Design and Implementation of 32-Element Monitoring Transducers for 3D Cavitation Monitoring of Ultrasound Therapy

Focused ultrasound is currently under evaluation as a treatment modality for a variety of medical indications, many of them involving cavitation. With the gaining interest in multielement therapy transducers enabling 3D beamsteering, the need for 3D cavitation monitoring is evident. Here, we propose two in-house built 32-element cavitation monitoring transducers for 3D passive acoustic mapping (PAM) with fixed co-registration. Based on previously in-house developed therapy matrix arrays, two different design approaches are used, one being an add-on transducer for a preclinical array and the other one being the integration of elements into the transmit aperture of a clinical array. For the experimental validation in the preclinical setup, the therapy transducer excited a microbubble solution flowing through a tube and the emissions were simultaneously recorded by the add-on transducer (1 mm elements, 4 MHz). For the clinical setup with the integrated elements (2 mm, 1 MHz), the cavitation was generated by a separate single-element focused transducer. For both design approaches, 3D PAM was successfully validated, with the experimental full width at half maximum values being in good agreement with the numerical ones. Additionally, the detection of cavitation signals through an ex vivo skull fragment was shown for the clinical design. For the integrated elements, cross coupling remains an issue which needs to be solved to enable monitoring during therapy pulses.