MRI detection of the thermal effects of focused ultrasound on the brain

This study tested the hypothesis that MRI thermometry can be correlated wit h the different degrees of tissue damage observed after focused ultrasound (US) exposure of brain. The brains of 6 rabbits were sonicated to calibrate the MRI proton resonant shift with temperature, In addition, 13 rabbits we re sonicated at acoustic powers ranging from 3.5 to 17.5 W. The experiments were performed in a 1.5-T MRI scanner with the temperature-sensitive phase imaging used during the sonications of 4-5 different locations in each rab bit. MR images mere obtained 2 h and 2 days after the sonications, dependin g on when the animals were sacrificed. Whole brain histologic evaluation wa s performed by sectioning the brain and performing a microscopic investigat ion. The MRI-derived temperature elevation was found to correlate well with the degree of tissue damage. In addition to the common histology findings, apoptotic cells were observed in the lesions, The T1-weighted contrast enh anced and T1-weighted scans both detected the brain damage. The applied aco ustic power did not correlate well with the degree of damage. As a conclusi on, the results showed that the measurement of temperature elevations by MR T during sonications can improve the accuracy and safety of clinical US bra in surgery. (C) 2000 World Federation for Ultrasound in Medicine & Biology.