Temperature imaging of laser-induced thermotherapy (LITT) by MRI: evaluation of different sequences in phantom

The purpose of this study was to evaluate magnetic resonance (MR) temperature imaging of the laser-induced thermotherapy (LITT) comparing the proton resonance frequency (PRF) and T (1) thermometry methods. LITT was applied to a liver-mimicking acrylamide gel phantom. Temperature rise up to 70 A degrees C was measured using a MR-compatible fiber-optic thermometer. MR imaging was performed by a 1.5-T scanner utilizing fast gradient echo sequences including a segmented echo planar imaging (seg-EPI) sequence for PRF and the following sequences for T (1) method: fast low-angle shot (FLASH), inversion recovery turbo flash (IRTF), saturation recovery turbo flash (SRTF), and true fast imaging (TRUFI). Temperature-induced change of the pixel values in circular regions of interest, selected on images under the temperature probe tip, was recorded. For each sequence, a calibration constant could be determined to be -0.0088 +/- 0.0002 ppm A degrees C-1 (EPI), -1.15 +/- 0.03 A degrees C-1 (FLASH), -1.49 +/- 0.03 A degrees C-1 (IRTF), -1.21 +/- 0.03 A degrees C-1 (SRTF), and -2.52 +/- 0.12 A degrees C-1 (TRUFI). These constants were evaluated in further LITT experiments in phantom comparing the calculated temperatures with the fiber optic-measured ones; temperature precisions of 0.60 A degrees C (EPI), 0.81 A degrees C (FLASH), 1.85 A degrees C (IRTF), 1.95 A degrees C (SRTF), and 3.36 A degrees C (TRUFI) were obtained. Furthermore, performing the Bland-Altman analysis, temperature accuracy was determined to be 0.23 A degrees C (EPI), 0.31 A degrees C (FLASH), 1.66 A degrees C (IRTF), 1.19 A degrees C (SRTF), and 3.20 A degrees C (TRUFI). In conclusion, the seg-EPI sequence was found to be more convenient for MR temperature imaging of LITT due to its relatively high precision and accuracy. Among the T (1) method sequences, FLASH showed the highest accuracy and robustness.