Effect of Pulse Repetition Frequency of HIFU on in Vitro Thrombolysis
Vascular occlusion by the thrombi is the main reason for ischemic stroke and deep vein thrombosis. High-intensity
focused ultrasound (HIFU) and histotripsy or microtripsy pulses can effectively dissolve the blood clot with no use of
thrombolytic agent and ultrasound contrast agent. In this study, HIFU bursts at the same duty cycle (2%) but varied pulse
repetition frequency (PRF) from 1 Hz to 1000 Hz were delivered to in vitro porcine blood clot for 30 s. Thrombolysis efficiency
initially increases slightly with the PRF, 86.4±10.3%, 89.9±11.9, and 92.9±12.8% at the PRF of 1 Hz, 10 Hz, and 100 Hz,
respectively, without significant difference, but then drops dramatically to 37.9±6.9% at the PRF of 1000 Hz. The particle size
in the supernatant of dissolution is 547.1±129.5 nm. Thrombi motion during HIFU exposure shows violent motion and
significant curling at the low PRF, rotation about its axis with occasional curling at the moderate PRF, and localized vibration
at the high PRF due to the generation of acoustic radiation force and streaming. Quantitative analysis of recorded motion
shows the axial displacement decreases with the PRF of delivered HIFU bursts, from 3.9±1.5 mm at 1 Hz to 0.7±0.4 mm at
1000 Hz. Bubble cavitation during HIFU exposure to the blood clot was also monitored. The increase of PRF led to the
increase of inertial cavitation but the decrease of stable cavitation.
Index Terms- thrombolysis, high-intensity focused ultrasound (HIFU), pulse repetition frequency (PRF), clot motion.