Читать книгу Interventional Cardiology - Группа авторов - Страница 237

Atherosclerotic plaque components can be categorized by utilizing various optical properties of different tissues (Figure 9.2). Fibrous plaques are identified as homogenous, highly backscattering, low attenuation lesions. The emitted light is absorbed more intensely by lipids which leads to a high level of posterior signal attenuation; plaques appear as low backscatter regions with poor delineation of the external contour. Calcifications are also regions of low backscatter (signal poor), but they let light filter rather than absorb it, so that they maintain well‐delineated external borders. However, differentiation of signal‐poor areas is not always straightforward and calcium can be misinterpreted as lipids, particularly if the calcification is situated deep in the vessel wall.

Figure 9.2 Plaque characterization with optical coherence tomography. (a) Normal coronary anatomy organized in three layers. (b) Lipid‐rich plaque; note possible macrophage accumulations around 3 o’clock. (c) Lipid‐rich plaque in a saphenous vein graft. (d) Fibrous plaque. (e) Calcification extending deep into the vessel wall between 4 and 11 o’clock. Outer border of the calcification cannot be delineated. (f) Calcification between 12 and 4 o’clock. (g) Thrombus protruding into the lumen. (h) Neovascularization (arrows).

An early ex vivo study established a sensitivity and specificity ranging 71–79% and 97–98% for fibrous plaques, 95–96% and 97% for fibro‐calcific plaques, and 90–94% and 90–92% for lipid‐rich plaques with low inter‐observer and intra‐observer variability [18]. Other studies showed less impressive results, with only 45% of lipid‐laden atheromas identified, with higher but still suboptimal identification success in fibro‐calcific and fibrous plaques (68% and 83%, respectively) [19]. Misinterpretation in this study was mainly caused by low OCT signal penetration, which precluded the detection of lipid pools or calcium behind thick fibrous caps and by misclassification of calcium deposits for lipid pools and vice versa [19]. Furthermore, artifacts such as superficial shadowing and tangential signal dropout can produce images with signal‐poor regions covered by a thin signal rich layer mimicking thin‐cap fibroatheromas (TCFA). Rather than relying only on subjective visual interpretations, algorithms based on the optical attenuation coefficient to classify plaques quantitatively have been proposed; however, as yet, these algorithms are not sufficiently robust to be used in the clinical setting [20].

OCT imaging can also demonstrate thrombi as protrusions or floating masses. Red and white thrombi can be identified via the differences in attenuation intensity, with red thrombi showing high attenuation and complete wall shadowing and white thrombi appearing as low attenuation intraluminal masses or layers [21].