In this study, the feasibility of using magnetic resonance imaging (MRI) to detect tissue necrosis induced by focussed ultrasound beams was investigated. It was shown that lesions produced in dog's thigh muscle in vivo were clearly visible in T2-weighted images and that the lesion dimensions measured from the images correlated with the postmortem measurements of the visible tissue damage. It was also shown that the sonications can be done in the magnet and that the lesions are visible immediately after the sonications with increasing image contrast as a function of time. These results showed that MRI can be used to direct and monitor on-line noninvasive ultrasound surgery. This may have a major impact in future patient treatments.