Advances in CT imaging technology, including the introduction of multidetector row systems with electrocardiographic gating, have made imaging of the heart and the coronary arteries feasible. The potential to obtain information noninvasively comparable to that provided by invasive coronary angiography has been the major driving force behind the rapid growth and dissemination of cardiac CT imaging. In the future, the ability of CTA to provide information not currently available from invasive angiography may provide the basis for a major shift in how patients with atherosclerotic cardiovascular disease are classified and managed. Currently, cardiac CTA can provide information about coronary anatomy and left ventricular (LV) function that can be used in the evaluation of patients with suspected or known CAD. The technology for performing coronary CT angiograms is evolving at a rate that often outpaces research evaluating its incremental benefits. Multidetector CT technology prior to 64-channel or "slice" systems should now be considered inadequate for cardiac imaging (except for studies limited to assessing coronary calcium). The incremental value of recently introduced CT hardware with 128-, 256-, and 320-channel systems over 64-channel systems has not yet been determined. As with any diagnostic technology, coronary CTA has technical limitations with which users should be familiar, and proper patient selection and preparation are important to maximize the diagnostic accuracy of the test. Most cardiac CTA examinations result in a large 4-dimensional (4D) dataset of the heart obtained over the entire cardiac cycle. Physicians who interpret these examinations must be able to analyze the image data interactively on a dedicated workstation and combine knowledge of the patient with expertise in coronary anatomy, coronary pathophysiology, and CT image analysis techniques and limitations. In addition, integration of coronary CTA data into clinical practice requires that the results be evaluated in terms of what was known diagnostically and prognostically before the test was performed and, thus, what incremental information the test provides. The ability of a test such as coronary CTA to provide incremental diagnostic information that alters management (as contrasted with increasing diagnostic certainty alone) is heavily dependent both on the pretest probability and on the alternative diagnostic strategies considered. The published literature on the diagnostic accuracy of 64-channel coronary CTA compared with invasive coronary angiography as of June 2009 consists of 3 multicenter cohort studies along with over 45 single-center studies, many of the latter involving fewer than 100 patients. This literature reflects careful selection of study subjects and test interpretation by expert readers, typically with exclusion of patients who would be expected to have lower quality studies, such as those with irregular heart rates (e.g., atrial fibrillation), obesity, or inability to comply with instructions for breath holding. In addition, because the cohorts for these studies were assembled from patients referred for invasive coronary angiography, they do not necessarily reflect, in terms of obstructive CAD prevalence or clinical presentation, the population to which coronary CTA is most likely to be applied in clinical practice. Accepting these caveats, some consistent conclusions emerge from this literature that may be useful in clinical decision making. In these studies, overall sensitivity and specificity on a per-patient basis are both high, and the number of indeterminate studies due to inability to image important coronary segments in the select cohorts represented is less than 5%. In most circumstances, a negative coronary CT angiogram rules out significant obstructive coronary disease with a very high degree of confidence, based on the post-test probabilities obtained in cohorts with a wide range of pretest probabilities. However, post-test probabilities following a positive coronary CT angiogram are more variable, due in part to the tendency to overestimate disease severity, particularly in smaller and more distal coronary segments or in segments with artifacts caused by calcification in the arterial walls. At present, data on the prognostic value of coronary CTA using 64-channel or greater systems remain quite limited. Furthermore, no large-scale studies have yet made a direct comparison of long-term outcomes following conventional diagnostic imaging strategies versus strategies involving coronary CTA. As with invasive coronary angiography, the results of coronary CTA are often not concordant with stress single-photon emission computed tomography (SPECT) myocardial perfusion imaging (MPI). The differences in the parameters measured by MPI ("function" or "physiology") and CTA ("anatomy") must be considered when making patient management decisions with these studies. Of note, a normal MPI does not exclude the presence of coronary atherosclerosis although it does signify a very low risk of future major adverse events over the short to intermediate term. Conversely, coronary CTA allows detection of some coronary atherosclerotic plaques that are not hemodynamically significant. The optimal management of such disease has not been established. Neither test can presently identify with any reasonable clinical probability nonobstructive coronary plaques that might rupture in the future and cause acute myocardial infarction (MI). Invasive coronary angiography has a similar limitation. Studies comparing coronary CTA with fractional flow reserve (FFR) measured as part of invasive coronary angiographic studies complement the MPI comparisons described in the preceding text by showing that coronary CTA anatomic data do not provide very accurate insights into the probability that specific lesions will produce clinically significant ischemia. Similar observations have been made about the relationship of FFR data and the anatomic information provided by invasive coronary angiography. In the context of the emergency department evaluation of patients with acute chest discomfort, currently available data suggest that coronary CTA may be useful in the evaluation of patients presenting with an acute coronary syndrome (ACS) who do not have either acute electrocardiogram (ECG) changes or positive cardiac markers. However, existing data are limited, and large multicenter trials comparing CTA with conventional evaluation strategies are needed to help define the role of this technology in this category of patients. Coronary CTA imaging of patients with prior coronary bypass surgery yields very accurate information about the state of the bypass grafts but less accurate information about the native arteries distal to the bypasses and the ungrafted arteries. Because chest pain after bypass surgery might be associated with disease progression in either a graft or a native coronary artery, the difficulty of accurately assessing the native vessels is an important limitation for the clinical use of coronary CTA in the post-bypass patient. Coronary stents pose some significant technical challenges for coronary CTA, since the metal in the stents may create several types of artifacts in the images. Special algorithms are now routinely used that may reduce some of these artifacts during image reconstruction. The literature suggests that in patients who have large diameter stents, good image quality, and whose clinical presentation suggests low-to-intermediate probability for restenosis, 64-channel coronary CTA can be used to rule out severe in-stent restenosis. There are no studies that directly compare a coronary CTA strategy with an invasive coronary angiography strategy in patients with coronary stents, and such data will be required to understand the efficiencies and tradeoffs of these 2 strategies in this population. The literature on the assessment of LV function using cardiac CTA in patients with suspected or known CAD is much smaller than that for diagnostic coronary imaging. One likely reason is that echocardiography already provides a readily available, noninvasive means of assessing ventricular function and wall motion and does so without exposing patients to ionizing radiation or iodinated contrast agents. Available comparisons with cardiovascular magnetic resonance (CMR) suggest that CTA estimation of LV ejection fraction is accurate over a wide range of values. Accuracy may, however, be reduced at higher heart rates due to difficulties in capturing end-systolic and end-diastolic phases accurately. Use of some newer strategies to reduce the radiation dose of coronary CTA studies, such as sequential scanning, will eliminate the ability to assess LV function with the same study. The writing committee considered several emerging applications where empirical data were deemed insufficient to support development of a consensus. Imaging of noncalcified coronary plaques may in the future become a useful application for coronary CTA, but it has no role in current practice since there are insufficient data to assess its clinical utility. CTA assessment of total atherosclerotic burden and potential plaque vulnerability similarly will require substantial additional technical development and clinical investigation to define their potential value in patient management. The writing committee identified 3 areas without consensus: the interpretation of incidental noncardiac findings on the CT examination, the use of coronary CTA in asymptomatic subjects, and the "triple rule-out" examination of patients with acute chest pain in the emergency department. Use of coronary CTA raises 2 important safety issues: 1) the amount of radiation absorbed by the body tissues; and 2) the exposure to iodinated contrast agents that have the potential to produce allergic reactions and acute renal injury.
Mark, D. B., Berman, D. S., Budoff, M. J., Carr, J. J., Gerber, T. C., Hecht, H. S., … Schwartz, R. S. (2010, June 8). ACCF/ACR/AHA/NASCI/SAIP/SCAI/SCCT 2010 Expert Consensus Document on Coronary Computed Tomographic Angiography. A Report of the American College of Cardiology Foundation Task Force on Expert Consensus Documents. Journal of the American College of Cardiology. https://doi.org/10.1016/j.jacc.2009.11.013