Preliminary investigation of dual mode CMUT probe for ultrasound image guided HIFU therapy

This preliminary study focuses on Capacitive Micro-machined Ultrasound Transducers (CMUTs) for dualmode ultrasound therapy and imaging. A planar probe design is being investigated in the context of Ultrasound-guided High Intensity Focused Ultrasound (USgHIFU) designed for endocavitary HIFU tissue ablation of the prostate gland. The hypothesis is that this CMUT design could offer potential advantages over currently used piezoelectric probes such as ease of miniaturization, increased elements for improved imaging and more control over electronic focusing allowing focal therapy. A planar dual mode CMUT probe consisting of a 3 MHz, 64-element annular array for therapy and 7 MHz, 256-element linear array for imaging were investigated in simulation and experimentally. Numerical models of the pressure field and tissue heating simulations were performed and compared with an existing piezo-based geometrically focused annular array HIFU probe (16 rings) with a hole in the center that possesses a curvilinear imaging array (128 elements). To validate the modelling and the focusing capabilities, pressure field hydrophone measurements were performed. Simulations showed that the planar annular CMUT design could dynamically focus from 3-7cm which matched well with the experimentally obtained pressure field data. Radiation force balance measurements confirmed the desired 5 W/cm² surface acoustic intensity. This CMUT prototype will require further investigation for determination of the robustness of this technology and the ability to create lesions, yet according to the modelling and experimental validation thus far, this CMUT design shows promise for improved USgHIFU treatment strategies.

I. Introduction

High Intensity Focused Ultrasound (HIFU) has become a widely used clinical tool for the thermal ablation of tissues, such as cancerous tumors [1]. It allows for a non-invasive or minimally-invasive method for treatment, often performed in a single treatment session, which leads it to be a desirable treatment option for patients as well as clinicians. One clinical application of HIFU is treatment of the prostate using an endocavitary approach [2], [3]. The most common method of using HIFU to treat prostate cancer is the complete thermal ablation of the entire prostate gland, however, a more desirable treatment, and the current trend, is towards a focal treatment where only a portion of the prostate is treated [4], [5]. There is evidence that focal HIFU therapy can be a safe alternative to the treatment of the entire prostate in cases where the tumors are localized [6]–[8]. This evidence showcases a need for improved focal therapy devices which have the ability to accurately localize treatment under realtime image guidance. Any modifications to current tools that improve upon the accuracy of HIFU targeting or imaging, will help to advance future generations of focal treatments. In order to accomplish this goal improved imaging and detection of the tumor area is needed along with accurate, finely tunable, generation of thermal lesions.