A noninvasive method of tissue ablation that is guided and monitored with magnetic resonance (MR) imaging has been developed. The method uses sharply focused ultrasound transducers of different focal lengths to induce a localized temperature elevation during a short exposure (1-20 seconds). A hydraulic, computer-controlled positioning device moves the transducer in an MR imager. The positioner is built into a standard cradle in the imager. The system includes cavitation detection and power monitoring circuitry for patient safety. The target volume is outlined with cross-sectional MR images obtained immediately before sonication. By means of the software, the focus is moved to ablate the volume defined with the images. The temperature elevation during the exposure is monitored by means of the proton resonance frequency shift with fast gradient-echo sequences, and the necrosed volume is demonstrated with T2-weighted fast spin-echo images. This method has been extensively tested in in vivo animal experiments and is now undergoing clinical trial.