Usefulness of MR imaging-derived thermometry and dosimetry in determining the threshold for tissue damage induced by thermal surgery in rabbits

PURPOSE: To investigate in vivo the feasibility of using magnetic resonance (MR) imaging-derived temperature and thermal dose measurements to find the threshold of thermal tissue damage. MATERIALS AND METHODS: Sonications were delivered in rabbit thigh muscles a t varying powers. Temperature-sensitive MR images obtained during the sonic atitions were used to estimate the temperature and thermal dose. The temper ature, thermal dose, and applied power were then correlated to the occurren ce of tissue damage observed on postsonication images. An eight-element pha sed-array transducer was used to produce spatially flat temperature profile s that allowed for averaging to reduce the effects of noise and the voxel s ize. RESULTS: The occurrence of tissue damage correlated well with MR imaging de rived temperature and thermal dose measurements but not with the applied po wer. Tissue damage occurred at all locations with temperatures greater than 50.4 degrees C and thermal doses greater than 31.2 equivalent minutes at 4 3.0 degrees C. No tissue damage occurred when these values were less than 4 7.2 degrees C and 4.3 equivalent minutes. CONCLUSION: MR imaging thermometry and dosimetry provide an index to predic t the threshold for tissue damage in vivo. This index offers improved onlin e control over minimally invasive thermal treatments and should allow for m ore accurate target volume coagulation.