Page 104 - Libro vascular I

Chap-08.qxd 1~9~04 16:41 Page 95
ULTRASOUND ASSESSMENT OF THE EXTRACRANIAL CEREBRAL CIRCULATION
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ICA
ICA
CCA
AB
CCA
ICA
CD
ICA
CCA
CCA
Figure 8.16 A: Anechoic plaque (arrow) with a thin cap, type 1. B: Type 2 plaque. C: Type 3 plaque. Types 2 and 3 are heterogeneous, with type 3 appearing more echogenic than type 2. D: Homogeneous echogenic plaque, type 4.
have attempted to use image analysis methods to objectively quantify features in images of plaques, which may in time improve plaque characterization.
Large areas of atheroma will often be seen in the origin of an occluded ICA and, if the occlusion is long-standing, the occluded ICA may appear much smaller as its lumen contracts over time (Fig. 8.18).
COLOR IMAGING
Normal appearance
Flow in normal carotid arteries is pulsatile, with forward flow present throughout the cardiac cycle. With the appropriate PRF selected, color should continually fill the vessel lumen up to the walls. The flow may appear more pulsatile in the ECA than in the ICA and CCA. At the bifurcation, changes in the vessel geometry often lead to areas of flow reversal within the bifurcation, on the opposite side of the
vessel to the ECA origin, giving the normal appearance seen in Chapter 5 (see Fig. 5.12A).
Abnormal appearance
The lack of color filling to the vessel wall may indi- cate the presence of atheroma. However, it is important to ensure that filling defects are not due to a poor Doppler angle, inappropriately high PRF or high-pass filter setting or to the presence of an image artifact preventing the color from being dis- played. The presence of a filling defect at the ves- sel wall, where atheroma is not apparent on the B-mode image, may indicate an area of anechoic atheroma.
Increased velocity within a stenosis usually causes a change in the color displayed on the image, often associated with aliasing (Fig. 8.17A) and some- times with flow recirculation (see Fig. 5.18). The color flow image can help to locate the area of