Design of Noninvasive Therapies Utilizing Nonlinear Focused Ultrasound With Shocks

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PROJECT SUMMARY Renal carcinoma (RCC) and hepatocellular carcinoma (HCC) are two of the most common abdominal solid organ malignancies in the US, accounting for nearly 100,000 combined new cases and nearly 40,000 deaths in 2015 [Siegel 2015]. With recent imaging advances leading to early diagnosis, medical practice is shifting toward the use of minimally invasive focal therapies (e.g., radiofrequency ablation, cryotherapy) for treating tumors. However, these treatments possess limitations: They require invasive deployment and rely on thermal effects that are poorly controlled, especially near vascular structures that can act as heat sinks. In addition, real-time treatment monitoring capabilities are minimal because thermal lesions are not easily visualized on standard imaging techniques. High intensity focused ultrasound (HIFU) offers an alternative focal therapy that can be delivered noninvasively. At present, clinical HIFU treatments involve thermal ablations that are subject to these same limitations; however, boiling histotripsy (BH) is a noninvasive HIFU modality recently invented by our group that can potentially overcome these limitations by delivering high-amplitude shock waves to mechanically ablate tissue. BH has many potential clinical advantages over existing focal therapies, including thermal HIFU: 1) generation of precise, controllable lesions with sharp margins while sparing critical structures; 2) targeting and real-time monitoring of treatments through ultrasound-based imaging, which utilizes the strong acoustic reflectivity of bubbles; and 3) potentially faster resorption of liquefied BH lesions. In previous years of NIH support, we have developed metrology tools for characterizing HIFU fields with shocks, invented the BH method and elucidated its physical mechanisms, identified effective pulse sequences, implemented real-time B-mode imaging of treatments, and designed a HIFU array optimized for abdominal BH applications. However, two scientific challenges remain in order to reliably deliver safe and effective BH treatments in humans: First, the impact of tissue inhomogeneities on shock formation is not yet quantitatively understood. Second, dose metrics and corresponding treatment strategies have not been defined and validated for ablating tissue volumes comprising multiple target sites. The first two aims in this project seek to address these challenges by 1) extending our nonlinear metrology tools to include modeling in heterogeneous tissues to predict in situ shock formation, and 2) conducting experimental studies in ex vivo liver and kidney tissue to determine dose metrics for use in designing volumetric BH treatments. The third aim involves the performance of rigorous pre-clinical studies using a prototype system to treat in pig kidney and liver in vivo. Successful completion of these aims will aid the design and execution of BH treatments, providing the framework needed to conduct clinical trials for RCC and HCC. Beyond the specific clinical targets, these studies will facilitate the advancement of BH for other clinical applications as well as the development of novel ultrasound-based therapies that utilize shocks.