The arterial wall has three layers: (1) intima, typically white (echodense); (2) media, a thin black band; (3) adventitia, a thick, white, onion-skin external layer. The lumen is black (echolucent). In normal vessels, the intima is very thin, thinner than the media. In atheromatous arteries, the intima is essentially composed of atheroma, and its thickness corresponds to the plaque thickness; the media undergoes atrophy and becomes thinner than the intima. When one is looking at an IVUS image, the first step is to find the black band inside the arterial wall. This black band is the media; inside it are the intima and the lumen (Figures 37.1, 37.2). Because of a blood stasis artifact, the lumen may look white and “foggy” and the lumen–intima boundary may be blurry, making luminal measurement difficult. In addition, the “foggy” lumen may be confused with a thrombus or with an echolucent intima, e.g., lipid-rich intima or intima with a necrotic core, falsely suggesting unstable disease. In order to improve blood stasis artifact, flush the coronary artery with contrast or saline whenever there is luminal blurriness (Figure 37.3). One may also use the color signal feature available with the Volcano system, wherein blood echogenicity is assigned a red color. Vascular structures seen in the surroundings of the imaged artery may be arterial branches (e.g., septal, diagonal branches) or coronary veins. If, upon pullback, the structure enters the intima and joins the main vessel, the structure is a branch. Otherwise, it is a vein. Arterial branches are useful landmarks that identify the disease location and correlate it with angiography (Figures 37.4, 37.5, 37.6). Also, in the case of dissection, if the wire position is in question (true lumen vs. false lumen), seeing branches that join the lumen confirms the true luminal position of the wire. In the presence of atherosclerosis, the media undergoes expansion in such a way that the luminal area remains normal. This is called positive remodeling or the Glagov phenomenon (Figures 37.7, 37.8). When the plaque area occupies more than 40% of the total vessel area (external elastic membrane area), luminal narrowing is seen. However, ~10–20% of atherosclerotic vessels undergo negative remodeling, wherein the media constricts and further narrows the lumen beyond what is expected from atherosclerosis. Thus, luminal narrowing depends on the amount of atherosclerosis but also the type and extent of remodeling. Figure 37.1 The left image is duplicated on the right with blue shading highlighting the intima and a white line highlighting the media. Figure 37.2 The left image is duplicated on the right with blue shading highlighting the intima. A wire artifact is seen as a black shadow. An adjacent vessel is seen. The imaged vessel is the proximal LAD and the adjacent vessel is the ramus. Figure 37.3 (a) Intima is marked in blue in the right-hand image. Note that the lumen is hazy, and one cannot definitely rule out echolucent (dark) plaque within this lumen. (b) After flushing the lumen, its boundaries become sharper, which makes luminal and intimal calculations easier and rules out echolucent plaque or intraluminal thrombus. Figure 37.4 (a) Ostial LAD with LCx adjacent to it. The blue shading marks the intima, the gray shading marks the lumen. The black line surrounding the intima is the media. Always start by identifying the media, then identify the intima and lumen. (b) The severe LAD disease with constrictive remodeling extends until the very ostium, just as the LCx meets the LAD. The LAD lumen is narrow as a result of atherosclerosis but also as a result of vessel constriction. In fact, the media-to-media diameter of this ostial LAD (external elastic membrane [EEM] diameter) is only 3 mm (the distance between two dots being 1 mm) Figure 37.5 Diseased LAD at the level of first septal and first diagonal branches. Figure 37.6 (a, b) Two IVUS images of a left main bifurcating into LAD and LCx. In (b), note that the ostial LAD is heavily calcified with a luminal area of 4 mm2. Figure 37.7 Positive and negative remodeling. Figure 37.8 Example of positive remodeling (EEM area has expanded to accommodate atherosclerosis). Remodeling index = EEM area at the level of the lesion (right)/EEM area at the level of the reference (left). IVUS identifies three types of plaques, i.e., three types of intima: In addition, one should look for signs of plaque instability (Figures 37.9–37.12):
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Intracoronary Imaging
1. INTRAVASCULAR ULTRASOUND (IVUS)
I. Image basics
II. Plaque types
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