Depth-sensitive reflectance measurements using obliquely oriented fiber probes

Abstract

Computer simulation is used to facilitate the design of fiber-probe geometries that enable enhanced detection of optical signals arising from specific tissue depths. Obtaining understanding of the relationship between fiber-probe design and tissue interrogation is critical when developing strategies for optical detection of epithelial precancers that originate at known depths from the tissue surface. The accuracy of spectroscopic diagnostics may be enhanced by discretely probing the optical properties of epithelium and underlying stroma, within which the morphological and biochemical features vary as a function of depth. While previous studies have investigated controlling tissue-probing depth for fluorescence-based modalities, in this study we focus on the detection of reflected light scattered by tissue. We investigate how the depth of optical interrogation may be controlled through combinations of collection angles, source-detector separations, and numerical apertures. We find that increasing the obliquity of collection fibers at a given source-detector separation can effectively enhance the detection of superficially scattered signals. Fiber numerical aperture provides additional depth selectivity; however, the perturbations in sampling depth achieved through this means are modest relative to the changes generated by modifying the angle of collection and source-detection separation.

Investigation of a probe design for facilitating the uses of the standard photon diffusion equation at short source-detector separations: Monte Carlo simulations

“…This result is caused by the oblique incidence angle of photons traveling from the diffusing layer to the tissue layer in the MTL geometry. Similar phenomenon has been reported by Wang et al32…”

“…Both the MTL geometry and the semiinfinite geometry lose the dependence of interrogation depth on the NA of detection fiber when the source detector separation is increased to 5 mm. Although the NA has been shown to have moderate influence on the interrogation depth of the semiinfinite probing geometry, the weak influence of NA on the interrogation depth of the MTL geometry makes it a less effective parameter in modulating the probing depth compared to the source detector separation and the scattering layer thickness layer thickness 32. Note again that, as shown in Figs.…”

Combined Monte Carlo and finite-difference time-domain modeling for biophotonic analysis: implications on reflectance-based diagnosis of epithelial precancer

“…33 Extensive efforts have recently been made to design probe geometries with increased selectivity for optical signals from the top epithelial layer, since early identification of changes in epithelial cells is expected to be of significant importance in cancer diagnostics. The second and third designs are based on previous studies that suggest using tilted source and detector fibers to limit the penetration depth of photons, [9][10][11][12]14,[34][35][36] or coupling fibers to a half-ball lens such that the flat surface of the lens faces the tissue. 10 The three different probe geometries modeled are illustrated in Fig.…”

“…[6][7][8][9][10][11][12][13][14] Most MC studies, however, are generally performed using simplified phase functions that only approximate the angular scattering probability distributions of microscopic tissue constituents. A common example is the wellknown Henyey-Greenstein phase function, which is given by a simple analytical expression and resembles scattering patterns computed on the basis of the Mie theory for homogeneous spherical scatterers.…”

“…33 The second design involves source and detector fibers tilted at an angle with respect to the tissue surface. [9][10][11][12]14,[34][35][36] Finally, the third design involves lens coupled source and detector fibers. 10 Simulations are performed to determine the extent of the influence of using FDTD-generated cellular phase functions when these different probe geometries are employed for optical measurements.…”