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 resonanc e imaging (MRI)-guided focused ultrasound (PUS) with a spherical transducer that can be moved along two directions inside the bed of a clinical MR ima ger and that has an adjustable focal length in the third dimension. Absorpt ion of PUS gives rise to a highly localized thermal buildup, which then spr eads by heat diffusion and blood perfusion. A uniform temperature within a large target volume can be obtained using a double spiral trajectory of the transducer focal point together with constant and maximum PUS power. Diffe rences between the real and target temperatures during the first spiral are evaluated in real time with temperature MRI and corrected for during the s econd spiral trajectory employing PUS focal point velocity modulation. Once a uniform temperature distribution is reached within the entire volume, PU S heating is applied only at the region's boundaries to maintain the raised temperature levels. Heat conduction, together with the design and timing o f the trajectories, therefore ensures a similar thermal dose for the entire 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 r abbit thigh muscle. Edema in muscle was visible 1 hour after hyperthermia a s a spatially uniform rise of the signal Intensity In T-2-weighted images. J. Magn. Reson. Imaging 2000; 12:571-583. (C) 2000 Wiley-Liss, Inc.