Shortwave-infrared spatial frequency domain imaging for quantifying water and lipid concentrations in tissue

Studying the concentrations of water and lipids in human tissue can give insights into biological processes and diseases. This study shows that shortwave-infrared (SWIR) light from light-emitting diodes (LEDs) can be used in spatial frequency domain imaging (SFDI) to quantify water and lipid concentrations in tissue. In contrast to near-infrared (NIR) wavelengths, the SWIR wavelength range offers deeper tissue penetration and coincides with strong absorption bands of water and lipids. The system developed in this work uses 970 nm, 1050 nm, and 1200 nm LEDs with a digital micromirror device for DC and AC illumination. An InGaAs camera and optics image the diffusely reflected light. A 10% Intralipid phantom was used to calibrate the system, allowing conversion of demodulated pixel values to diffuse reflectance. Measurement of sample lipid and water concentrations was performed for several different known dilutions of Intralipid. Water content in biological tissue was measured using SWIR-SFDI in ex vivo porcine skin tissue samples and validated by measuring the change in mass due to water during desiccation, showing a mean error of 0.9% in prediction of initial water content. SWIR-SFDI measurements were taken in human subjects before and after light exercise, showing distinct changes in tissue absorption and reduced scattering. These results show the potential of a LED-based SWIR-SFDI system for noninvasive quantification and mapping of important tissue chromophores.