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.