Interpretation and Reporting in Obstructive Coronary Disease

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CHAPTER 35 Interpretation and Reporting in Obstructive Coronary Disease

Since its introduction in the early 1970s, computed tomography (CT) has become a well-established imaging technology and the modality of choice for noninvasive evaluation of the aorta and pulmonary, carotid, renal, and peripheral arteries.

In CT angiography (CTA), the use of rapidly injected contrast agents raises the density in the blood pool, thereby allowing easy differentiation between the lumen, vessel walls, and surrounding tissues.

CT imaging of the coronary arteries became practical with the arrival of the 4-slice scanners. The ongoing advancement and innovation in CT technology over the years provided faster scans with improved temporal and spatial resolutions. As a result, the role of CT in the imaging of coronary arteries progressed from simple determination of the presence of arterial calcifications to demonstration of the atherosclerotic plaque itself and quantification of luminal stenoses.

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.

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.

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