Acoustic emissions associated with cavitation and other bubble activity have previously been observed during ultrasound ablation experiments. Since detectable bubble activity may be related to temperature, tissue state, and sonication characteristics, these acoustic emissions are potentially useful for monitoring and control of ultrasound ablation. To investigate these relationships, ultrasound ablation experiments were performed with simultaneous measurements of acoustic emissions, tissue echogenicity, and tissue temperature, on fresh bovine liver. Ex vivo tissue was exposed to 0.9-3.3 s bursts of unfocused, continuous-wave, 3.10 MHz ultrasound from a miniaturized 32-element array, which performed B-scan imaging with the same piezoelectric elements during brief quiescent periods. Exposures employed pressure amplitudes of 0.8-1.4 MPa for exposure times of 6-20 min, sufficient to achieve significant thermal coagulation in all cases. Acoustic emissions received by a 1 MHz, unfocused passive cavitation detector, beamformed Aline signals acquired by the array, and tissue temperature detected by a needle thermocouple were sampled 0.3-1.1 times per second. Tissue echogenicity was quantified by the backscattered echo energy from a fixed region of interest within the treated zone. Acoustic emission levels were quantified from the spectra of signals measured by the passive cavitation detector, including subharmonic signal components at 1.55 MHz, broadband signal components within the band 0.3-1.1 MHz, and low-frequency components within the band 10-30 kHz. Tissue ablation rates, defined as the thermally ablated volumes per unit time, were assessed by quantitative analysis of digitally imaged, macroscopic tissue sections. Correlation analysis was performed among the averaged and time-dependent acoustic emissions in each band considered, B-mode tissue echogenicity, tissue temperature, and ablation rate. Ablation rate correlated significantly with broadband and low-frequency emissions, but was uncorrelated with subharmonic emissions. Subharmonic emissions were found to depend strongly on temperature in a nonlinear manner, with significant emissions occurring within different temperature ranges for each sonication amplitude. These results suggest potential roles for passive detection of acoustic emissions in guidance and control of bulk ultrasound ablation treatments.

Ultrasound ablation experiments were performed, with simultaneous measurements of acoustic emissions, tissue echogenicity, and tissue temperature, on fresh, degassed bovine liver. The tissue was exposed to bursts of unfocused, continuous-wave, 3.10 MHz from a 3 mm-diameter, 32-element array, which performed B-scan imaging with the same piezoelectric elements during quiescent periods. Exposures employed pulse lengths of 0.9-3.3 s with pressure amplitudes of 0.8-1.4 MPa and duty cycles of 97-99% for exposure times of 6-20 min, sufficient to achieve significant thermal coagulation in all cases. RF echo traces from the array, time-domain signals received by a 1 MHz, unfocused passive cavitation detector, and tissue temperature detected by a needle thermocouple were sampled 0.3-1.1 times per second. Tissue echogenicity was quantified, within the region of significant tissue heating, from the amplitude of RF signals received by the array. Cavitation dose was quantified from the spectra of signals measured by the passive cavitation detector, including subharmonic signal components at 1.55 MHz, broadband signal components within the band 0.3-1.1 MHz, and low-frequency components within the band 10-30 kHz. Tissue ablation effects were assessed by quantitative analysis of digitally imaged, macroscopic tissue sections. Correlation analysis was performed among the averaged and time-dependent acoustic emissions in each band considered, tissue echogenicity, tissue temperature, and ablation rate. Tissue echogenicity correlated significantly with subharmonic and low-frequency emissions as well as tissue temperature, but was uncorrelated with broadband emissions. Ablation rate correlated significantly with broadband and low-frequency emissions, but was uncorrelated with subharmonic emissions.