Optimization of histotripsy for kidney stone erosion

Previous work has demonstrated that histotripsy can effectively comminute model kidney stones, eroding them to sub-100 µm particulate debris via a cavitational bubble cloud. We now focus on optimization of the histotripsy parameter space and treatment strategies to yield optimal stone erosion. Ultracal-30 model stones were sonicated using a 750 kHz piezocomposite transducer. Pulses of 5 cycles in duration delivered at pulse repetition frequencies (PRFs) of 10 Hz, 100 Hz, and 1 kHz were tested. For each PRF, the erosion characteristics were investigated at increasing intensity levels corresponding to peak negative pressures (p-) of 10, 15, 19, 22, and 26 MPa. Additionally, the effect of exposed stone surface area was explored through the treatment of the same model stones that were first mechanically sectioned into 1/8's. Results show the following: (1) At PRF of 10 Hz, stone erosion rate was observed to increase linearly with p-; at 100 Hz and 1 kHz, erosion rate saturation was observed. (2) Increasing the PRF at a given intensity resulted in an increase in stone erosion; however, a 10fold increase in PRF did not produce an equivalent 10-fold increase in erosion rate. (3) Treatment of model stones sectioned into 1/8's produced a 2-fold rate increase relative to their unsectioned counterparts. (4) The maximum observed histotripsy stone treatment rate (88 mg/min) rivals that previously measured for a piezoelectric lithotripter (111 mg/min); this fact, coupled with histotripsy being a surface erosion phenomenon that produces ultra-fine debris, suggests that histotripsy offers a potential adjunct to conventional shock wave lithotripsy.