Acoustic Emissions During 3.1 MHz Ultrasound Bulk Ablation In Vitro

Abstract

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.

Passive imaging with pulsed ultrasound insonations

“…18,27 Unfocused PCDs can record cavitation emissions over a large area but have decreased sensitivity and spatial specificity. 28,29 More recently, PCDs using multiple-element arrays have been shown to reduce the limitations of focused and unfocused PCDs by allowing for spatial resolution over a large area. Farny et al 23 and Salgaonkar et al 30 used modified diagnostic scanners with the ultrasound array transmit turned off and the array in a passive detection state.…”

Magnetic resonance imaging of boiling induced by high intensity focused ultrasound

“…16,20 Several acoustic techniques have been reported to differentiate the two phenomena. 15,16,21,22 However, few of these techniques have yet been applied in bio-effect studies.…”

Conformal drug delivery and instantaneous monitoring based on an inverse synthesis method at a diagnostic ultrasound platform

“…In terms of the cavitation detection, standard passive cavitation detection (PCD) using a focused or unfocused single-element transducer has been employed to detect cavitation arising from ultrasound-UCA interaction [1], which, however, limits the spatial sensitivity [2] as well as clinical applicability [3] due to the confocal setting between devices. Recently, multi-element ultrasound array detectors have been used to passively image cavitation during continuous-wave high intensity focused ultrasound (HIFU) ablation [4] or other pulsed-wave therapies such as drug delivery [5] under the development of customized beamforming methods.…”