Diffuse photon-remission associated with the center-illuminated-area-detection geometry. II, an approach to the time-domain model

Steady-state diffuse reflectance associated with a center-illuminated-area-detection (CIAD) geometry has shown potentials for tissue assessment and elucidating the patterns of single-fiber reflectance (SFR) via a simple scaling of the photon remission between the two geometries. Part I has demonstrated a new algebraic model approach to the steady-state diffuse photon remission associated with the CIAD geometry, by means of area-integration of a radially resolved diffuse photon remission projected by a master-slave dual-source scheme. This Part II proposes a model of time-dependent diffuse photon remission for the CIAD geometry, by virtue of area-integration of the radially resolved time-dependent diffuse photon remission formulated with the master-slave dual-source scheme. Monte Carlo (MC) simulations, limiting to only the Heyney-Greenstein scattering phase function, are used to examine the outputs of the terminally algebraic model of the time-dependent photon remission associated with the CIAD geometry. The time-domain model is assessed against MC for CIAD of physical scales and medium properties that may be relevant to SfR and over a 2 ns duration in compliance with the timespan of the only experimental report of SfR demonstrated with a 50 μm gradient index fiber. The time-domain model-MC assessments are carried out for an absorption coefficient ranging 3-orders of magnitude over [0.001, 0.01, 0.1, 1] mm-1 at a fixed typical scattering, and a reduced scattering coefficient ranging 3-orders of magnitude over [0.01, 0.1, 1, 10] mm-1 at a fixed typical absorption, among others, in association with a CIAD geometry of up to 1mm in diameter. Photons of shorter and longer propagation times, relative to the diameter of the area of collection, respond differently to the scattering and absorption changes, which indicates the benefit of time-domain information. Limited comparisons of MC between CIAD and a top-hat illumination-collection overlapped geometry reveal that the time-domain photon remissions of the two geometries differ appreciably in only the early arriving photons