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In this work, a new therapeutic ultrasound device is presented that is designed for performing minimally invasive thermal ablation of brain tumors under guidance with magnetic resonance imaging (MRI). The device consists of an array of ultrasound transducers, oriented on multiple faces of a flexible sheath with an integrated cooling system that can be directly inserted into the brain through a small burr hole in the skull. Heating can be monitored using real-time MRI and conformed to the tumor volume by varying the power to the individual elements on the probe. In this work, preliminary testing of the device was performed and included acoustic characterization, numerical simulations, and experiments in a clinical MRI system. Numerical simulations of the acoustic field and temperature rise during heating were compared with results of in vitro testing using bovine brain samples. The results demonstrate that the device has good MRI compatibility and is capable of generating output surface intensities of greater than 20 W/cm2, which is sufficient to ablate tissue at depths of more than 10 mm from the probe in less than four minutes of heating. [ABSTRACT FROM AUTHOR]