Читать книгу Interventional Cardiology - Группа авторов - Страница 215

Grayscale IVUS has some ability to differentiate plaque composition based on different echoreflectivity of the tissue. Atherosclerotic plaques are rarely homogeneous and contain a mixture of plaque components with different impedance (density). A standard approach is to compare the echointensity or “brightness” of the plaque to the surrounding adventitia that is used as a reference. Three basic types of lesions are distinguished according to plaque echogenicity: (i) “soft” or hypoechoic plaque does not reflect much ultrasound and appears dark with less echointensity compared to the adventitia (Figure 8.5), (ii) fibrous, and (iii) calcific plaques are characterized by equal or greater intensity than the adventitia. A plaque that is not so reflective as to cause shadowing is labeled “hard” or hyperechoic and is composed primarily of fibrous tissue. The presence of acoustic shadowing along with the brightest echoes and reverberations are characteristic of the presence of calcification (Figure 8.4). Extensive target lesion calcification may adversely impact the PCI procedure by affecting the ability for effective dilatation of a coronary stenosis and is associated with greater likelihood of stent underexpansion. In lesions with maximum circumferential extension of calcium >180 degree by IVUS, greater calcific burden was associated with a smaller stent area and greater stent eccentricity [4].

Figure 8.4 A pure soft or hypoechoic plaque is uncommon because atherosclerotic plaques are rarely homogeneous. (a) shows an example of a predominantly soft plaque – a thin fibrous cap (small arrows) and lipid core underlying it; the plaque is less bright than the adventitia a. In (b), fibrous plaque or hyperechoic plaque is shown. Hyperechoic plaque is as bright as or brighter than the adventitia a without shadowing. In this eccentric plaque, the thickness of the media behind the thickest part of the plaque b is an artifact caused by attenuation of the beam as it passes through the hyperechoic plaque. In reality, the media becomes thinner with increasing atherosclerosis. Note that the media behind the thinnest part of the plaque is also thinner – without artifacts. (c) shows superficial calcium – defined as calcium a that is closer to the intima than it is to the adventitia. Calcium shadows the deeper arterial structures; in this case, the arc of calcification is ~180°.

Intimal hyperplasia due to in‐stent restenosis often appears to have low echogenecity depending, in part, on age and adjunct therapies (i.e. brachytherapy).

The identification of thrombus is difficult by IVUS. It may appear as lobulated hypoechoic mass within the lumen, scintillating echoes, a distinct interface between the presumed thrombus imaging and underlying plaque, and blood flow through the thrombus (Figure 8.5j).

Figure 8.5 Diagnostic intravascular ultrasound was performed to assess the angiographic filling defect at the right coronary artery. The intravascular ultrasound images are shown from proximal (A) to distal (J). There is marked segmental positive remodeling (B to I) with severe malapposition and underexpansion throughout the entire length of the stent, which appear “sized” just to the smallest lumen (panel B). Notice the space between the stent strut and the intima and the blood speckle/thrombus behind the stent struts in the axial (B through I) as well as the longitudinal view (at the bottom). At the site of maximum stent malapposition (I), the stent area (4.99mm²) was smaller than lumen area (15.22mm²) and external elastic membrane (26.64mm²). The entire distal 20+mm of the stent was thrombus filled with additional thrombus on the abluminal side of the stent partly filling the area of malapposition and causing the linear filling defect on the angiogram. The small intraluminal mass on IVUS (J, small arrows) represented the tail of the thrombus with the bulk being proximal to that slice.

Source: From Caixeta A et al. Einstein 2013;11: 364‐366.