Determination of laser-induced temperature distributions using echo-shifted TurboFLASH

An echo-shifted TurboFLASH sequence implemented on a clinical whole body MR scanner was used to determine thermal changes in tissue. With this snapshot-like data acquisition, temperature-related phase shifts were measured with a temporal resolution of 1.3 s. For different types of tissue (postmortem porcine brain, liver, and muscle) the temperature coefficients of the proton chemical shift were recorded during uniform heating of the specimen in a water bath. The specific temperature-dependent frequency shifts appeared similar to the proton chemical shift of free water (-0.01 ppm/degrees C). With this method, laser-induced ablation in postmortem porcine brain was monitored by temperature mapping. Comparison of the induced temperature profiles measured with NiCrNi-thermocouples with the MR calculated profiles demonstrated excellent temperature sensitivity and accuracy for this method of MR thermometry, with a maximum deviation of the determined temperatures of only 1.8 degrees C. This investigation was designed as a feasibility study for this rapid version of the phase mapping method, and no in vivo studies were performed.