T1 relaxation time at 0.2 Tesla for monitoring regional hyperthermia: Feasibility study in muscle and adipose tissue

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Abstract
The purpose of this study was to characterize T1, particularly in the hyperthermia temperature range (ca. 37–44°C), in order to control regional hyperthermia with MR monitoring using 0.2 Tesla, and to improve T1 mapping. A single‐slice and a new multislice “T One by Multiple Read‐Out Pulses” (TOMROP) pulse sequence were used for fast T1 mapping in a clinical MRI hyperthermia hybrid system. Temporal stability, temperature sensitivity, and reversibility of T1 were investigated in a polyamidacryl gel phantom and in samples of muscle and adipose tissues from turkey and pig, and verified in patients. In the gel phantom a high linear correlation between T1 and temperature (R2 = 0.97) was observed. In muscle and adipose tissue, T1 and temperature had a linear relationship below a breakpoint of 43°C. Above this breakpoint muscle tissue showed irreversible tissue changes; these effects were not visible in adipose tissue. The ex vivo results were confirmed in vivo under clinical conditions. T1 mapping allows the characterization of hyperthermia‐related tissue response in healthy tissue. T1, in combination with fast mapping, is suitable for controlling regional hyperthermia at 0.2 T within the hybrid system. Magn Reson Med 47:1194–1201, 2002.