The effects of beam shape on the ability to predict changes in vessel sizefrom Doppler signal power

Theory suggests that, under certain ideal circumstances, the power of a Dop pler signal is proportional to the size of the vessel from which it is reco rded and can, therefore, be used to assess the scale of any changes in vess el size that occur during clinical recordings of cerebral blood flow. Howev er, the relationship between signal power and vessel size depends on the in tensity of the incident beam being uniform across the vessel. This is unlik ely to be the case for cerebral vessels insonated by commercial transducers , A model was used to estimate the signal power received from vessels inson ated by a beam passing through a homogeneous medium, and also by beams pass ing through each of five samples of temporal bone. In each case, the effect s of initial vessel size and various changes in cross-sectional area were i nvestigated for different vessel positions in the beam. The results for the beam paths through bone predict that the power change arising from a chang e in vessel cross-sectional area is between 5% and 75% smaller than that oc curring in a uniform beam. If these results are representative of those ari sing for an in vivo change in middle cerebral artery size, then the potenti al magnitude of the error illustrates the caution that may need to be appli ed when interpreting changes in Doppler signal power. (C) 2000 World Federa tion for Ultrasound in Medicine & Biology.