An MR-compatible fiber-optic probe for measuring focused ultrasound-induced temperature rises without viscous heating artifacts

Focused ultrasound (FUS) beam interactions with thermocouples and fiber optic (FO) probes cause a “viscous heating artifact” (VHA), which prevent accurate temperature measurements during acoustic sonication. This work demonstrates a novel fiber-optic temperature probe which is insensitive to VHA, validated in an MR-guided FUS treatment setting. The FO probe (OSENSA Innovations; PRB-140, 0.14 mm diameter) was inserted with an 18G catheter into a tissue-mimicking gelatin phantom. The FO probe tip was located with high-resolution MR images (0.25 × 0.25 × 0.5 mm3) for targeting with FUS. Continuous-wave FUS sonications (50 W, 20 s) were delivered at a distance of ∼1.5 mm from the FO probe tip using a 256 phased-array transducer (Imasonic, France; 1 MHz, 2.1 × 2.3 × 9.8 FWHM spot size) and FUS-induced heating was measured with 3D MR temperature imaging (MRTI; 0.5 × 0.5 × 1 mm3 resolution, 3.9 s acquisition). The VHA effect was not observed in the PRB-140 FO probe measurement data. The FO probe heating and cooling curves closely matched those measured by MRTI, with a root mean-squared error (RMSE) of 0.61 °C. In contrast, the RMSE of VHA-sensitive FO probe was 6.00 °C for a similar acoustic power output. This ultrasound artifact-immune FO temperature probe is highly advantageous for MR temperature sequence development and precise temperature monitoring in FUS treatment applications.