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Fig. 9B. —78-year-old man with retrograde flow in left external carotid
artery caused by complete occlusion of left common carotid artery. Spectral
Doppler sonography tracing from internal carotid artery shows flow away from
transducer (display has been inverted to show waveform above baseline; note
negative velocity measurements on scale). Therefore, flow in internal carotid
artery is antegrade, reconstituted via retrograde flow in external carotid
artery. Note that reconstituted flow in external carotid artery and internal
carotid artery displays low-resistance waveform with diminished systolic
amplitude and delayed systolic upstroke similar to parvus–tardus
waveform. This waveform is seen in 70% of proximal occlusions, presumably
resulting from filtering out of higher frequency velocities by collateral
network supplying external carotid artery, often from contralateral external
carotid artery system.
