Local hyperthermia with MR-guided focused ultrasound: Spiral trajectory ofthe focal point optimized for temperature uniformity in the target region

The objective of hyperthermia treatment is to deliver a similar therapeutic thermal dose throughout the target volume within a minimum. amount of time . We describe a noninvasive approach to this goal based on magnetic resonan ce imaging (MRI)-guided focused ultrasound (FUS) with a spherical transduce r that can be moved along two directions inside the bed of a clinical MR im ager and that has an adjustable focal length in the third dimension. Absorp tion of FUS gives rise to a highly localized thermal buildup, which then sp reads by heat diffusion and blood perfusion. A uniform temperature within a large target volume can be obtained using a double spiral trajectory of th e transducer focal point together with constant and maximum FUS power. Diff erences between the real and target temperatures during the first spiral ar e evaluated in real time with temperature MRI and corrected for during the second spiral trajectory employing FUS focal point velocity modulation. Onc e a uniform temperature distribution is reached within the entire volume, F US heating is applied only at the region's boundaries to maintain the raise d temperature levels. Heat conduction, together with the design and timing of the trajectories, therefore ensures a similar thermal dose for the entir e target region. Good agreement is obtained between theory and experimental results in vitro on gel phantoms, ex vivo on meat samples, and in vivo on rabbit thigh muscle. Edema in muscle was visible I hour after hyperthermia as a spatially uniform rise of the signal intensity in T-2-weighted images. (C) 2000 Wiley-Liss, Inc.