Aim: To evaluate magnetic resonance sequences for T(1) and proton resonance frequency (PRF) thermometry during laser-induced thermotherapy (LITT) in liver tissue.
Materials & methods: During LITT (1064 nm; 30 W; 3-cm diffuser; 2-3 min) in ex vivo porcine liver, temperature was measured (25-70°C) utilizing a fiberoptic thermometer and MRI was performed with a 1.5-T scanner through the following sequences: segmented echo planar imaging (seg-EPI) for the PRF method; fast low-angle shot (FLASH), inversion-recovery turbo FLASH (IRTF), saturation-recovery turbo FLASH (SRTF) and true-fast imaging (TRUFI) for the T(1) method. Phase angle and signal amplitude (regarding PRF/T(1)) was recorded in regions of interest, on images under fiberoptic probe tips. Sequences' thermal coefficients were determined by calibrating phase angle and signal amplitude against temperature and subsequently validated.
Results: Coefficients of -0.0089 ± 0.0003 ppm °C(-1) (seg-EPI) and -0.917 ± 0.046, -1.166 ± 0.058, -1.038 ± 0.054 and -1.443 ± 0.118°C(-1) (FLASH, IRTF, SRTF and TRUFI, respectively) were obtained. Precisions of 0.71, 1.34, 2.07, 2.44 and 3.21°C and, through Bland-Altman analysis, accuracies of -0.67, 0.79, 1.65, 1.57 and 2.13°C (seg-EPI, FLASH, IRTF, SRTF and TRUFI, respectively) were determined.
Conclusion: The PRF method with seg-EPI sequence is preferred for thermometry during LITT owing to higher precision and accuracy. Among T(1)-method sequences, FLASH showed higher accuracy and robustness.