Measurements of cavitation dose, echogenicity, and temperature during ultrasound ablation

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.