A passive method that uses time exposure acoustics (TEA) to map inertial cavitation activity in real time during therapeutic ultrasound exposure was recently presented [Gyongy and Coussios, IEEE Trans. Biomed. Eng. 57, 48–56 (2010)]. While this approach provides sub-millimetric spatial resolution transversely to the high-intensity focused ultrasound (HIFU) beam, its axial resolution requires improvement. Furthermore, the TEA method is expected to have a lower overall resolution and diminished ability to reject interference and noise compared to an adaptive beamforming approach. To address these limitations, we propose the use of the adaptive, robust Capon beamformer (RCB), which has been previously shown in the context of active, 2-D ultrasound imaging to provide high resolution, good interference suppression, and robustness against steering vector errors. Using a multi-element tomographic cavitation sensor being developed for quality assessment of clinical HIFU transducers, the effectiveness of RCB is demonstrated for improved 3-D passive localization of inertial cavitation compared to TEA using simulated and experimental data.

Objective: To test the hypothesis that shock wave lithotripsy machines vary in their ability to fragment standardized artificial urinary calculi. Materials and Methods: An in vitro test configuration was used to fragment synthetic U-30 Gypsum (U.S. Gypsum, Chicago, IL) stones (mean length 7.1 -0.2 mm, mean diameter 6.5 -0.07 mm, mean mass 299 -16 mg) using the Sonolith i-sys (EDAP TMS, Vaulx-en-Velin, France), Modulith SLX F2 (Storz Medical AG, Tägerwilen, Switzerland), and Piezolith 3000 (Richard Wolf GmbH, Knittlingen, Germany) lithotripters. Gypsum stones were placed at the nominal focus and treated with 250, 500, or 1000 shocks. The residual mass following passage through a 2-mm wire mesh was measured and compared using ANOVA and the TukeyKramer HSD test. Results: There was no statistically significant difference between the Modulith SLX F2 and Piezolith 3000 lithotripters for 250 and 1000 shock treatments ( p = 0.34 and 0.31, respectively). The Piezolith 3000 demonstrated the most favorable stone mass reduction for 500 shock treatments (187.4 -45.2 mg). The Sonolith i-sys was found to be significantly less effective than the other lithotripters for all shockwave conditions. Furthermore, performance of the Sonolith i-sys decreased beyond a threshold generator electrode age of 6000 shocks. Conclusions: This in vitro study found considerable variability in the ability of lithotripters to fragment synthetic urinary calculi. Synthetic stones were employed to provide a repeatable means of assessing variability in fragmentation efficiency of lithotripters. The Modulith SLX F2 and Piezolith 3000 are broadly equal and resulted in greater fragmentation efficiencies than the Sonolith i-sys. The performance of the Sonolith i-sys deteriorates at 6000 shocks, before the specified lifetime of 20,000 shocks.

High-intensity focused ultrasound (HIFU) fields are known to nucleate and excite inertial and noninertial cavitation in tissue and tissue-mimicking materials once a threshold negative acoustic pressure is reached. In the context of ablative HIFU treatment, inertial cavitation has been correlated with significantly enhanced rates of heating, while in histotripsy, cavitation is the very mechanism that causes tissue damage. Characterizing the extent of the cavitation region produced by clinical HIFU devices is therefore important to ensure safe, efficient, and effective treatment. A novel, multielement sensor is being developed to enable accurate axial and radial mapping of the cavitation region during HIFU exposure by passive detection and tomographic reconstruction of the broadband emissions arising from bubble collapse. Computational modeling has shown that the application of a cross-correlation algorithm to simulated received signals has the potential to localise individual sources of emissions with submillimeter accuracy. Initial experimental validation of models has been conducted using a prototype device developed in collaboration with the National Physical Laboratory. Future work will involve the refinement of the sensor design and reconstruction algorithm to improve spatiotemporal resolution, along with the development of a tissue mimicking material which matches the acoustic properties and cavitation threshold of real tissue.

Key to the success of high-intensity focused ultrasound (HIFU) as a clinical tool is the development of standardized quality assessment procedures to assess the safety and efficacy of HIFU transducers. The present work details the development of a cylindrical sensor array to be positioned around the HIFU focus during pre-treatment quality assessment of clinical transducers, which is designed to localize cavitation activity in three dimensions by passive mapping of the broadband emissions arising from inertial cavitation. The propagation of sound from a collection of broadband sources was first modeled to determine the optimum size, number and distribution of array elements for accurate mapping, and characterization of the cavitation dynamics produced during HIFU exposure. The optimal array configuration was then manufactured from PVDF using a novel printed circuit board technique, and theoretical predictions of the spatial resolution that it could achieve were validated experimentally. Because inertial cavitation is a pressure driven phenomenon, the ability to map cavitation activity using this array could provide a novel tool for rapid mapping of pressure fields produced by clinical HIFU transducers, in addition to providing invaluable information about the evolution of cavitation dynamics during HIFU exposure.

Sufficiently robust and reliable quality assessment (QA) procedures are vital in assuring the widespread adoption of high intensity focused ultrasound (HIFU) for use in both thermal ablation and enhanced drug delivery. Mapping of broadband cavitation emissions in a tissue-mimicking material with a repeatable cavitation threshold offers the potential for rapid, 3-D, cavitation-based pressure mapping of the field produced by a given HIFU transducer. Previous work has demonstrated the viability of this concept, including the design and optimization of a 50-element, cylindrical array capable of mapping a collection of broadband sources distributed throughout a region comparable to the size of a typical HIFU focal volume. The work presented here relates to in vitro experimentation using the array to map cavitating fields produced by a number of HIFU transducers at a range of insonation amplitudes. Results are compared to the predicted size of the cavitation region determined via hydrophone-based characterizations of the HIFU field. Future work will involve using the array to map the instigation and evolution of cavitating fields in three-dimensions in ex vivo tissue during HIFU exposure.