Measurement of the local optical properties of turbid media by differential path-length spectroscopy

Abstract

We report on the development of an optical-fiber-based diagnostic tool with which to determine the local optical properties of a turbid medium. By using a single fiber in contact with the medium to deliver and detect white light, we have optimized the probability of detection of photons scattered from small depths. The contribution of scattered light from greater depths to the signal is measured and subtracted with an additional fiber, i.e., a collection fiber, to yield a differential backscatter signal. Phantoms demonstrate that, when photons have large mean free paths compared with the fiber diameter, single scattering dominates the differential backscatter signal. When photons have small mean free paths compared with the fiber diameter, the apparent path length of the photons that contribute to the differential backscatter signal becomes approximately equal to 4/5 of the fiber diameter. This effect is nearly independent of the optical properties of the sample under investigation.

Implicit and explicit prior information in near-infrared spectral imaging: accuracy, quantification and diagnostic value

“…Such approaches can be categorized into several different data types, as outlined in table 1. These [7][8][9] spectral derivative data multiple analyte analysis [10][11][12] fluorescence/excitation ratio fluorescence imaging [13][14][15] multi-distance data estimation of slopes with linear approximations robust tissue spectroscopy systems insensitive to slight shape changes [16][17][18] small-volume sampling scatter spectroscopy fibre probes with size less than effective scatter distance [19][20][21][22] absorption spectroscopy fibre probes sensitive to absorption only [23,24] fluorescence spectroscopy fibre probes sensitive to fluorescence and not tissue [25,26] temporal signals millisecond variations in tissue oxygen saturation and haemodynamic sampling [8,27,28] microsecond sampling flash photolysis analysis of biochemical changes in vivo [29,30] picosecond sampling ultrafast signals to reduce model dependence [31][32][33][34][35] high-frequency phase shift derivative with distance frequency-domain spectroscopy of tissue [36,37] lifetime-based signals fluorophore identification or microenvironment analysis [38] include: (i) ratiometric or derivative data at two or more different wavelengths, (ii) multiple-distance ratio or derivative data, (iii) small spatial volumes that either limit the effect of physical boundaries through scatter and/or absorption, or allow simpler empirical modelling, and (iv) temporal signals that are less sensitive to boundaries and/or more robustly insensitive to shape changes. The use of prior information about the tissue to be sampled is still essential in the design process with these systems, but can be implemented in the very first step o...…”

Autofluorescence and diffuse reflectance spectroscopy of oral epithelial tissue using a depth-sensitive fiber-optic probe

“…Variations in fiber size, illumination-collection fiber separation, and probe-sample spacing have been shown to influence sensitivity to different fluorophore layers in turbid media [28]. Differential pathlength spectroscopy has been demonstrated as a technique for preferential detection of photons scattered from shallow depths [29].…”

Algorithm for automated selection of application-specific fiber-optic reflectance probes

“…9 We simulated the DPS geometry with two fibers spaced 1.2d fiber apart owing to cladding separation. 42 In contrast to Ref. 42, we also took into account the numerical aperture of the delivery and collection fiber Table 1 Fitting coefficients for PGS depth models.…”