The effects of non-uniform insonation on ultrasound monitoring of cerebral blood flow changes

Transcranial Doppler is widely used to measure flow velocity in the middle cerebral artery. These measurements are sometimes combined with Doppler power for use as an indicator of cerebral blood flow changes. This is based on the assumption that the power of the Doppler signal depends on the number of scatters within the beam, which is proportional to the vessel area, so any changes in vessel area will be matched exactly by changes in Doppler signal power. A three-dimensional numerical model was developed to incorporate data from beam shape measurements with various vessel shapes and sizes in order to study the appropriateness of this assumption. It was found, using the model, that for a typical, clinical positioning of the ultrasound beam with respect to the vessel the ultrasound field was not uniform and Doppler signal power changes significantly underestimated vessel area changes. A flow phantom was set up to investigate the effects in vitro, and in vivo recordings from a number of volunteers were also studied. Evidence of non-uniform insonation was seen in both situations, indicating that power and area changes are not equivalent. Using the spectra from the in vitro and in vivo recordings, it was possible to estimate the shape of the insonating beam and its size relative to the insonated vessel. However, the variance in the beam estimate prevented a direct measurement of changes in vessel size. Instead, a look-up table was derived to allow a correction of signal power measurements to account for non-uniform insonation effects.