a-

b-

Normal spectral waveforms from intracranial arteries. (a) Spectrum from the anterior cerebral artery in an 8-month-old girl shows a low-resistance waveform with continuous forward diastolic flow. (b) Spectrum from the posterior cerebral artery in a 6-day-old girl shows similar low resistance but less diastolic flow than in a. Note the complete filling of the spectral window because of the small diameters of these vessels.

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