Optical property recovery with spatially-resolved diffuse reflectance at short source-detector separations using a compact fiber-optic probe

Abstract

We describe a compact fiber-optic probe (2 mm outside diameter) that utilizes spatially-resolved diffuse reflectance for tissue optical property recovery. Validation was performed in phantoms containing Intralipid 20% as scatterer, and methylene blue (MB), MnTPPS, and/or India ink as absorbers. Over a range of conditions, the reduced scattering coefficient was recovered with a root mean square error (RMSE) of 0.86-2.7 cm^-1 (average error = 3.8%). MB concentration was recovered with RMSE = 0.26-0.52 µM (average error = 15.0%), which did not vary with inclusion of MnTPPS (p=0.65). This system will be utilized to determine optical properties in human abscesses, in order to generate treatment plans for photodynamic therapy.

Fig. 1.

(a) Schematic of optical probe face with source fiber (S) and detector fibers (D1-D8) indicated. (b) Photograph of manufactured optical probe face with each fiber illuminated. Note that final image was stitched together from separate images with each fiber illuminated independently. (c) Schematic of spectroscopy system.

Fig. 2.

Monte Carlo simulation library as a function of µ_a and µ_s’ for detectors fibers located at source-detector separations of (a) 288 µm and (b) 974 µm.

Fig. 3.

Recovery of (a) µ_a and (b) µ_s’ at 665 nm for phantoms containing India ink and Intralipid. Symbols represent mean values across phantoms with identical optical properties, with error bars corresponding to standard deviation. Solid red lines indicate perfect agreement. Note that error bars are included for all cases, but are not always visible.

Fig. 4.

Measured phantom data for µ_s=50 cm^-1 at 665 nm, after correction for dark background, system response, integration time, lamp power, and division by corresponding integration sphere calibration. Colors indicate detection fiber, while line styles indicate MB concentration in the phantom.

Fig. 5.

Recovered µ_a spectra from a single phantom, with fits performed (a) individually at each wavelength and (b) across the full spectrum using the known shape of MB absorption. Solid lines represent known spectra, with open circles corresponding to recovered spectra. Color corresponds to MB concentration.

Fig. 6.

Recovery of (a) MB concentration and (b) µ_s’ at 665 nm using either independent wavelength or full spectral fits. Symbols represent mean values across phantoms with identical optical properties for each method, with error bars corresponding to standard deviation. Solid red lines indicate perfect agreement. Note that error bars are included for all cases, but are not always visible.

Fig. 7.

Measured phantom data for [MnTPPS]=9 µM, after correction for dark background, system response, integration time, lamp power, and division by corresponding integration sphere calibration. Colors indicate detection fiber, while line styles indicate MB and MnTPPS concentration in the phantom.

Fig. 8.

Recovery of (a) MB concentration and (b) MnTPPS concentration in phantoms containing both MB and MnTPPS. Data points represent mean recovered concentrations, while error bars indicate standard deviation across phantoms with identical optical properties. Solid red lines indicate perfect agreement. Error bars are included for all points, but may not be visible in all cases.