Near-infrared scattering measurements of the iso-path-length point for endoscopic applications

Near-infrared (NIR) diagnosis permits sensing changes in the optical properties on an air-tissue interface. While imaging is bound to the surface due to the high tissue scattering, many diagnosis methods, such as the photoplethysmograph (PPG) and pulse oximeter, focus on sensing instead of imaging. These methods are commonly used in cylindrical tissues, such as fingertips, or half infinite tissues, such as forehead. However, these methods are less common in endoscopic applications, where a cylinder of air is surrounded by tissue. Previously, we proposed measuring the full scattering profile (FSP), which is the angular intensity distribution, of cylindrical tissues. MC simulations revealed that the FSP has a fixed intensity point, named the iso-path-length (IPL) point, which does not depend on the reduced scattering coefficient. The location of this point depends solely on geometry. Using an analytics photon diffusion method, we have discovered that the IPL point exists in the endoscopic geometry as well. In this study, we present Monte Carlo (MC) simulations of the FSP in endoscopic geometry, where a cylinder of air is surrounded by tissue. This geometry, as the cylindrical geometry, has two IPL points; along the azimuthal direction and the longitudinal direction. We will demonstrate that while the position of the azimuthal IPL point depends on the diameter of the air hole, the position of the longitudinal one stays constant. The IPL point in endoscope applications may open a new path of diagnosis of change in cells of the digestive system.