Temperature monitoring for cooled LITT applicators using a specially designed magnetic resonance sequence

Motivation: Recently, new applicators have been designed for laser-induced interstitial thermo-therapy to induce large (greater than 25 mm in diameter) tissue lesions. These systems combine laser-induced tissue heating with cooling of areas adjacent to the optical fiber. The resulting complex tissue temperature profiles, however, require an adequate temperature monitoring during therapy. Aim of the study: Evaluation of a new MR-sequence to determine the temperature distribution in laser-irradiated tissue. Materials and Methods: A fresh sample of bovine liver was irradiated (Nd:YAG laser, (lambda) equals 1064 nm, cw, 15 min, 11.8 W) under MRI control uscg a specially designed cooled laser applicator. The MR sequence used is based on the measurement of the temperature-dependent shift of the proton resonance frequency. To increase the sensitivity, a standard TurboFLASH sequence (slice thickness 6 mm, flip angle 5 degrees, TR/TE equals 10/3 ms, matrix 128 by 128) has been modified. The gradient scheme used leads to a total dephasing of the primary echo resulting in an effective echo time of TE(eff) equals TE plus TR. Results: Temperature maps were obtained every minute during laser irradiation. Regions with maximum temperatures were located in a radial distance of 5 to 10 mm from the cooled applicator representing the typically induced temperature distribution. Conclusion: Echo shifted TurboFLASH sequences based on the determination of the temperature dependent shift of the proton resonance frequency allow an adequate monitoring of the complex heating patterns induced by cooled LITT- applicators.