476
J. Eur. Opt. Society-Rapid Publ. 22, 48( 2026)
provided in the Supplementary Materials, Section C: Diffusive Photon Transport Simulations. Specifically, Table S2 summarizes the detected photon counts across all investigated combinations of pore diameters and vertical pore positions. Additionally, the complete set of 66 photon path length histograms is presented in Figures S157 – S222. These distributions demonstrate that the temporal characteristics of the detected signal remain consistent with expected diffusive behavior for the specified source – detector separation, regardless of the surface pore geometry.
The statistical distribution of detected photon counts and mean photon weights across the eight cadaveric heads is summarized in Table 2. Asshown in Figure 5, shifting the pore position from 1.0 mm to 0.5 mm off-axis resulted in a decrease in the median detected photon count. For both alignment positions, head # 3 was identified as a consistent outlier, falling below the lower whisker of the cohort distribution. A parallel trend was observed for the mean photon weights( Fig. 6), where the median weight decreased as the pore was moved closer to the optical source center. Unlike the photon count data, the distribution of mean photon weights exhibited a larger interquartile range across the cohort, and no individual subjects were identified as outliers. These results indicate that while the vertical positioning of the skin pore modulates signal intensity, the underlying anatomical differences across the eight cadaveric heads introduce a more significant variance in these energy metrics than the localized pore alignment itself. This characterizes the variance in signal detection across the MRIsegmented models relative to localized pore-induced energy modulation, establishing that anatomical variability remains the primary determinant of total detected energy.
4 Discussion
This study systematically evaluates the influence of skin pore microgeometry on NIR photon transport within anatomically realistic, MRI – segmented cadaveric head models. By employing a cascaded simulation approach, the present work effectively bridges the gap between porescale surface realism and macro-scale photon propagation. This sequential integration where ballistic photon propagation simulations through truncated semi – elliptical, roughwalled pores define the initial photon states for subsequent diffusive photon transport simulations provides a robust framework for assessing multi-scale optical interactions. Thereby, study clarifies the conditions under which microscopic surface features can be safely neglected in optical modeling, while simultaneously identifying specific regimes where anatomical and pore-scale variability contribute measurably to the detected signal.
Ballistic photon propagation simulations reveal that skin pore geometry modulates transmitted optical energy in a pore diameter and position-dependent manner, with the effect becoming more pronounced as pore diameter increases and as the vertical distance between the pore and the optical source center decreases. Within this regime, the modeled pore geometries act as localized perturbations at the optical source – scalp interface, redistributing the angular spectrum of incident rays. Examination of the
Figure 4. Representative DS profile for a 30 mm source – detector separation with a 500 lm diameter pore( positioned at 0.5 mm off-axis relative to the optical source center). The DS curves obtained across all individual cadaveric models and pore positions were found to be virtually identical, following the same spatial distribution. For visual clarity, this single representative profile is shown to illustrate the system’ s sensitivity to underlying tissue volumes, which remains stable despite surface-level intensity fluctuations.
angular trajectories( Figs. 2 and S1 – S66) indicates that photons interacting with the rough pore structure tend to acquire larger propagation angles relative to the surface normal. Consequently, rather than remaining confined within a narrow forward-directed cone toward deeper tissues, these photons are redirected outward, increasing beam divergence and reducing the concentration of ballistic energy delivered to deeper layers. As pore diameter increases, this divergence effect influences a larger fraction of the incident beam profile, leading to progressively stronger spatial spreading of transmitted optical energy.
Building upon the initial phase of the study, the final photon states from the ballistic simulations characterized by both pore-induced angular redirection and photon weight reduction were used to initialize the subsequent diffusive photon transport simulations. The results demonstrate a clear bifurcation in how these perturbations propagate: while the high-scattering nature of the scalp and skull induces a rapid loss of directional memory within the first few millimeters [ 19, 20 ], the initial attenuation of photon weights persists as a dominant factor( Figs. 3 and S67 – S148). Consequently, localized angular perturbations do not propagate into deeper tissue layers [ 42, 43 ] and produce no considerable changes in DS profile( Figs. 4 and S149 – S156) or path length statistics( Figs. S157 – S222). While the stochastic scattering regime effectively randomizes the angular information imparted by localized surface features [ 17, 18 ], it cannot recover the optical energy lost during the ballistic phase( Fig. 6). Although this poreinduced weight reduction is an order of magnitude smaller than the inter-subject anatomical variance, it represents a permanent loss of incident energy that scales systematically with pore geometry.
The identification of head # 3 as a consistent outlier in detected photon counts( Figs. 5 and 6) provides a mechanistic example of this sensitivity; its exceptionally thick CSF