Frequency domain broadband short-wave infrared spectroscopy for measurement of tissue optical properties from 685 to 1300 nm.

Significance: Extending frequency domain diffuse optical spectroscopy (FD-DOS) into the short-wave infrared (SWIR) region has the potential to improve measurements of key biological tissue chromophores such as water and lipids, given their higher absorption in SWIR compared with near-infrared wavelengths. Few studies have explored FD-DOS in the SWIR range.

Aim: We present the first demonstration of a frequency domain broadband SWIR spectroscopy (FD-Bb-SWIRS) system to measure optical properties from 685 to 1300 nm.

Approach: A custom hybrid system was developed, combining discrete frequency domain measurements from 685 to 980 nm with broadband continuous wave measurements from 900 to 1300 nm. This setup provided absolute absorption ( ${\mu }_a$ ) spectra from 685 to 1300 nm. Validation was performed using mineral oil-based solid phantoms, deuterium oxide ( $D_{2}O$ ) liquid phantoms, and desiccating porcine tissue.

Results: The FD-Bb-SWIRS system was sensitive to changes in ${\mu }_a$ from varying concentrations of absorbers in solid and liquid phantoms. Ex vivo measurements of ${\mu }_a$ spectra indicated differences in tissue water content across different porcine tissue samples during baseline and desiccation.

Conclusions: FD-Bb-SWIRS is highly sensitive to ${\mu }_a$ in the 685 to 1300 nm range and enables precise quantification of water in biological tissues. It represents a significant step forward in advancing SWIR-based optical spectroscopy for clinical applications.