Measurement of tissue temperature by MRI

The authors investigated the possibility of making non-invasive temperature measurements using MRI. Excised samples of beef muscle and brain were imaged with sequences for rapidly measuring the T/sub 1/, T/sub 2/, and proton density as the tissue temperature was varied from 37/spl deg/C to 43/spl deg/C and above. The signal strength varied by as much as 1.7% per /spl deg/C. For the imaging protocol used, the minimum detectable temperature change (1/spl sigma/) in a 1 cm/sup 3/ voxel over a 6 minute time interval was 1/spl deg/C. Some improvement is expected if the measured dependencies of T/sub 1/, T/sub 2/, and proton density on temperature are used to design a sequence that maximizes the change in signal strength with temperature. The signal strength vs. temperature data show some hysteresis, even for samples held below 43/spl deg/C-the maximum temperature to which healthy tissue is generally raised in hyperthermia. If live tissue shows the same behavior, it may be impossible to do true thermometry based on changes in T/sub 1/, T/sub 2/, and density. There may remain the possibility of accurately measuring small temperature changes over a narrow range.<
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