Effects of pulse repetition frequency on bubble cloud characteristics and ablation in single-cycle histotripsy

Abstract

Histotripsy is a cavitation-based ultrasound ablation method in development for multiple clinical applications. This work investigates the effects of pulse repetition frequency (PRF) on bubble cloud characteristics and ablative capabilities for histotripsy using single-cycle pulsing methods. Bubble clouds produced by a 500 kHz histotripsy system at PRFs from 0.1-1000 Hz were visualized using high-speed optical imaging in 1% agarose tissue phantoms at peak negative pressures, p-, of 2-36 MPa. Results showed a decrease in the cavitation cloud threshold with increasing PRF, ranging from 26.7±0.5 MPa at 0.1 Hz to 15.0±1.9 MPa at 1000 Hz. Bubble cloud analysis showed cavitation clouds generated at low PRFs (0.1-1 Hz) were characterized by consistently dense bubble clouds (41.7±2.8 bubbles/mm2 at 0.1 Hz), that closely matched regions of the focus above the histotripsy intrinsic threshold. Bubble clouds formed at higher PRFs measured lower cloud densities (23.1±4.0 bubbles/mm2 at 1000 Hz), with the lowest density measured for 10 Hz(8.8±4.1 bubbles/mm2). Furthermore, higher PRFs showed increased pulse-to-pulse correlation, characteristic of cavitation memory effects; however, bubble clouds still filled the entire volume of the focus due to their initial density and enhanced bubble expansion from the restimulation of residual nuclei at the higher PRFs. Histotripsy ablation assessed through lesion analysis in red blood cell (RBC) phantoms showed higher PRFs generated lesions with lower adherence to the initial focal region compared to low PRF ablations; however, no trend of decreasing ablation efficiency with PRF was observed, with similar efficiencies observed for all the PRFs tested in this study. Notably, this result is different than what has previously been shown for shock-scattering histotripsy, which has shown decreased ablation efficiencies at higher PRFs. Overall, this study demonstrates the essential effects of PRF on single-cycle histotripsy procedures that should be considered to help guide future histotripsy pulsing strategies.

Ultrasound-Guided Mechanical High-Intensity Focused Ultrasound (Histotripsy) Through an Acoustically Permeable Polyolefin-Based Cranioplasty Device

“…With AC, more predictable cavitation was observed at 100 Hz, but significant peripheral cavitation was still seen at 500 Hz PRF. TBME-00252-2024.R1 expected given the comparatively lower p-) and took multiple histotripsy pulses to fully develop, mimicking recent findings which describe the behavior of histotripsy bubble clouds generated using single cycle pulses at PRFs ≥ 100 Hz [49]. In the referenced study, dense bubble clouds were generated by the tenth pulse at p-as low as 20 MPa for PRFs of 100 Hz and 1000 Hz [49].…”