Non-invasive MR thermometry in liver undergoing laser ablation: multi-parameters analysis

The aim of this work is to define suitable parameters to perform non-invasive magnetic resonance (MR)-thermometry during laser ablation (LA) on biological tissues. Fast MR scans were acquired all along the LA procedure performed on ex vivo animal model; simultaneously, the temperature of the ablated region was measured by means of MR-compatible sensors (fiber Bragg Grating, FBG) at several distances from the laser fiber tip (from 4 to 12 mm). The intensity signal values of MR images were correlated with the temperature values measured by the reference FBGs to perform a calibration of the MR images, evaluating the thermal sensitivity $(\beta\,\,\, { [^{\circ } \mathbf {C}} ^{\mathbf {-1}}])$ of the MR sequence. The higher the tissue temperature, the lower the MR signal intensity: the pixel value turns from gray, at room temperature, to black for higher temperature. Three different T1-weighted MR sequences were studied (IRTF, SRTF, and FLASH), and, for each, images with three different pixel number and size (265x265, 192x192, and 128x128 pixels) were tested during the Nd:YAG laser ablation on two porcine livers. Results showed that the bigger the number of pixels, the bigger the RMSE for all the sequences. The images with 192x192 pixels provided better results for IRTF and FLASH in terms of thermal sensitivity ${(\beta _{\mathbf {IRTF}} = {-1.24 ^{\circ } \mathbf {C}} ^{\mathbf {-1}},\mathbf {RMSE}_{\mathbf {IRTF}} =2.4,\beta _{\mathbf {FLASH}} = {-1.74 ^{\circ } \mathbf {C}} ^{\mathbf {-1}},\mathbf {RMSE}_{\mathbf {FLASH}} =1.8)}$. On the other hand, SRTF exhibits the lower values of RMSE for all the pixel number settings. These results are useful to guide the choice of the suitable MR imaging parameters for MR-based thermometry in liver, and potentially in other organs undergoing LA for tumor removal.