Spatially resolved thermometry during laser ablation in tissues: Distributed and quasi-distributed fiber optic-based sensing

This work assesses quasi-distributed Fiber Bragg Gratings (FBGs) and Rayleigh scattering-based distributed sensing for accurate and millimeter resolved thermometry in media undergoing laser ablation. Mono-, two- and three-dimensional temperature measurements were performed with a network of 1 mm-long FBGs in 10-25-40-FBGs-arrays and distributed-sensing fiber with gauge lengths (GL) of 5.2 and 1.3 mm. Results confirm that the lower the spatial resolution of the sensors, the highest the root mean square error (RMSE) and temperature underestimation with respect to reference sensors (25-FBGs array). Compared to 1 mm-long FBGs, distributed sensing with 5.2 mm GL provided RMSE >7 °C and underestimation >16 °C in agar-gel phantom. Signal-to-noise ratio over time and temperature standard deviation over temperature show better performances of FBGs over distributed sensors. Indeed, any improvement of spatial resolution for distributed sensors determines an increase of noise in the signal. The three-dimensional measurement performed with 420 FBGs with 1 mm-sensing length demonstrated the spatial and thermal gradient occurring in the pancreas undergoing LA, reaching 28 °C over 5 mm. Millimeter-resolved FBGs proved to be effective for accurate thermometry in laser irradiated tissues, and potentially usable in clinical applications.