Doppler power variation from porcine blood under steady and pulsatile flow

Although a number of recent studies have demonstrated that the echogenicity of blood varies as a function of time under pulsatile flow, the fundamenta l mechanisms responsible for it are still uncertain. To better understand t his phenomenon, the Doppler power from porcine blood and polystyrene micros phere suspensions was measured at the center of the tube as functions of tw o crucial parameters, flow velocity and stroke rate (for pulsatile flow), u nder steady and pulsatile flow in a mock flow loop. In the present study, t he experimental results were obtained with a 10-MHz pulsed Doppler system w ith a frequency response estimated more accurately by electronic injection, and validated by comparing to the radiofrequency (RF) signal acquired from the same Doppler instrument. The results show that the Doppler power from microspheres and porcine red blood cell (RBC) suspensions did not vary appr eciably (<2 dB), with either the speed or stroke rate (for pulsatile flow o nly) under steady and pulsatile flow. It was found that the Doppler power f rom porcine whole blood under steady flow decreased with the speed by appro ximately 13 dB from 3 to 33 cm/s and was only 3 dB higher than that from RB C suspension at 33 cm/s, suggesting minimal RBC aggregation in whole blood at this speed. The apparent cyclic variation from whole blood was observed at 20 and 40 beats/min (BPM). The cyclic variation became more obvious as t he speed and stroke rate decreased. The mean Doppler power over a cycle inc reased as the peak speed decreased. The Doppler power reached a maximum nea r peak systole and a minimum at late diastole at the center of the tube. Th is pattern cannot be explained by RBC aggregation due to the shear rate alo ne, and may be attributed to acceleration and deceleration along with aggre gation. The cyclic variation was not observed at 60 BPM, probably because o f a lack of time for aggregation to occur.