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

This study tested the hypothesis that MRI thermometry can be correlated with 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 were 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 rabbit. MR images were obtained 2 h and 2 days after the sonications, depending on when the animals were sacrificed. Whole brain histologic evaluation was performed by sectioning the brain and performing a microscopic investigation. 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 enhanced and T2-weighted scans both detected the brain damage. The applied acoustic power did not correlate well with the degree of damage. As a conclusion, the results showed that the measurement of temperature elevations by MRI during sonications can improve the accuracy and safety of clinical US brain surgery.