Appropriate patient selection and preparation are the predominate determinants of CCTA image quality and diagnostic utility. Image quality is markedly improved by achieving a slow, regular heart rate for imaging, avoiding CCTA in the very obese and by selecting patients who are able to understand and comply with directions and who are able to properly perform a breath hold. In addition, avoiding CCTA in patients known to have excessive coronary calcium will make the interpretation simpler and more accurate. All of these factors can be ascertained well in advance of the procedure except determining the amount and location of coronary calcium which is assessed on the pre-contrast calcium scoring scan.

Diagnostic imaging quality is more difficult to obtain in the morbidly obese due to attenuation artifact and reduced visualization of contrast in the coronary tree. As a general rule, CCTA is not optimal in patients whose body mass index is >40 kg/m². However, patients should be considered on a case by case basis. For example, patients whose body mass resides predominately in the abdomen, hips and lower extremities may still undergo a diagnostic CCTA due to the absence of fat in the chest region. Additionally, for specific diagnostic questions such as evaluation of the left main or proximal coronary arteries, a diagnostic CCTA may still be obtained in the morbidly overweight.

If a large patient is imaged, steps should be taken to improve the image quality, which make a diagnostic image more likely. First, the milliampere setting should be maximized to increase the signal to noise ratio. Additionally, using thicker slices will also improve the signal to noise ratio, albeit at the expense of less z axis resolution. Maximization of the iodine flux is also necessary to increase contrast opacification. The iodine flux is related to the iodine concentration of the contrast where contrast agents with increased iodine concentration will increase coronary artery opacification. Increasing the rate of bolus administration will also improve the iodine flux. The usual rate of contrast administration is 5 ml/s. However, with larger bore (18 guage) intravenous catheters, rates as high as 6–7 ml/s may be used. It should be noted that iodine flux is also related to uncontrollable factors such as cardiac output. Patients with diminished cardiac output will have decreased iodine flux. It should be noted that low cardiac output may not be coincident with low ejection fraction.

Coronary calcification may interfere with the interpretation of a CCTA. This is due to the blooming effect of calcium, which appears, on CCTA, to be five times larger than it’s actual size. This “blooming” effect is due to partial volume averaging. This concept is illustrated in cartoon format in Fig. 3.3. Partial volume averaging is the result of the limitations of CCTA and is due to the methods of image creation. The reason for partial volume averaging is that, while excellent, the spatial resolution of CCTA is still approximately 0.4 mm. The edges of the calcium are smaller than 0.4 mm and cannot be accurately resolved by current CT scanners. Thus, the size of a voxel is larger than the size of the calcium edge. Consequently, when the calcium partially encompasses a voxel, the CT number of the entire voxel will exceed the threshold for calcium and the entire voxel will be displayed as the highest CT number despite the fact that the calcium encompasses only part of the voxel. Said another way, the resolution of the computer matrix representation of the image can be no smaller than the voxel size. The result of calcium blooming is that the coronary artery lumen may be partially or fully obscured and when this phenomenon is extensive and diffuse or even focally severe and in these instances, gradations of a particular coronary lesion may be difficult.

Methods to help resolve the issue of calcium blooming include widening the contrast window and using a sharper filter. If the calcium completely obscures the vessel lumen in all views, there is approximately a 50 % chance of an obstructive lesion in this location. If any lumen is visualized around the calcium, there is approximately a 2 % chance of an obstructive lesion [3]. How an individual reader chooses to report calcified lesions depends on the reader’s decision regarding how sensitive versus how specific he or she wishes to be. It is my opinion that sensitivity is more important and that obstructive lesions should not be missed. Therefore, my practice is to report that an obstructive lesion cannot be excluded if no lumen is visualized around the calcium and that an obstructive lesion is unlikely if lumen can be seen. There are no definitive guidelines regarding what level of coronary calcium precludes a diagnostic CCTA. Since my anecdotal experience indicates that very high calcium score scans may still be diagnostic, I do not use a calcium score threshold for cancelling a scan. Presently, there is no absolute consensus on the threshold of calcium that precludes a technically adequate and diagnostic scan [1].

Pacemakers and defibrillators also do not preclude the use of CCTA. However, it should be recognized that pacemaker and defibrillator leads may create artifacts that could obscure sections of the coronary arteries.

Regular, slow heart rhythms are mandatory for diagnostic CCTA imaging. Patients with arrhythmia during the pre-scan assessment period should, in general, not be imaged. Exceptions do exist and at times, in fact, patients in atrial fibrillation are imaged. In addition, ECG editing techniques (discussed later) can salvage scans performed during limited ectopy. Extensive ECG editing, however, will eliminate too much data for a full and diagnostic reconstruction. In general, for the evaluation of coronary arteries, only patients with slow, regular heart rhythms should be imaged.

CCTA is excellent for diagnostic imaging of bypass grafts and yields excellent diagnostic information [1]. In fact, CCTA for this purpose is an appropriate criteria for imaging [2]. However, native coronary arteries in patients who have had bypass surgery degenerate and calcify quickly and CCTA yields much less accurate information about the native coronary arteries in these patients and is an important limitation of CCTA in evaluating the patient who has had bypass surgery [1]. Additionally, the imaging of stents also pose significant challenges for CCTA. The metal in the stents create many CCTA imaging artifacts, which include but are not limited to blooming. The literature suggests that in patients with larger stents (>3.0 mm) who have a high likelihood for good image quality, whose clinical presentation suggests low to intermediate risk for in stent stenosis, CCTA can be used to rule out significant in stent restenosis [1]. However, in most instances CCTA is not the test of choice for the sole purpose of evaluating for in stent stenosis. While the lumen within larger stents (>3.0) may often be adequately visualized, CCTA solely for stent evaluation is not routinely advisable at this time. Discussion about CCTA for bypass patients and in patients who have had coronary stents placed will be discussed in detail later.

Proper patient selection accounts not only for individual patient characteristics that provide a higher likelihood for a diagnostic scan but also for appropriate patient indications that result in diagnostic utility. While data are not sufficient to justify guidelines on the topic of data driven CCTA indications, appropriate use criteria have been published and while individual judgment should be employed for each single patient, these criteria are recommended as a guide to the proper indications for CCTA [2]. For a full account of the appropriate uses of CCTA, please consult the referenced appropriate use criteria document [2]. Tables 3.1a–g categorize the current appropriate use criteria for CCTA.

Proper patient preparation both before and during the CCTA examination is essential to creating a useful CCTA image. Controlling for noncardiac motion (breathing and patient movement) as well as cardiac and coronary motion is the critical factor in preparing the patient for the CCTA examination and begins in the days prior to the patient presenting to the CT suite and continue throughout the procedure itself. Strategies to control for movement and motion involve both technology (discussed later) and pharmacology. The heart and coronary arteries move in complex patterns very rapidly through space throughout the cardiac cycle [4]. Each coronary artery moves in a different pattern and with a different velocity and degree. In fact, different segments of the same artery move differently [3]. In addition, the coronary velocity and acceleration during the cardiac cycle increase at higher heart rates [4]. See Fig. 3.4. Efforts to slow the heart rate down both before and during the procedure are critical.

Patients should present to CT suite well hydrated to avoid sinus tachycardia resulting from dehydration. Patients should be instructed to eat a light breakfast the morning of the scan. Additionally, patients should be instructed to take all scheduled medications. Missing scheduled beta blocker doses, for example, may result in withdraw tachycardia which will make the scan difficult to perform. Subjects should also be asked to avoid significant physical activity such as exercise before their scan to avoid inappropriate sympathetic activity at the time of the scan. B- blockers are most frequently used to affect heart rate control. Oral b blockers are prescribed for home use before the procedure and IV b blockers may be used at the time of the scan if needed. Pre-procedural b blockers should be prescribed based on the patient’s age, resting heart rate and whether or not the patient has ever been exposed to b blockers. We predominantly use oral (at home) and IV (just before the scan if necessary) Lopressor because we find it most affective. Calcium blockers may be used if b blockers are contraindicated as in severe reactive airway disease. As the heart rate slows, the time where the least coronary motion occurs is during isovolumic relaxation (at approximately 60–80 % of the R-R interval). Optimal right coronary images may be found at different phases than optimal left coronary images. Often the right coronary is seen best in the systolic phases. Table 3.2 illustrates the pre procedural b blocker protocol employed in our laboratory.

Metformin should be avoided for 24 h before the scan and for 48 h after the scan to avoid potential adverse affects of concomitant iodinated contrast administration. Viagra, Cialis and Levitra should be avoided 48 h before the scan to eliminate the potential adverse reaction when used in combination with sublingual nitroglycerin, which is necessary for an optimal CCTA. Pre-procedural hydration as well as pre-procedural mucomyst is suggested in those with renal insufficiency. CCTA should be avoided in patients with creatinine >1.5. If an iodinated contrast allergy is present, premedication protocols should be employed.