2-MHz range-gated pulsed Doppler insonation through thinner parts of skull to determine blood velocity in the basal cerebral arteries. TCD measures the velocity and direction of blood flow, but does not measure flow rate or perfusion directly. However, elevated cerebral arterial blood velocity correlates well with angiographic vasospasm.

The MCA is the vessel best suited to TCD evaluation because of its location, size and orientation. In addition, MCA is an end artery with relatively limited leptomeningeal collateral supply. Sensitivity and specificity of TCD in demonstrating spasm in the MCA are 60-85 and 89-98%, respectively. Baseline MCA blood flow velocities range from 50 to 74 cm/s, with an average of 62 cm/s [19]. Blood flow velocity increases with progressive narrowing of vessels and, when critical narrowing occurs, blood flow is actually reduced, with the development of neurological deficits. Velocities greater than 120 cm/s indicate mild-to-moderate vasospasm, as seen by angiography, often with impending symptomatology, whereas velocities greater than 200 cm/s correlate with severe spasm. Infarction rarely occurs if the velocity is less than 140 cm/s, whilst velocities of greater than 200 cm/s are frequently associated with ischemia and infarction.

Xenon CT

Stable xenon CT is a relatively new technique, which is being increasingly used to demonstrate quantitative CBF measurements superimposed on anatomical information gained from plain CT. The patient undergoes CT scanning at limited levels through the brain (slices can be chosen to demonstrate anterior, middle and posterior cerebral vascular territories) before, during and after inhalation of non-radioactive xenon in oxygen. Patients with moderate or severe angiographic spasm have been shown to have globally reduced CBF and a poorer outcome than patients without angiographic vasospasm. However, there is often no direct relationship between vessel calibre on angiog-raphy and the corresponding regional cerebral blood flow (rCBF) on xenon CT.

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