Cavitation is known to affect therapeutic ultrasound applications such as tissue ablation, where it may complicate heat deposition and make treatment control difficult. In this thesis, acoustic emissions from cavitating bubbles are measured and imaged to serve as indicators of thermal ablation progress. Cavitational acoustic emissions were measured using a 1-MHz transducer during thermal ablation of excised bovine livers with a 32-element linear array (3.1 MHz, 0.8-1.4 MPa pressure amplitude). Broadband, subharmonic and low-frequency emissions consistent with inertial, stable and vaporous cavitation respectively were observed. Broadband (r = 0.848) and low-frequency (r = 0.747) emissions exhibited statistically significant linear correlations with coagulated tissue volumes. Statistical models based on multinomial logistic regression were implemented to predict tissue temperature based on measured cavitational emission signals.
To perform spatially sensitive measurements of cavitation activity, images were created from beamformed bubble emission signals received by a diagnostic imaging array. This method was called passive cavitation imaging. Analytic models for point spread functions were developed to test this imaging method. It was implemented on a 192-element linear array (7.5 MHz) and separate images of stable and inertial cavitation activity were created in free field and tissue media, with mm-level resolution along the array azimuth. Passive cavitation imaging techniques were used to record emissions during ablation of ex vivo bovine liver with 1.1-MHz (1984 W/cm2 focal intensity) focused ultrasound. Spatial correspondence was observed between harmonic emissions and tissue lesioning, along the array azimuth. This was assessed by a statistically significant correlation (r = 0.684) and area under a receiver operating characteristics (ROC) curve (AUROC = 0.71).
The present cavitation detection and imaging techniques, implemented in this thesis to monitor ultrasound ablation, can potentially be extended to other therapeutic ultrasound procedures that are significantly influenced by cavitation.