Figure 8
From: Pilot in vivo studies on transcutaneous boiling histotripsy in porcine liver and kidney
Ex vivo investigation of the aberrative properties of fatty layers. (A) Schematic of the experimental setup. A 3–4 cm thick segment of the body wall overlying the liver, consisting of alternating layers of fat and muscle of varying thickness, was freshly collected from the experimental animals. Fiber Optic Probe Hydrophone (FOPH) was used to record ultrasound waveforms at the HIFU transducer focus in free field in water and behind the segment of the body wall. (B) Representative focal waveforms in shock forming regime (acoustic power 400 W) produced by individual elements and all elements of the array measured in water in free field. Each element of the array produced a small amplitude shock wave at the focus, and the shock fronts were temporally aligned. When all elements were activated simultaneously, a shock front of sufficient amplitude for BH ablation was formed at the focus (red arrow). (C) Representative focal waveforms from individual array elements and the entire array recorded in water behind a 3 cm thick segment of the body wall at higher acoustic power (1430 W) that was expected to compensate for tissue attenuation (1.7 dB/cm). Although the amplitude of the signals from individual array elements was similar to that measured in free field as expected, the shock fronts were not phased-in. This is likely due to the difference in propagation time through different regions of the body wall that corresponded to the positions of the elements. When the entire array was activated, the focal waveform (bottom, blue line) contained multiple shock fronts (red arrows) of smaller amplitude, that would preclude the initiation of boiling within the BH pulse. When phase delays were introduced for individual array elements based on the FOPH measurements, the shock front was restored to its expected amplitude (green line). It is thus concluded that the main limiting factor in performing extracorporeal BH is aberration induced primarily by fat layers. These measurements were performed four times with similar results.
