Broadband ultraviolet-visible optical property measurement in layered turbid media

Abstract

The ability to accurately measure layered biological tissue optical properties (OPs) may improve understanding of spectroscopic device performance and facilitate early cancer detection. Towards these goals, we have performed theoretical and experimental evaluations of an approach for broadband measurement of absorption and reduced scattering coefficients at ultraviolet-visible wavelengths. Our technique is based on neural network (NN) inverse models trained with diffuse reflectance data from condensed Monte Carlo simulations. Experimental measurements were performed from 350 to 600 nm with a fiber-optic-based reflectance spectroscopy system. Two-layer phantoms incorporating OPs relevant to normal and dysplastic mucosal tissue and superficial layer thicknesses of 0.22 and 0.44 mm were used to assess prediction accuracy. Results showed mean OP estimation errors of 19% from the theoretical analysis and 27% from experiments. Two-step NN modeling and nonlinear spectral fitting approaches helped improve prediction accuracy. While limitations and challenges remain, the results of this study indicate that our technique can provide moderately accurate estimates of OPs in layered turbid media.

Keywords: (170.3660) Light propagation in tissues; (170.3890) Medical optics instrumentation; (170.6510) Spectroscopy, tissue diagnostics; (170.6935) Tissue characterization; (170.7050) Turbid media.

Fig. 1

Diagram of diffuse reflectance spectroscopy system for OP measurement.

Fig. 2

Theoretical evaluation of OP prediction accuracy for D = 0. 22 mm (a, c, e, g) and D = 0.44 mm (b, d, f, h). TAR: target values from Beer’s law and Mie theory; 1NN: values from NN#1; 2NN: values from NN#1 and NN#2; FIT: fitted values based on 2NN.

Fig. 3

Theoretical estimates of OP prediction accuracy based on data from Fig. 2.

Fig. 4

Theoretical evaluation of 2NN and FIT approaches based on reflectance data with added noise (D = 0.22 mm).

Fig. 5

Summary of the theoretical accuracy of our OP predication method using (a) noise-free and (b) noise-added reflectance data.

Fig. 6

Simulation results showing reflectance as function of distance from center of illumination fiber and OPs when D = 0.44 mm: numbers in the legends have units of cm^–1. (a) Change in µ_s1' and (b) change in µ_s2'

Fig. 7

Experimental evaluation of OP estimates (FIT) as compared to target/true (TAR) values for phantoms representing normal mucosal tissue [D = 0.22mm: (a), (c); D = 0.44mm: (b), (d)].

Fig. 8

Experimental evaluation of OP estimates based on phantoms representing dysplastic mucosal tissue [D = 0.22mm: (a), (c); D = 0.44mm: (b), (d)].

Fig. 9

OP predictions based on experimental measurements of single-layer phantoms representing the (a) top and (b) bottom layers of normal tissue.