Cyclic and radial variation of ultrasonic backscatter from flowing porcine blood

by Paeng, Dong-Guk.

The ultrasonic backscattering from flowing blood was investigated using several hemodynamic parameters and a physiological parameter. An emphasis was placed on the cyclic variation of the backscattering power and the origin and mechanisms responsible for it. Acceleration was hypothesized to enhance the aggregation of red blood cells (RBCs), and this is the first time that acceleration is suggested and experimentally verified as having an effect on aggregation of RBC. Two interesting phenomena, the ‘Black Hole (BH)’ phenomenon and the ‘Bright Collapsing Ring (BCR)’ phenomenon, were observed under pulsatile flow in B-mode cross sectional images. The BH phenomenon describes a dark hypoechoic hole at the center of the tube surrounded by a bright hyperechoic zone in B-mode cross sectional images, and the BCR phenomenon describes the appearance of a bright hyperechoic ring at the periphery of the tube at early systole and its convergence from the periphery to the center of the tube, finally collapsing as flow develops. These two phenomena were analyzed by the RBC aggregation due to the combined effects of the shear rate and acceleration, and this analysis could provide an integrated explanation of the cyclic and radial variation of the backscattered power. The origin of the Doppler power variation was investigated using a 10 MHz pulsed Doppler system with a single element transducer from three different fluid media: a rigid polystyrene microsphere solution, deformable porcine RBC suspension, and aggregating porcine whole blood. The Doppler power variation was observed only from porcine whole blood, which led to a conclusion that the ultrasonic backscattering was mainly dependent on the RBC aggregation under steady and pulsatile flow. The pattern of the cyclic variation of the Doppler power to have a maximum power at peak systole was mainly due to the enhanced rouleaux formation by acceleration. The BCR phenomenon was observed from the cyclic variation pattern of the Doppler power at different radial positions; the Doppler power peak was observed at early systole at the periphery of the tube and lagged the flow as close from the periphery to the center of the tube. The BH iv phenomenon from the Doppler power measurements was also observed during some parts of a cycle. The BCR phenomenon from porcine whole blood in a mock flow loop was further examined in real time in B-mode images under pulsatile flow. The BCR phenomenon was found to be dependent on the flow speed, stroke rate and hematocrit, but independent on the transducer frequency from 9 to 13 MHz. The BCR phenomenon was stronger as systolic peak speed at the center of the tube increased from 10 to 25 cm/s, and as stroke rate decreased from 60 to 20 BPM. At low hematocrit of 12 %, no BCR phenomenon was discernable although it was observed at higher hematocrits. The pattern of the nonlinear relationship between echogenicity and hematocrit varied with radial positions. The BH phenomenon was also observed under certain hemodynamic conditions and varied over a pulsatile cycle. The BCR phenomenon was also observed from human carotid arteries from 10 subjects only in the harmonic images. In order to better understand these phenomena, the cyclic and radial variation of echogenicity under oscillatory flow was measured and the results showed a different pattern from that under pulsatile flow. The echogenicity at the center of the tube was enhanced during acceleration and degraded during deceleration, while the expansion and collapse of the ‘Bright Ring’ was observed twice per cycle. The cyclic and radial variation of echogenicity was dependent on stroke volume, stroke rate, mean steady flow added to the pure oscillatory flow, and transducer angle. The rouleaux distribution and orientation across the tube during flow acceleration were proposed based on the experimental results of the transducer angle, reaching a maximum of the echogenicity variation at about 25°. The cyclic variation of echogenicity was also observed from the porcine RBC suspensions, independent of the transducer angle and stroke rate but changed with the mean steady flow added to the pure oscillatory flow. The strong variation of echogenicity from oscillatory flow seemed to be caused by the rouleaux formation in addition to the echogenicity variations from one cell based deformation of RBC suspensions. v