MR TEMPERATURE MAPPING OF FOCUSED ULTRASOUND SURGERY

Deep lying soft tissue tumors may be treated by a nonincisional surgic al procedure executed inside an MR imaging system using a thermal effe ct delivered by a focused ultrasound transducer. A prototype system is constructed to assess MRI thermal monitoring and the localization of the heat zone in muscle. The temperature distribution of the focal spo t is imaged with MRI while mechanically moving the transducer with an hydraulic 3-axis positioner. Acoustic power is applied with a spherica l shell transducer using 1- to 10-s duration pulses at frequencies of 1.5 MHz to selectively coagulate tissue at 60-70 degrees C. The proced ure is monitored with a series of fast second gradient echo, T-1-weigh ted, temperature sensitive MR sequences. Acquisitions are optimized fa r high temperature sensitive images that yield the thermal diffusivity , heat flow time constant and the focal spot size in muscle. MR temper ature maps of muscle provide localization and dosimetry both in the fo cal region and near field.