Читать книгу Interventional Cardiology - Группа авторов - Страница 213

The ultrasound appearance of normal human arteries in vitro and in vivo has been reported [6–8]. In muscular arteries such as the coronary tree there are three layers: intima, media, and adventitia. Normal intima thickness increases with age, from a single endothelial cell at birth to a mean of 60 μm at five years to 220–250 μm at 30–40 years of age [19]. The definition of abnormal intimal thickness by IVUS is still controversial; in general, the threshold of “normal intimal thickness” is <300 μm (0.3 mm). The innermost layer of the intima is relatively echogenic compared with the lumen and media and displayed on the screen as a single bright concentric echo. The lower ultrasound reflectance of the media is due to its homogeneous smooth muscle cells distribution and smaller amounts of collagen, elastic tissue, and proteoglycans. The thickness of media histologically averages 200 μm, but medial thinning occurs in the presence of atherosclerosis [9]. In advanced atherosclerotic disease, the media may not appear as a distinct layer around the full circumference of the vessel; media thickness of coronary arteries is inversely related to lesion thickness [10]. The intima–media border is poorly defined because the intimal layer reflects ultrasound more strongly than the media. Conversely, the media–adventitia border, consistent with the location of the external elastic membrane (EEM), is accurately defined because a step‐up in echo reflectivity occurs without blooming. The outermost layer, the adventitia, is composed of collagen and elastic tissue; it is 300–500 μm thick. The outer border of the adventitia is also indistinct due to echo reflectivity similar to the surrounding peri‐adventitial tissues [6]. Therefore, the normal coronary artery is either (i) “mono‐layered” in cases of intimal thickness <100 μm because (if in case a 20 or 40 MHz is used) IVUS catheter resolution is less than 100 μm; or (ii) “three‐layered” to include a bright echo from the intima, a dark zone from the media, and bright surrounding echoes from the adventitia (Figure 8.2). The “three‐layered” has been better recognized with the 60 MHz IVUS catheter.

Figure 8.2 Normal coronary artery morphology in cross‐sectional view. In the magnified image on the right, the bright inner layer (intima), middle echolucent zone (media), and outer bright layer (adventitia) are representative of the “three‐layered” appearance of intravascular ultrasound (IVUS). In the magnified image on the left, only the outer bright adventitial layer is representative of the “mono‐layered” appearance.