Modified optical coefficient measurement system for bulk tissue using an optical fiber insertion with varying field of view and depth at the fiber tip

Abstract

To measure the few millimeter-scale macroscopic optical properties of biological tissue, including the scattering coefficient, while avoiding the instability that originates from sample slicing preparation processes, we performed propagated light intensity measurements through an optical fiber that punctures the bulk tissue while varying the fiber tip depth and the field of view (FOV) at the tip; the results were analyzed using the inverse Monte Carlo method. We realized FOV changes at the fiber tip in the bulk tissue using a variable aperture that was located outside the bulk tissue through a short high-numerical aperture (high-NA) multi-mode fiber with a quasi-straight shape. Using a homogeneous optical model solution, we verified the principle and operation of the constructed experimental system. A 200-μm-core-diameter silica fiber with NA of 0.5 and length of 1 m installed in a 21G needle was used. The detection fiber's shape was maintained over a radius of curvature of 30 cm. The dependences of the detected light intensity on the FOV and the depth showed better than 1.4% accuracy versus calculated dependences based on the measured optical properties of the solution. Adaptation of the method for use with complex structured biological tissue, particularly in the presence of a thick fascia, was not completely resolved. However, we believe that our specific fiber puncture-based measurement method for use in bulk tissue based on variation of the FOV with inverse Monte Carlo method-based analysis will be useful in obtaining optical coefficients while avoiding sample preparation-related instabilities.

Keywords: Bulk tissue; Field of view; Inverse Monte Carlo method; Optical coefficient measurement; Ray tracing simulation.