Verification of MR thermometry via in-vivo intralesional fluorooptic temperature measurement for laser-induced thermotherapy of liver metastases

Purpose: To evaluate the correlation of MR-measured changes of signal intensity and invasive fluoroptic temperature measurements during MR-guided LITT of liver metastases. Materials and methods: In 15 patients with proven liver metastases of colorectal carcinoma, MR-guided LITT was performed with a percutaneous approach in a multiapplicator technique. Two temperature sensitive T-1-weighted sequences (FLASH-2 D- and TurboFLASH-sequences) were used to map the spatial and temporal distribution of Nd:YAG laser effects. Parallel fluoroptic temperature measurements were carried out by means of an inserted probe in a distance of 5-26 mm (mean: 14 mm) from the laser applicator. Results: In both sequences a gradually increasing signal toss could be documented during laser application which proved to be reversible after cessation of energy deposition. The percentage of decrease in signal intensity correlated directly with the measured increase of temperature. Invasive fluoroptical evaluation of temperature distribution after 10 min exposure time showed at 5 mm distance from the applicator an increase of temperature of 35 degrees C, in 10 mm distance a mean increase of 9 degrees C+/-1.7, in 15 mm a mean increase of 7 degrees C+/-1.6 and in 20 mm a mean increase of 3 degrees C+/-0.5. This is evidence of thermal tissue damage up to 3 cm in diameter with laser monoapplication. The qualitative evaluation revealed a reproducible correlation of the extent of signal loss around the applicator and the finally induced degree of necrosis. Conclusion: Invasive fluoroptical temperature measurements prove the diagnostic reliability of MR thermometry for the online monitoring of LITT of liver metastases.