Endoscopic iso-pathlength self-calibration for direction-resolved retrieval of tissue optical properties

Medical examination of human tissue is preferably performed by imaging the tissue surface. Optical imaging techniques are limited by low penetration depth due to high tissue scattering, whereas sensing techniques can detect changes deeper inside the tissue. Near-infrared sensing methods such as oximetry and fNIRS are already used clinically but have not yet been applied in endoscopy.

We investigate the existence of iso-pathlength (IPL) points in endoscopic geometry, with the goal of extending the concept of IPL points from cylindrical and half-infinite geometries into hollow cylindrical tissue relevant to endoscopy. In addition, we demonstrate the ability to extract the absorption properties of a tissue at this structure by the IPL and demonstrate it by ex vivo experiment.

The IPL point is a unique position in the full scattering profile, independent of tissue scattering and dependent only on the tissue absorption and geometry. We studied two directions in cylindrical endoscopic geometry: azimuthal and longitudinal. First, diffusion theory with extrapolated zero-boundary conditions was applied to predict IPL positions. These predictions were then tested using Monte Carlo simulations of photon distribution and validated experimentally using phantoms with cylindrical air holes measured by endoscopy. Finally, using the experimentally identified IPL point and applying the same procedure to a standard phantom, a hemoglobin-agar phantom, and chicken breast tissue, we were able to estimate the absorption coefficient of the chicken tissue.

Both azimuthal and longitudinal IPL points were identified. The experimental azimuthal IPL point was found at an angle of $144\mathrm{deg}\pm 3\mathrm{deg}$ , whereas the longitudinal IPL point appeared at a distance of $0.33\pm 0.05\mathrm{cm}$ from the laser spot center. These findings confirm the theoretical and simulation predictions. Moreover, from the ex vivo experiment of a chicken breast, the IPL point enables us to calculate the absorption coefficient and get ${\mu }_{\text{a}}=0.94{\mathrm{cm}}^{-1}$ , within the range of $0.2{\mathrm{cm}}^{-1}\le {\mu }_{\text{a}}\le 2{\mathrm{cm}}^{-1}$ .

The demonstration of IPL points in endoscopic geometry provides a new framework for depth-resolved optical sensing in hollow cylindrical tissues. This approach may enable self-calibrated absorption measurements and open the way for improved diagnostic tools in the digestive system, esophagus, and other hollow organs where conventional endoscopy lacks depth information.