Portable, Fiber-Based, Diffuse Reflection Spectroscopy (DRS) Systems for Estimating Tissue Optical Properties

Abstract

Steady-state diffuse reflection spectroscopy is a well-studied optical technique that can provide a noninvasive and quantitative method for characterizing the absorption and scattering properties of biological tissues. Here, we compare three fiber-based diffuse reflection spectroscopy systems that were assembled to create a light-weight, portable, and robust optical spectrometer that could be easily translated for repeated and reliable use in mobile settings. The three systems were built using a broadband light source and a compact, commercially available spectrograph. We tested two different light sources and two spectrographs (manufactured by two different vendors). The assembled systems were characterized by their signal-to-noise ratios, the source-intensity drifts, and detector linearity. We quantified the performance of these instruments in extracting optical properties from diffuse reflectance spectra in tissue-mimicking liquid phantoms with well-controlled optical absorption and scattering coefficients. We show that all assembled systems were able to extract the optical absorption and scattering properties with errors less than 10%, while providing greater than ten-fold decrease in footprint and cost (relative to a previously well-characterized and widely used commercial system). Finally, we demonstrate the use of these small systems to measure optical biomarkers in vivo in a small-animal model cancer therapy study. We show that optical measurements from the simple portable system provide estimates of tumor oxygen saturation similar to those detected using the commercial system in murine tumor models of head and neck cancer.

Fig. 1

Pictures of the (A) SkinSkan (JY Horiba) (B) USB4000+HL2000, and (C) USB4000+LED systems.

Fig. 2

Linearity of lamp intensity for (A) System 1 (USB+HL), (B) System 2 (USB+LED), and (C) System 3 (Ava+HL) with different integration times. In each plot, stars, diamonds and triangles represent data at 460 nm, 550 nm, and at 610 nm, respectively. (D) The spectral profile of the two light sources (dashed line: LED; solid line: halogen).

Fig. 3

SNR of the systems as a function of measured signals: (A) System 1 (USB+HL), (B) System 2 (USB+LED), and (C) System 3 (Ava+HL) with increasing signal. In each plot, stars, diamonds, and triangles represent data at 460 nm, 550 nm, and at 610 nm, which are fit to the shot-noise equation, as shown by the solid, dashed-dotted, and dashed lines, respectively (see text).

Fig. 4

Extracted vs. expected values of the absorption coefficient and scattering coefficients from measurements using Systems 1–3 on phantom Set 3. (A) Absorption (triangles) and (D) scattering (circles), respectively, for System 1; (B) absorption and (E) scattering for System 2; and (C) absorption and (F) scattering for System 3. The dashed lines shows the ideal-fit line (y = x).

Fig. 5

Extracted vs. expected values of the absorption coefficient (triangles; A and B) and scattering coefficient (circles; C and D) measurements using System 1 (USB+HL; A and C) and System 2 (USB+LED; B and D). The phantoms measured in (A) and (C) are from Phantom Set 1 while the data in (B) and (D) are from Phantom Set 2 (see Table I).

Fig. 6

Longitudinally measured trends in the baseline-corrected oxygen saturation (ΔSO₂) measured in small-animal studies using (A) the commercial instrument (Skinskan) and (B) System 1. In each figure, the symbols represent the mean value of ΔSO₂ computed across all animals within a given group, at that time-point, while the error bars represent the standard error in those data. The squares show these trends for the group of animals that showed long-term local control, triangles for the treated animals that had local recurrence and circles for the control group. (C) and (D) depict the fiber-probe geometries used to collect the data in (A) and (B), respectively. The hollow circles indicate source fibers and filled circles are the detector fibers. Each fiber in (C) had a 200 µm core, while each fiber in (D) had a 1 mm core.

Fig. 7

(A) The correlation in SO₂ obtained using System 1 (USB+HL) and Skinskan, where these measurements were obtained from different sites on the tumor. (B) The correlation in SO₂ data measured using System 1 at two different sites, on each subject. (C) and (D) The correlation in SO₂ values obtained from repeated scans on the same site using the Skinskan and System 1, respectively.