In transtemporal Doppler, which vessel at about 55-65 mm depth presents bidirectional flow with velocity near 55 cm/s?

• A. Middle cerebral artery
• B. Posterior cerebral artery
• C. Terminal internal carotid artery
• D. Anterior cerebral artery

Answer: (C)

In transtemporal Doppler, you identify vessels by where they lie in depth and how their waveforms look. The depth of about 55–65 mm lines up with the terminal internal carotid artery just before it splits into the middle and anterior cerebral arteries. The velocity around 55 cm/s fits the internal carotid’s characteristic flow speed—faster than most distal intracranial branches and with a strong systolic component plus continued diastolic flow.

Seeing bidirectional flow at this site makes sense if your sample is near the bifurcation point where flow is feeding two downstream branches. The spectral display can then show components directed toward each branch, which may appear as bidirectional flow depending on the angle of insonation and how the waveform is presented. This combination of depth and velocity, along with the bifurcation geometry, points to the terminal internal carotid artery as the vessel in question.