Three-dimensional thermal therapy using multiple planar ultrasound transducers with real-time MR temperature feedback in gel phantoms

High intensity ultrasound delivered transurethrally is a promising approach for the treatment of localized prostate cancer. The use of multiple planar ultrasound transducers mounted on an MR-compatible applicator with rotational capability can provide precise control over the spatial deposition of energy within the gland. Using MR thermometry for adaptive temperature feedback, accurately shaped three-dimensional heating patterns can potentially be achieved. The goal of this study was to evaluate the feasibility of simultaneously controlling multiple elements with real-time MR temperature feedback in gel phantoms in a 1.5T MR imager. Numerical simulations were used initially to determine treatment delivery strategies and appropriate tuning of the temperature feedback control algorithm. Two typical prostate shapes were then treated in tissue-mimicking polyacrylamide gel phantoms using a prototype system to demonstrate conformity of the thermal damage patterns to the target boundaries. Five planar gradient-echo MRI slices with a spatial resolution of 1.7x3.4x5mm and a temporal resolution of 5s were obtained. Each slice was centered on a transducer element which had a length of 9mm operating at 7.7MHz. Results showed high correlation between the desired target boundary and the 55°C isotherm with an average error of 1.0 ± 1.5mm (n=5) for shape # 1 and 1.0 ± 1.2mm (n=5) for shape # 2 across five slices with target volumes of approximately 53cm ³ and 58cm ³ respectively. The feasibility of MRI-guided active feedback for accurate, 3D, multi-planar treatments has been demonstrated and further investigation in vivo will be done.