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Chap-08.qxd 1~9~04 16:41 Page 91
ULTRASOUND ASSESSMENT OF THE EXTRACRANIAL CEREBRAL CIRCULATION
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Vein CCA
ECA
Figure 8.8 B-mode image of the ECA showing the superior thyroid branch (arrow).
A
the flow from the proximal CCA up into the ICA and ECA. Identification of ECA branches (either on B-mode or color imaging) serves as a further indication as to which vessel is the ECA, as the ICA has no branches below the jaw (Fig. 8.8). Color flow imaging can provide evidence of disease, such as velocity changes due to steno- sis, areas of filling defects due to the presence of atheroma and the absence of flow due to occlu- sion. Diagnosis should not be made based on the color flow imaging alone, but it greatly aids the sonographer in selecting areas that require close investigation with the spectral Doppler.
4. The spectral Doppler is now used to observe the inflow to the carotid arteries by placing the sam- ple volume in the proximal CCA at the base of the neck. The shape of the waveform may reveal the presence of proximal or distal disease, such as an ICA occlusion. In the absence of signifi- cant distal or proximal disease, the left and right CCA waveforms should appear symmetrical.
B
5. The examination so far has provided many clues C
as to which of the two vessels beyond the bifurcation is the ICA, such as the relative size and position of the two vessels and the presence of ECA branches. Spectral Doppler can now be used to confirm the identification of the ICA and ECA, as the ICA waveform shape is less pulsatile and has higher diastolic flow than the ECA (Fig. 8.9). Differentiation of the vessels may be further helped by tapping the temporal artery, an ECA branch (which runs in front of
Typical normal Doppler spectra obtained from the CCA (A), the ICA (B) and the ECA (C). The effect of temporal tapping on ECA diastolic flow is marked with arrows.
Figure 8.9
the upper part of the ear), as this will cause changes in the ECA flow during diastole (Fig. 8.9C) but will have little effect on the ICA. It is imperative that the ICA and ECA should be correctly identified, as it is the presence of