BLOOD-FLOW MEASUREMENT BY DOPPLER ULTRASOUND - A QUESTION OF ANGLES

Ultrasound Doppler equipment is widely used to estimate blood velocity and volume flow. Recently there has been correspondence concerning th e origin of the Doppler shift in blood vessels. The assumption that on ly the movement of the target need be considered is challenged by the observation that Doppler shifts occur where there is relative motion. In blood there is unlikely to be relative motion between a reflecting blood corpuscle and its supporting plasma. In the simple case of plug flow the relative movement takes place at the vessel wall. An investig ation is described which uses a flow rig Doppler phantom to assess wha t effect, if any, this phenomenon would have on Doppler shift results obtained by insonating a vessel such as the aortic arch and range gati ng through a curved section where flow velocity gradients are encounte red. It has been assumed that if the sample volume were to lie in that part of the vessel in which the blood velocity vector were directly t owards the probe, then an angle of 0-degrees could be assumed for calc ulations using the Doppler equation. Our results indicate that the obs erved Doppler shifts lie midway between those expected if the shift we re to occur solely at the first moving fluid boundary and those origin ating only from scatterers within the sample volume.