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Transcranial Doppler measurements of blood flow velocity in the middle cerebral artery are often used as the basis for non-invasive monitoring of changes in cerebral blood flow. Flow index techniques that include a measurement of signal power to take account of possible diameter changes may be hampered in this aim by non-uniform insonation of the vessel. A numerical model, created from in vitro measurements of ultrasound sensitivity and in vivo measurements of middle cerebral artery position, was used to calculate the magnitude of the effect. The calculated power change always underestimated the change in area, typically by 14%, but by 36% in the worst case.