PROJECT SUMMARY/ABSTRACT Histotripsy is a non-invasive, ultrasound based tissue ablation therapy which relies on the targeted generation of cavitation events to mechanically fractionate and liquefy tissues. Quantifiable metrics by which the outcomes of histotripsy therapy can be predicted as a function of therapy inputs are essential for ensuring reliable and repeatable treatments, but do not currently exist. Although histotripsy-generated cavitation and liquefied tissue can be detected in ultrasound imaging, there is no established metric to quantify induced tissue damage versus cavitation exposure, which is known to vary with tissue properties, as well as among patients. With clinical translation of histotripsy ongoing, it is critical to establish a dose metric by which cavitation energy deposited to tissue during histotripsy can be monitored in situ to accurately predict therapy-generated damage. In this project we propose to develop metrics for monitoring histotripsy-induced tissue fractionation by monitoring the acoustic cavitation emission (ACE) signals generated by the cavitation events responsible for therapy during histotripsy. The ACE signals encode information about the dynamics and energetics of the cavitation events from which they are emitted, which depend on the mechanical properties/integrity of the media in which the cavitation events were generated. As a result of exposure to cavitation during histotripsy, targeted materials are mechanically disrupted which alters their mechanical properties, which can thus affect the dynamics of the cavitation events. By developing methods to monitor features of the ACE signals the mechanical state of the material in which the cavitation events were generated can be assessed in situ. We will carry out experiments in which histotripsy will be used to generate cavitation in a range of tissue- mimicking gel phantoms and tissues with a wide range of mechanical properties to ablate them. During treatment, the ACE signals will be recorded. Following treatment, generated damage will be assessed optically and histologically and the recorded ACE signals will be analyzed to identify the features in them that can be correlated with the induced damage observed in images or histology. Establishing such correlations will allow the ACE signals to be used as a metric for monitoring induced material fractionation during histotripsy treatment. To enable robust monitoring, the ACE signals can be monitored using the transmitting elements of the array as receivers in addition to hydrophones. This will ensure that an acoustically accessible path to the generated cavitation events will always be available to provide accurate monitoring of the ACE signals, but will require the development of sophisticated real-time algorithms to process owing to the large amount of data that will be generated. Once correlations between features of the ACE signals and induced damage in gel phantoms and ex vivo tissues have been identified, and real-time algorithms for monitoring them developed, they will be validated in vivo in a swine model. The results of this work will be essential for establishing a histotripsy dose metric and for histotripsy to obtain FDA approval for clinical use.

PROJECT NARRATIVE Metrics by which induced tissue damage during histotripsy – a non-invasive, ultrasound based, mechanical tis- sue ablation modality – can be monitored in situ during treatment do not currently exist. This project seeks to develop such metrics, as well as strategies for monitoring them in situ, by measuring the acoustic cavitation emission (ACE) signals emitted by the histotripsy-generated cavitation events during therapy, and correlating features of these ACE signals with material properties and damage induced to targeted gel phantoms and tissues in ex vivo and in vivo settings. Developing metrics to assess induced tissue damage, and to monitor them in situ, are vital for ensuring histotripsy treatment efficacy and for securing future FDA approval for histotripsy’s clinical use.