All plaques identified by OCT are characterized by the loss of the layered structure observed in normal vessels or vessels with intimal hyperplasia. As the various components of atherosclerotic plaques have different optical properties, OCT makes it possible to differentiate them to a great extent. Identification of plaque components by OCT depends on the penetration depth of the incident light beam into the vessel wall. The depth of penetration is greatest for fibrous tissue and least for thrombi with calcium and lipid tissue having intermediate values [3, 8, 9].

Calcifications within plaques are identified by the presence of well-delineated, low-backscattering heterogeneous regions (Figs. 13.1 and 13.2) [3, 6–9]. Superficial microcalcifications , considered to be a distinctive feature of plaque vulnerability, are revealed as small superficial calcific deposit. The contrast between calcifications and the surrounding vessel wall is often well-defined in IVUS images. However, the bright IVUS signal from calcifications can cause difficulty in accurate assessment of neighboring plaque composition due to saturation artifact. In contrast, OCT images allow improved evaluation of the extent of calcifications within plaques and visualization of plaque microstructure adjacent to calcifications. Fibrous plaques are typically rich in collagen or muscle cells and consist of homogeneous high-backscattering area (Figs. 13.2 and 13.3) [3, 6–9]. Necrotic lipid pools are less well-delineated than calcifications and exhibit decreased signal density and more heterogeneous backscattering than fibrous plaques (Fig. 13.2) [3, 6–9]. The strong contrast between lipid-rich cores and fibrous regions in OCT images allows fibrous caps to be easily identified.

Intracoronary thrombi might take a critical role in the pathogenesis and the clinical manifestations of acute myocardial infarction (AMI). But coronary angiography and IVUS cannot reliably identify thrombus, and OCT is able to visualize the intracoronary thrombus clearly [8]. Thrombi are identified by the masses protruding into the vessel lumen discontinuous from the surface of the vessel wall. White thrombi consist mainly of platelets and white blood cells and are characterized by a signal-rich, low-backscattering billowing projections protruding into the lumen (Figs. 13.4 and 13.5). Red thrombi consist mainly of red blood cells, and relevant OCT images are characterized as high-backscattering protrusions with signal-free shadowing (Figs. 13.2 and 13.5) [10].