Abstract : Focused ultrasound is a unique and non-invasive technique for the local deposition of thermal energy deep inside the body. MRI guidance offers the additional benefits of excellent target visualization and continuous temperature mapping. However, treating a moving target poses severe problems because 1) motion related thermometry artifacts must be corrected, 2) the ultrasound focal point must be relocated according to the target displacement. In this paper, a complete MRI compatible High Intensity Focused Ultrasound (HIFU) system is described together with adaptive methods allowing continuous MR thermometry and therapeutic ultrasound with real-time tracking of a moving target, on-line motion correction of
the thermometry maps, and regional temperature control based on the Proportional, Integral, and Derivative method. The hardware is based on a 256 element Phased-Array transducer with rapid, electronic, displacement of the focal point. The exact location of the target during ultrasound firing is anticipated using automatic analysis of periodic motions. The methods were tested with moving phantoms undergoing either rigid body or elastic periodical motions. The results show accurate tracking of the focal point. Focal and regional temperature control is demonstrated with a performance, similar to that obtained with stationary phantoms.