Interpretation of power changes in Doppler signals from human blood-in vitro studies

Extensive in vitro measurements of the received Doppler power have been made in order to understand the origin of the large power changes that have been previously reported for carotid and umbilical Doppler signals over a cardiac cycle. Variations in the backscattered Doppler power with flow have been investigated using both fixed human red cells at a hematocrit of 4% and heparinized human blood at hematocrits of up to 42%. Measurements were made under both steady and pulsatile flow conditions using a waveform similar to that seen in the carotid artery. A pulsed Doppler system was used to study the power variations at various radial positions in the vessel, while a continuous-wave system provided a measure of the power across the vessel. These studies show that the backscattered power is independent of the flow rate provided that the flow is laminar. However, as the flow rate is increased, it was found that at the onset of turbulence the Doppler power increases and the statistical properties of the signal change. It is also shown that, under normal clinical pulsatile flow conditions, any power change that is observed to occur over the cardiac cycle is unlikely to be caused by changes in flow rate