**4. Plaque characterization**

The first attempts in evaluating atherosclerotic plaques via CT have already been made 1985 [43], but this approach did not gain acceptance due to insufficient resolution and image quality. Nowadays, with a spatial resolution up to 400 μm, noninvasive detection and characterization of atherosclerotic lesion and plaque characteristics can be performed by current CT scanners. Although intravascular ultrasound (IVUS) and optical coherence tomography (OCT) provide even higher spatial resolutions up to 80 and 20 μm, respectively [44], and therefore are the reference standard, cCTA yields the advantage of its noninvasive character. This technique enables an evaluation and characterization of the individual plaque extent and composition in patients without the clear indication for invasive measures. Recent studies have shown the ability of cCTA to perform on a high level in comparison with earlier mentioned reference standards, thus making cCTA a promising noninvasive method in identifying high-risk atherosclerotic coronary plaques [45–47]. Plaque characterization is essential in risk stratification in patients with suspected or diagnosed CAD or ACS, hereby it is important to distinguish the terms "stable" and "vulnerable" plaque (**Figure 2**). The hazard in stable plaques, consisting mainly of calcifications, lies in their subsequent obstruction of vessel lumen, associated with hemodynamic insufficiency, whereas vulnerable plaques tend to rupture and can lead to occlusion of the affected vessel through the thrombogenic lesion [48]. The finding that major adverse cardiac events (MACEs) are a consequence of the hemodynamically insignificant vulnerable plaques in more of two-thirds has been already made in the end of the last century [49, 50], but only now it is possible to detect morphological correlates *in vivo* via noninvasive methods [51]. Certain morphological plaque features correlate with the presence of ruptureprone plaques, and it is yet to be examined, which of these are reliable markers of plaque vulnerability [47, 52]. Although cCTA can distinguish distinctly between calcified, noncalcified (lipid rich/fibrotic) and mixed plaques, direct visualization of thin-cap fibroatheroma (TCFA) is currently only possible via OCT. To make plaque characterization via cCTA less dependent on the examiner's experience, scoring systems [53] and semiautomated software are ready to be implemented in clinical use, increasing operator convenience of this promising method.
