Assessment of chirp-coded excitation to monitor histotripsy bubble clouds

Histotripsy is a non-invasive form of therapeutic ultrasound that ablates tissue via the generation of bubble clouds with short, nonlinear pulses. A reduction in treatment efficacy has been linked to bubble clouds that persist between subsequent pulses. In order to ensure the dissolution of bubble clouds, a high frame rate, contrast specific imaging sequence may provide the feedback necessary to dictate the application rate of histotripsy therapy. In this study, histotripsy bubble cloud dissolution was visualized with plane wave imaging. Both standard imaging pulses and chirped imaging pulses of twenty cycle duration were explored. For chirped pulses, the received signals were processed with match filters to compute image pixel values based on fundamental frequency bands and nonlinear frequency bands associated with bubble oscillations. Hyperechogenicity was observed for all sequences with imaging pulse peak negative pressures 0.2 MPa or greater. Faster grayscale dissolution profiles were observed for nonlinear frequency bands compared to imaging schemes based on the fundamental pulse bandwidth. The estimated bubble cloud dimensions were similar for all imaging sequences, indicating minimal error in assessing the extent of histotripsy treatment with long duration chirp-coded pulses. Overall, these findings reveal the potential of a chirped imaging sequence for monitoring histotripsy-generated bubble clouds.