MRI temperature quantification at 1.5T in vitro: Comparison of fast T-1 maps and a phase-sensitive sequence.

Purpose: To determine the value of fast T-1-maps and a phase-sensitive sequence for temperature quantification with MR imaging. Materials and methods: The experimental setup allowed both homogeneous heating as well as laser-induced interstitial thermotherapy (Nd:YAG, 1064 nm) of tissue specimens (pig brain, liver and muscle) in vitro. A total of 60 experiments were performed. T-1-maps were calculated by means of a multi-point-turbo-FLASH sequence. Additionally, the temperature dependent phase shift was determined with a ZD-FLASH sequence. Results: The T-1-relaxation times of four different aqueous solutions of gadolinium-DTPA (0.125-1.0 mmol/l) varied by less than 5%. During homogeneous heating, the T-1-maps revealed a linear correlation (r>0.98) between temperature and T-1-relaxation times. The temperature coefficients were 11.0 +/- 0.42 ms/degrees C. Variations of the linear correlation were observed during laser irradiation. There was only slight variation of the temperature coefficients of the chemical shift during homogeneous heating of different tissues (brain: 0.0098+/-0.0002 ppm/degrees C; muscle: 0.0109+/-0.0003 ppm/degrees C; liver: 0.0093+/-0.0002 ppm/degrees C). The temperatures calculated during laser therapy based on the phase shift correlated strongly (r=0.99) to the measured temperatures. Conclusion: Due to the considerable tissue independence and high accuracy, the phase mapping method is superior to T-1-maps for monitoring thermal therapy modalities at 1.5 T in vitro.