contrast and blood.

Figure 7. A 66-year-old female Two-phase CCTA reveals slow-flow state in the left atrial appendage.Early CT (A) shows low density lesions in the left atrial appendage, however late phase CT (B) shows disappeared low density lesions, which means slow-flow state, not thrombosis.

Left Atrium _ Recess

Septum Primum : In the developing heart, the cavity of the primitive atrium becomes subdivided into right and left chambers by a septum, the septum primum, which grows downward into the cavity. It eventually fuses with the endocardiac cushion, while perforations appear in the superior part, forming the ostium primum. This will eventually form part of the fossa ovalis.

Remnant of common pulmonary vein : Faulty incorporation of the common pulmonary vein leaves it as a distinct structure posteriorly, into which the pulmonary veins empty. This “chamber” is separated from the anterior “fetal” left atrium (containing the left atrial appendage and communicating with the mitral valve) by a diaphragm, and is known as cor triatriatum, one of the rarest of cardiac malformations. Less pronounced but still incomplete regression of this vein would result in the persistence of a portion of the common pulmonary vein appearing as a mass along the lateral wall of the left atrium at the junction of the left atrial appendage and left upper pulmonary vein.

Figure 3. A 58-year-old male.

(A-F) CCTA shows prominent moderator band (arrows), the line of union between septum and free wall of the right ventricle.

Figure 8. A 67-year-old male

(A, B) CCTA shows a left atrial flap (arrows) in the characteristic location of septum primum.

Figure 9. A 72-year-old female (A-F) CCTA shows low density structure (arrows) that is traversing within the left atrium. Linear calcification (arrowhead) is also seen along the remnant of common pulmonary vein. Incidentally a small accessory appendage (circle) is seen in the left atrium.

A B

C D

E

F

A B

C D E

A B

C D

E

F

A

B

A

B

C

D

A

A

B

B

A

B

C

D

E

F

A

B

Left Ventricle _ Diverticulum

Interatrial Septum _ Lipomatous Hypertrophy

Left Ventricle _ Nonischemic Hypoattenuation or Wall Thinning

Interatrial Septum _ Setpal Aneurysm

Interatrial Septum _ Patent Foramen Ovale

Left ventricular diverticulum is a rare congenital anomaly. In the adult population, the incidence was reported to be about 0.4% in an autopsy series of patients with cardiac death, and about 0.26% in nonselected patients who underwent cardiac catheterization. With increased use of CT imaging, small diverticula are more commonly identified in the wall of the left ventricle.Diverticula are usually localized near the apex and most often involve the inferior or anterior parietal walls of the left ventricle. A diverticulum of the left ventricle may be distinguished from a partial rupture of the myocardium by the presence of the normal thickness and function in the surrounding myocardium.

Lipomatous hypertrophy of the interatrial septum is a benign condition that is characterized by fat accumulation in the interatrial septum. It results from adipose-cell hyperplasia and is related to advanced age and obesity . Lipomatous hypertrophy of the interatrial septum is increasingly recognized with CT of the heart. In a prospective study using CT, lipomatous hypertrophy was identified in 2.2% of the patients. The tissue appears as a wedge-shape or diffuse septal thickening on CT scans with attenuation compatible to that of subcutaneous fat. It is generally a benign entity, though it may be associated with supraventricular arrhythmia, and rarely, lipomatous hypertrophy may present with superior vena cava obstruction or intractable arrhythmia.

Image Artifacts

1. Motion Related Artifact

2. Beam-Hardening Artifact

3. Structural Artifact

4. Technical Errors

Conclusion

References

Awareness of the CCTA findings of normal variations and imaging artifacts will help to avoid misinterpretation.

1. Roberson DA, et al. Double atrial septum with persistent interatrial space: echocardiographic features of a rare atrial septal malformation. J Am SocEchocardiogr 2006;19:1175-1181

2. Choi HS, et al. Pitfalls, artifacts, and remedies in multi- detector row CT coronary angiography. Radiographics 2004;24:787-8003. Hur J, et al. Left atrial appendage thrombi in stroke patients: detection with two-phase cardiac CT angiography versus transesophageal

echocardiography. Radiology 2009;251:683-6904. Nakanishi T, et al. Pitfalls in 16-detector row CT of the coronary arteries. Radiographics 2005;25:425-438; discussion 438-4405. Clinical Cardiac CT: Anatomy and Function. In: Halpern EJ, ed. Clinical Cardiac CT: Anatomy and Function: Thieme, 2008:151-158

Left ventricular end-diastolic wall thickness of 0.6 to 1.0cm is generally considered normal. Focal thinning at the ventricular apex is a normal

variation when associated with normal myocardial function.

And normal left ventricular myocardium is uniform in thickness. Deviations of the left ventricular morphology maybe characterized as thinning or hypertrophy in a segment or global distribution. Evaluation of the left ventricular wall motion is the key to assessing morphologic changes in left ventricular myocardium. Thinning of the myocardium associated with a segmental wall motion abnormality suggests an area of infarction or scarring. Segmental myocardial infarcts may appear lower in density relative to normally enhancing adjacent myocardium.

Although coronary angiographic images are obtained by using electrocardiographically gated reconstruction algorithm, image quality is affected by cardiac motion to some extent. Many factors can affect image quality like these; high heart rate greater than 70–75 beats per minute during imaging of the coronary arteries , variations in heart rate during image acquisition, arrhythmia, and inappropriate selection of pitch. Most of them has following features: stair -step effects (Fig 17) or motion blurring (Fig 18). Blurring occurred when movement in the cardiac structure of interest exceeded the temporal resolution of scanning, either because of a fast heart rate or because of an inappropriate selection of the reconstruction window for the particular coronary artery.Motion artifacts other than those from cardiac motion result either from respiratory motion (Fig 17) or from voluntary motion (Fig 18) . Images are routinely reconstructed from CTdata acquired at 75% of the R-R intervalwith a retrospectively electrocardiographicallygated algorithm. If the routine reconstructions ofdata from 75% of the R-R interval arenot sufficient to depict the coronary arteries, subsequent tailored reconstructions are performed with data acquired at 30%, 45%, 60%, and 80% of the R-R interval (Fig 19).

The most commonly encountered beam hardening are those produced by surgically implanted high-attenuating materials, such as metallic objects such as clips, markers, and wires used in coronary artery bypass surgery or by coronary stents (Fig 20), but they may also be caused by naturally occurring coronary dense calcifications (Fig 21). High-attenuating objects generally appear larger than they are. Although the depiction of coronary calcifications provides useful information regarding atherosclerosis, the resultant artifacts may interfere with the evaluation of luminal stenosis, usually causes false-positive result for the diagnosis of coronary artery stenosis. Low-attenuating artifacts (Fig 22,23), which are caused by high density calcifications in coronary arteries.

Artifacts caused by contrast-filled normal structures such as the right atrium (Fig 24) and the cardiac veins (Fig 25) may obscure the proximal segments of the left coronary arteries. Any negative effects of structure-related artifacts can be avoided with a review of the axial source images during image interpretation.

Artifacts that result from technical errors in image data acquisition may be avoided with appropriate planning and execution of the scanning procedure, including instruction and practice of the patient in breath holding, as well as the optimal selection of anatomic coverage, scanning delay, pitch, and reconstruction window.

Figure 10. A 62-year-old female.

(A) CCTA shows hypoattenuation in the subendocardium of the left ventricular myocardium. (B) Curved planar reformation image reveals normal left anterior descending coronary artery.

Figure 11. A 43-year-old female.

(A, B) CCTA shows segmental wall thinning (arrows) in the anteroseptal wall. The patient has no symptom and past history of myocardial infarction. The regional wall motion and myocardial function is within normal limit (not shown here).

Figure 12. A 48-year-old female.

(A ,B) CCTA shows localized diverticulum (arrows) in the left ventricular apex.

Figure 13. A 48-year-old female. (Body mass index; 26.4, TG; 145 mg/dl, LDL-C; 160 mg/d)

(A ,B) CCTA shows abundant fat deposition in the interatrial septum and epicardial space (arrows) . Epicardial fat is also prominent and right

ventricle has fatty deposition (arrowheads).

A redundancy of the interatrial septum is often present at the level of the fossa ovalis. When this redundancy is large, the finding is called an interatrial septal aneurysm. The diagnosis of atrial septal aneurysm is based upon a septal excursion of 15mm or an 11mm protrusion of the septum off midline, with an aneurysm base larger than 15mm in diameter. Atrial septal aneurysms may be related to an increased incidence of thromboembolic events and are associated with a patent foramen ovale.

Figure 14. A 64-year-old female.

(A ,B) CCTA demonstrates enlargement of the left atrium with a saccular pouch at the level of the fossa ovalis (arrows) secondary to the redundant tissue in the atrial septum.

In approximately 70% of the population, the primum and secundum septae of the interatrial septum fuse shortly after birth to form an intact barrier between the atria. However, in a significant proportion of the population, septal fusion fails or is incomplete. If the foramen ovale is covered but not sealed, the resulting condition is a patent foramen ovale. In this case, a potential channel exists between the atria that can be opened by a reversal of the interatrial pressure gradient.

Figure 15. A 52-year-old male. (A ,B) CCTA shows characteristic findings of PFO with left atrial flap (arrows), continuous contrast column connecting the left and right atria, and jet of contrast material diffusing out into right atrium

Interventricular Septum _ Recess

Figure 16. A 46-year-old male. (A) CCTA shows diverticulum in the interventricular septum. (B) Systolic view demonstrates normal contraction of the wall containing the diverticulum, confirming that this is a benign process, unrelated to ischemic coronary disease.

HU = 60

HU = 100

Fig 24 Fig 25

Fig 26

Figure 26. A 52-year-old male. (A ,B) CCTA shows characteristic findings of misregistration artifact which is due to inappropriate pitch for heart rate.

Fig 20 Fig 21

Fig 22 Fig 23

75% image, 40% stenosis in proximal RCA 45% image, No stenosis in proximal RCA

Fig 17 Fig 18

Fig 19