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 T₁, T₂, and proton density as the tissue temperature was varied from 37°C to 43°C and above. The signal strength varied by as much as 1.7% per °C. For the imaging protocol used, the minimum detectable temperature change (1σ) in a 1 cm³ voxel over a 6 minute time interval was 1°C. Some improvement is expected if the measured dependencies of T₁, T 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°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 T₁, T₂, and density. There may remain the possibility of accurately measuring small temperature changes over a narrow range