Screening coronary CT angiography: possibilities and pitfalls.

Abstract

Coronary artery calcium scoring (CACS) is a standard imaging tool to predict future cardiac events in asymptomatic subjects with intermediate Framingham risk score (FRS) due to its high accuracy for detecting coronary artery disease (CAD) [1]. In fact, the recently published 2013 guidelines of the AHA/ACC suggest an additional role for CACS in certain asymptomatic patients who are being considered for statin therapy but who are not included in four major statin benefit groups [2]. Notwithstanding these statements, it is undeniable that coronary CTA provides clearer anatomic definition of CAD than CACS and thus may have a role in screening for individuals at risk for CAD. The greatest advantage of coronary CTA over CACS is in visualization of the extent and type of coronary plaque. It has been estimated that CACS identifies only 20 % of total plaque volume that is present histologically [3]. Not only does coronary CTA visualize more of the total plaque amount, but it can provide superior information as to its location, whether the plaque is calcified, non-calcified or mixed, obstructive or non-obstructive and the status of vessel remodeling. Coronary CTA may also provide an advantage over CACS in identifying potential precursor lesions of the acute coronary syndrome (ACS), so-called vulnerable plaque [4]. In a cohort of symptomatic patients, Motoyama et al. [4] reported that the CT features of vulnerable plaque include positive remodeling and low attenuation plaque (\30 HU). ACS events were reported in only 0.49 % of patients lacking these features. In contrast, patients with plaques showing positive remodeling and/or low attenuation plaque had a very high hazard ratio of 22.8 for developing ACS compared with those lacking such plaques [4]. A more recently reported coronary CTA finding associated with risk is the so-called ‘‘napkin-ring sign’’, defined as plaque with low-attenuation center surrounded by a rim of higher attenuation. In one study comprising symptomatic and asymptomatic patients, the napkin-ring sign was an independent predictor of ACS, although the sign had a limited sensitivity of 41 % [5]. However, whether any of these findings that are associated with symptomatic patients can be generalized to an asymptomatic screening population is an open question. In addition, coronary CTA has other intrinsic limitations. It cannot reliably discriminate thin-cap fibroatheroma, an important feature of vulnerable plaque from thick-cap fibroatheroma because of limitations of spatial resolution [6]. CT density of a non-calcified plaque may also be altered by different kV settings or the degree of intracoronary enhancement after the contrast injection [7]. Thus, it is difficult to reliably differentiate lipid core from fibrous plaque based on the CT density itself. Other practical limitations to the use of screening coronary CTA instead of CACS include potentially increased radiation exposure, intravenous contrast use, and the concern that it will lead to increased unnecessary downstream testing. However, the gap between coronary CTA and CACS in terms of radiation exposure has been closed due to substantial progress in applying radiation sparing techniques [8, 9]. Thus, radiation S. M. Yoo Department of Diagnostic Radiology, CHA University Bundang Medical Center, Sungnam, South Korea