Characterization of plaque components based on tissue attenuation in Hounsfield units (HU) allows differentiation into broad groups of noncalcified, mixed, and calcified plaque types (Figs. 14.2, 14.3, 14.4, and 14.5). Comparison of the plaque types differentiated by CT with IVUS plaque characterization has revealed a correlation between IVUS echodensity and HU values, but significant overlap of attenuation values between the different noncalcified plaque types, in particular between lipid-rich and fibrous plaque components (Table 14.2). In addition, CT studies demonstrate that the attenuation of plaques is influenced by contrast density in the lumen and by contrast enhancement of the vessel wall itself. Furthermore, the visualization of relatively small HU differences between different plaque components is probably affected by the imaging technique used. The impact of low-dose CTA acquisition including imaging with lower tube voltage (kV), tube current (mA), and novel reconstruction algorithms, such as iterative reconstruction, on plaque imaging is incompletely understood. Importantly, it should be noted that IVUS echodensity has limited value as a reference standard because it correlates only modestly with histology.

The assessment of arterial remodeling is an important aspect of plaque identification and quantification. As described above, early atherosclerotic plaque accumulation is typically associated with expansion of the vessel wall, mitigating development of luminal narrowing. It is now well established that remodeling is related to plaque stability, with outward/positive remodeling constituting a marker of vulnerability. Remodeling is described by relating the vessel size at the lesion site to that at the adjacent reference site. The remodeling index is a quantitative measure calculated by dividing the vessel diameter at the lesion site by that at the adjacent normal reference site. Values higher than 1 or 1.05 indicate outward (positive) remodeling. While remodeling can be assessed with CT (Table 14.3), assessment of remodeling in clinical routine is limited by the focal nature of plaque and the fact that it is dependent on the definition of the lesion and reference site.

Quantification of overall plaque burden and its components with CT requires outlining the lumen/vessel wall and vessel wall/adventitia borders for segmentation of the wall, including atherosclerotic plaque. Segmentation allows quantification of plaque area or volume, and in a second step, the vessel wall and plaque contained between these borders can be characterized based on HU (Fig. 14.6 and 14.7).

Limitations are the lower spatial resolution relative to IVUS and optical coherence tomography (>0.4 mm for each voxel edge with CT) and difficulties in defining intimal and adventitial borders. Clinical experience with CT in comparison to histology and IVUS for quantification of coronary artery plaque (Table 14.4), including reproducibility of plaque volume quantification is accumulating (Table 14.5). Figure 14.8 outlines a systematic approach to plaque identification and semiquantitative grading.