Sectored interstitial ultrasound applicators for angular control of MR-guided thermal therapy (Invited Paper)

Hyperthermia and high temperature thermal therapy are currently used in the clinical treatment for a variety of cancers. Despite the increasing use of thermal therapies, heat treatments have not gained large-scale clinical acceptance, due in part to inconsistencies in controlling heat deposition in vivo and the lack of precise temperature measurement. Interstitial ultrasound applicators provide superior spatial control of power deposition and heating patterns compared to other interstitial techniques. Real-time MR temperature imaging (MRTI) provides thermal therapies with an accurate, non-invasive method for measuring temperature within the body during treatment. In this study, three MR-compatible water-cooled interstitial ultrasound applicators designs were developed and evaluated for dynamic angular control of the thermal dose to a target area. Two of the applicator designs utilize an ultrasound transducer separated into three individually powered sectors allowing the user to control heating deposition for the creation of odd size and shape thermal lesions. The third design utilizes a 90o sectored transducer that can be rotated to target the thermal treatment to a specified area. Comparisons and predictions of the in vivo performance of all applicators was examined with experiments in ex vivo tissue and simulations created by a biothermal model that incorporates changes in acoustic attenuation and perfusion as a function of thermal dose. Ex vivo experiments with real-time MRTI correlated well with results from the biothermal model. The results of this study bracket the feasibility and potential in vivo performance of the applicator designs for minimally invasive cancer treatment with MRTI.