Effects of acoustic parameters on bubble cloud dynamics in ultrasound tissue erosion (histotripsy)

Abstract

High intensity pulsed ultrasound can produce significant mechanical tissue fractionation with sharp boundaries ("histotripsy"). At a tissue-fluid interface, histotripsy produces clearly demarcated tissue erosion and the erosion efficiency depends on pulse parameters. Acoustic cavitation is believed to be the primary mechanism for the histotripsy process. To investigate the physical basis of the dependence of tissue erosion on pulse parameters, an optical method was used to monitor the effects of pulse parameters on the cavitating bubble cloud generated by histotripsy pulses at a tissue-water interface. The pulse parameters studied include pulse duration, peak rarefactional pressure, and pulse repetition frequency (PRF). Results show that the duration of growth and collapse (collapse cycle) of the bubble cloud increased with increasing pulse duration, peak rarefactional pressure, and PRF when the next pulse arrived after the collapse of the previous bubble cloud. When the PRF was too high such that the next pulse arrived before the collapse of the previous bubble cloud, only a portion of histotripsy pulses could effectively create and collapse the bubble cloud. The collapse cycle of the bubble cloud also increased with increasing gas concentration. These results may explain previous in vitro results on effects of pulse parameters on tissue erosion.

FIG. 1

Examples of light attenuation signals caused by the bubble cloud generated by histotripsy pulses using different pulse parameters. The axes for the wave form are the same for each row and shown on the right. Arrows indicate the arrival of the pulse. The pulse parameters and corresponding light attenuation results are listed in Table I.

FIG. 2

Images of the bubble cloud generated in free water created by a single histotripsy pulse at different pulse durations. The histotripsy pulse was delivered from the left to the right of each image. The overall size of the bubble cloud increased with increasing pulse duration. The ruler on the top of each image has markings of 1 mm on the right side and 0.5 mm on the left.

FIG. 3

Example of the light attenuation signal caused by a bubble cloud, recorded as the photodiode voltage output. The bubble cloud was generated by a three-cycle (4-μs) pulse at a tissue-water interface with 98%–100% gas concentration. The left arrow below “Attenuation Duration” indicates the arrival of the histotripsy pulse at the transducer focus where the laser beam was projected. The insert is an expanded view (expanded in the horizontal direction and compressed in the vertical) of the artifact in the light attenuation signal during the histotripsy pulse, which tracks the pulse wave form.

FIG. 4

Acoustic pressure wave form of a ten-cycle (14-μs) histotripsy pulse in water at the transducer focus. For this pulse, the peak rarefactional pressure was 21 MPa and the peak compressional pressure was 76 MPa.

FIG. 5

Diagram of the experimental arrangement for bubble cloud monitoring at a tissue-water interface using an optical attenuation method. Light source and camera (in dashed circle) are setup for high speed imaging in water. However, at a tissue-water interface, the light source was blocked by the tissue, and the imaging could not be used with the optical attenuation detection system.