DESCRIPTION OF TECHNICAL REQUIREMENTS

Challenges in noninvasive cardiac imaging by CT include small (typically 1 to 4 mm) and often tortuous vessels, complex and rapid cardiac motion, and variable, often unpredictable heart rates and respiratory motion. As a result, all cardiac scans are performed with ECG gating, either prospective or retrospective, to allow reconstruction of motion-free images.

CT scans of the heart should be fast and performed during a single, preferably short breath-hold to avoid breathing artifacts and to allow minimal contrast agent volume. Sub-second rotation time resulting in high temporal resolution is required to eliminate cardiac motion. High spatial resolution, preferably sub-millimeter, is necessary to adequately visualize small and complex anatomic structures such as the coronary arteries.

Adequate preparation of the patient is a must, and therefore a team made up of scheduling staff, technologist, physician, and nurse is required. Documentation of serum creatinine level and history of contrast allergy is essential at the time of scheduling. The risk of contrast nephropathy must be weighed against the benefits of the information gained by the procedure. Depending on the clinical indication, some degree of renal protection may be achieved with good hydration before and after injection of contrast material as well as the administration of N-acetylcysteine. In case of a known allergy to contrast material, and if the risk is acceptable to both patient and referring physician, pretreatment with corticosteroids may prevent reaction to the contrast agent.

The coronary arteries are best demonstrated when there is the least amount of motion. There are two relatively motionless phases during the cardiac cycle: diastasis in late diastole and isovolumic relaxation time in late systole. The diastasis period is longer; but at higher heart rates, it is almost eliminated. Isovolumic relaxation time, on the other hand, is shorter but unaffected by heart rate.

Lowering of the patient’s heart rate is suggested on the basis of comparison of coronary artery visibility in patients with low versus high heart rates. Most studies so far have been performed on 4-, 16-, and 64-slice scanners and have shown a higher number of unassessable segments in patients with rapid heart rates compared with patients with lower heart rates. Furthermore, a low heart rate, with a longer cardiac cycle, enables the use of tube current modulation (with full x-ray output only during a fixed period in diastole), thus reducing radiation dose. Preliminary experience with the latest generation of scanners (more than 64-slice and dual-source units) with improved temporal resolution shows promising results in patients with higher heart rates.

Heart rate can be controlled through the use of medication (e.g., oral or intravenous β blockers), by reassuring the patient (i.e., explaining the procedure, describing what to expect), and by rehearsing the breath-hold required during the scan. Practicing the breath-hold is also important to anticipate heart rate changes that may occur during the scan.

Sublingual nitroglycerin (if it is not contraindicated) should be administered immediately before the scan is started to improve vessel visualization. This is particularly important in women, smokers, diabetics, and patients who have had bypass graft surgery.

The radiation dose should be as low as possible with parameters adjusted to the patient’s size and expected heart rate for optimal visualization. In very large patients, a lower pitch should be considered to allow increased radiation dose.

TECHNIQUES

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.

TABLE 35-1 Artifacts, Their Appearance and Causes, and Measures to Avoid Them

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

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).