Effect of Frequency and Focal Spacing on Transcranial Histotripsy Clot Liquefaction, Using Electronic Focal Steering

This in vitro study investigated the effects of ultrasound frequency and focal spacing on blood clot liquefaction via transcranial histotripsy. Histotripsy pulses were delivered using two 256-element hemispherical transducers of different frequency (250 and 500 kHz) with 30-cm aperture diameters. A 4-cm diameter spherical volume of in vitro blood clot was treated through 3 excised human skullcaps by electronically steering the focus with frequency proportional focal spacing: λ/2, 2 λ/3 and λ with 50 pulses per location. The pulse repetition frequency across the volume was 200 Hz, corresponding to a duty cycle of 0.08% (250 kHz) and 0.04% (500 kHz) for each focal location. Skull heating during treatment was monitored. Liquefied clot was drained via catheter and syringe in the range of 6-59 mL in 0.9-42.4 min. The fastest rate was 16.6 mL/min. The best parameter combination was λ spacing at 500 kHz, which produced large liquefaction through 3 skullcaps (23.1 ± 4.0, 37.1 ± 16.9 and 25.4 ± 16.9 mL) with the fast rates (3.2 ± 0.6, 5.1 ± 2.3 and 3.5 ± 0.4 mL/min). The temperature rise through the 3 skullcaps remained below 4°C.