Indications

In recent years, technologic advancements have improved the temporal and spatial resolution of scanners as well as reduced acquisition time, resulting in reliable assessments of the heart and coronary arteries.

Potential indications for coronary CTA are many and continue to evolve. Recently, the American College of Cardiology Foundation Quality Strategic Directions Committee Appropriateness Criteria Working Group, the American College of Radiology, and others have published a joint review on the appropriateness of cardiac CTA.¹ This report scored the appropriateness of cardiac CTA in several clinical applications on a scale from 1 to 9 based on the available literature. The information and experience available with cardiac CTA at this time are not robust enough to support categorical recommendations and guidelines. This is so because the technology continues to change rapidly, there are not enough prospective studies on different groups of patients, and there are no outcome studies on the prognostic value of cardiac CTA. Therefore, the appropriateness criteria are not guidelines but rather temporary recommendations until official guidelines are published.

Among the potential indications for cardiac CTA, there are four clinical groups. The first group includes patients with an indication for invasive catheterization when the procedure is anticipated to be technically demanding (e.g., ascending aortic aneurysm, atherosclerotic arch, anticoagulant therapy), patients with a prior incomplete catheterization, and patients with suspected coronary anomalies. For patients in this group, cardiac CTA may be the optimal imaging modality and in certain cases the only one.

The second group consists of patients with an intermediate pretest probability for obstructive coronary artery disease. This includes patients with atypical symptoms, acute chest pain of uncertain origin, and inconclusive stress test or thallium scan as well as candidates for valve or noncardiac surgery. The purpose of cardiac CTA in this group is to exclude significant coronary stenosis, and a normal scan may obviate the need for a catheter-based intervention.

The third group includes patients with known coronary artery disease (e.g., patients after stent placement or bypass grafting) who present with a specific question, such as stent or graft patency. This group of patients tends to have frequent complaints; traditional noninvasive tests have limited predictive accuracy, and invasive procedures are commonly employed. Here, exclusion of significant stenosis may prevent unnecessary catheter-based intervention.

The fourth group is controversial and consists of asymptomatic individuals at risk for coronary artery disease. On the one hand, CT is able to identify subclinical atherosclerosis, and the findings themselves may lead to improved compliance with preventive regimens. On the other hand, CTA involves administration of contrast material and radiation exposure, and we currently have not established an approach to dealing with such findings.

Contraindications

There are only a few contraindications to cardiac CTA, the main being a contraindication to administration of iodinated contrast agents. Injection of contrast material is mandatory for visualization of the coronary arteries.

Inability to follow breath-hold instructions (8 to 20 seconds, depending on the scanner used) may also be considered a contraindication. Breathing-induced motion artifacts may render a study unassessable.

Irregular heart rate is considered a relative contraindication, depending on scanner type and available algorithms for dealing with variable R–R intervals.

Excessive calcium within the wall of the coronary arteries is sometimes considered a contraindication to cardiac CTA. It may lead to overestimation of stenosis severity and prompt unnecessary intervention.

Technique Description

Cardiac CTA is a contrast-enhanced spiral study performed with ECG gating to eliminate cardiac motion. It is technically complex and requires high spatial resolution, high temporal resolution, good low-contrast resolution, intravascular contrast enhancement, and short scanning time. Advanced scanners have increased coverage, faster gantry rotation time, and sophisticated algorithms for reconstruction of partial scan data or combination of data from various phases (multicycle reconstruction) to further reduce temporal resolution.

Compared with invasive catheterization, cardiac CT evaluates the coronary arteries noninvasively, displays its wall as well as the lumen, and allows quantification and characterization of atherosclerotic plaques even before there is luminal narrowing.

Initially, scout images are acquired to determine anatomic span. For native coronary arteries, the CTA scan extends from the level of the carina to the base of the heart. In patients who have had bypass surgery, scans are started at the level of the clavicles. The scan is performed during a single breath-hold with timed injection of contrast material to achieve optimal enhancement of the coronary arteries.

Images are typically reconstructed from data acquired at 70% to 75% and 40% to 45% of the R–R interval. If needed, additional phases are reconstructed to find a “quiet” (motionless) phase.

Pitfalls and Solutions

In CT, the term artifact applies to any systematic discrepancy between the CT numbers in the reconstructed image and the true attenuation coefficients of the object. CT images are inherently prone to artifacts because the image is reconstructed from multiple independent detector measurements. The reconstruction algorithm assumes that all these measurements are consistent, so any error of measurement will usually be reflected as an error in the reconstructed image.^2,3

Four types of artifact can occur: streaking, due to an inconsistency in a single measurement; shading, due to a group of channels or views deviating gradually from the true measurement; rings, which are due to errors in an individual detector calibration; and distortion, due to the helical reconstruction.

Artifacts can seriously degrade the quality of CT images, sometimes to the point of making them diagnostically unusable. Furthermore, accuracy of cardiac CTA for detection of stenoses depends highly on image artifacts,³ which are a major cause of false-positive and false-negative interpretations.

To optimize image quality, it is necessary to understand why artifacts occur and how they can be prevented (Table 35-1). CT artifacts originate from a range of sources. Physics-based artifacts result from the physical processes involved in the acquisition of CT data. Patient-based artifacts are caused by such factors as movement of the patient or the presence of metallic materials inside or on the patient.

Careful preparation of the patient and optimum selection of scanning parameters are, therefore, important factors in avoiding CT artifacts.

Beam Hardening

An x-ray beam is composed of individual photons with a range of energies. As the beam passes through the patient, it becomes “harder”; its mean energy increases because the lower energy photons are absorbed. As a result of this effect, dark bands or streaks can appear on the image adjacent to dense objects such as calcifications, dense contrast material, or metallic clips (Fig. 35-1). Beam hardening can be minimized by filtration.

FIGURE 35-1 Beam hardening causes dark areas (arrows) adjacent to metallic sternal wires and surgical clips in this curved multiplanar reformation of a left internal mammary artery to left anterior descending coronary artery graft.

Partial Volume (Blooming)

CT numbers are based on attenuation coefficients for a voxel of tissue. If the voxel contains more than one tissue type, the CT number will be based on an average of the different tissues involved. For example, a dense calcified plaque or stent, lying off-center, may appear larger than it actually is in the reconstructed image because of partial volume effects and create an exaggerated luminal narrowing (Fig. 35-2). This problem is magnified if there are also motion artifacts (Fig. 35-3).

FIGURE 35-2 Partial volume effects cause this mid left anterior descending coronary artery stent to appear larger than it actually is in the volume rendered image (A) and curved multiplanar reformation (B), with overestimation of luminal narrowing. Use of a wider window with a higher center (C) may improve lumen visualization.

FIGURE 35-3 Blooming artifacts are aggravated by motion, as seen in this patient with distal right coronary artery stent who was breathing during the scan. Volume rendered image (A), axial slice (B), and curved multiplanar reformation with cross-sectional images through the stent (C) show doubling of the stent as a result of motion (breathing).

Partial volume artifacts can be best avoided by use of a thin-slice acquisition width and appropriate computer algorithms.

Metallic Materials (Streaks)

The presence of metal objects (such as pacemakers, electrodes, stents, and surgical clips) in the scan field can generate severe streak artifacts (Figs. 35-4 and 35-5). They occur because the metal object absorbs the radiation, resulting in incomplete projection profiles.

FIGURE 35-4 Metal objects, such as surgical wires (A), pacemaker electrodes (B), and dense contrast material (C), can generate severe streak artifacts.

FIGURE 35-5 Post-bypass metallic wires and clips can cause dense streak artifacts (white arrows) that may affect lumen visualization (B, black arrow) of adjacent vein graft.

External metallic objects should be removed from the field of view when possible. Use of a sharp kernel reconstruction filter may help improve visualization of the vessel’s lumen within metallic stents.

Patient Motion (Blurring)

Motion of the patient can cause misregistration artifacts, which usually appear as shading or streaking in the reconstructed image (Fig. 35-6). Steps can be taken to prevent voluntary motion (due to movement or breathing during the scan), but some involuntary motion may be unavoidable. Prescan instructions about the expected breath-hold are critical to minimize motion artifacts as well as to decrease scan time. Involuntary motion artifacts may be caused by heart rate irregularities during the scan and will appear as blurring or stair-step artifacts. Looking for a motion-free phase can sometimes help improve visualization of the coronary arteries (Fig. 35-7).

FIGURE 35-6 Two pairs of axial slices (in lung and soft tissue windows) show motion artifacts due to breathing (A and C), causing blurring and streaking of the coronary arteries (B and D).

FIGURE 35-7 Involuntary motion artifacts may be caused by heart rate irregularities during the scan. The right coronary artery and, to a lesser extent, the circumflex artery are more susceptible to motion artifacts. Comparison of axial slices (A) and oblique coronal maximum intensity projection images (B) from phases 75% (top) and 0% (bottom) shows improved right coronary artery visualization with phase 75% (arrows).