(a) Spectrum from the external carotid artery shows a high-resistance waveform with reversal of flow in early diastole. (b) Spectrum from the internal carotid artery displays a low-resistance waveform with continuous forward diastolic flow and with a spectral line that ascends farther above the baseline than that from the external carotid artery. (c) Spectrum from the common carotid artery represents a composite of waveforms from the external carotid artery and internal carotid artery but more closely resembles the waveform from the internal carotid artery, because 80% of the common carotid artery flow goes into the internal carotid artery.

(d) Spectrum from the vertebral artery shows low-resistance forward flow above the baseline throughout diastole. Spectral broadening is due to the small diameter of this vessel.

http://radiographics.rsna.org/content/28/3/691.full