Spatial-temporal dynamics of cavitation bubbles induced by pulsed HIFU thrombolysis within a vessel and parameters optimization for cavitation enhancement

Abstract

In this paper, conventional B-mode imaging and pulsed Doppler were employed to investigate spatial-temporal distribution of residual cavitation bubbles generated by pHIFU thrombolysis within a vessel. The HIFU pulse sequences had various acoustic parameters including pulse duration (PD = 10, 20, 30 µs), duty cycle (DC = 1∶10, 10∶20, 1∶30) and the number of pulses (n = 5, 10, 20). A non-transparent wall-less flow phantom with a diameter of 4 mm and porcine blood clot were employed to mimic an embolism and its flow environment with various velocities from 0 to 5 cm/s. The results showed that when a single pulse sequence was used, residual bubble cloud firstly emerged at the focal region, then expanded along the vessel axis with an initial velocity, and gradually dissolved. When subject to multiple pulse sequences, the bubble cloud generated by each single pulse sequence accumulated persistently, and expanded with a much higher volume than that by one single pulse sequence. The initial velocity, the fluid velocity and the effects of vessel wall mainly determined the distribution and movement of cavitation bubbles in the vessel. The velocity of a whole bubble cloud moving along the vessel axis was 0 to 9.5 cm/s. The velocity of bubbles immediately after HIFU treatment was highest with both forward and reverse flows, and then decreased until close to fluid velocity. The higher initial velocity and volume of cavitation cloud were achieved with PD of 10 µs, 20 µs and 30 µs, and DC of 1∶10, 1∶30 and 1∶20, and n of 5, 10, 20, suggesting that the optimal parameter to enhance cavitation was PD of 30 µs, DC of 1∶20, and n of 20. Conventional B-mode imaging and pulsed Doppler may supply the possibility to monitor spatial-temporal distribution of cavitation bubbles during a clinical ultrasound thrombolysis, and be useful for parameters optimization to improve thrombolysis efficiency by cavitation enhancement.