Dual-slope broadband spectroscopy

Broadband spectroscopy of highly scattering media that aims to measure absorption spectra that properly take into account optical scattering can be performed with a hybrid CW & FD approach. The idea is to perform multi-distance FD-NIRS at a few discrete wavelengths \((\lambda_i)\) for absolute measurements of the reduced scattering coefficient \((\mu_s^\prime(\lambda_i))\). Then a broadband scattering spectrum \((\mu_s^\prime(\lambda))\) is obtained by extrapolating the discrete wavelength scattering measurements through the power law \(\mu_s^\prime\left(\lambda\right)=a{(\lambda/\lambda_0)}^{-b}\), where a and b are constants, and \(\lambda_0\) is a reference wavelength. Then, dual-slope broadband CW-NIRS measurements are performed to obtain self-calibrated intensity slopes that are converted into absolute absorption spectra after considering the broadband scattering spectra obtained as described above. We validated this approach in highly scattering phantoms containing milk and water (MW) and the addition of different dyes: India ink (II: featureless absorption spectrum), NIR746A (N7: absorption peak at 746 nm), and NIR869A (N8: absorption peak at 869 nm). The measured spectra are shown in Fig. 1(a). Measurements of tissue absorption spectra with this approach are reported in Fig. 1(b) for three healthy subjects (A: 27 y.o. male; B: 30 y.o. male; C: 24 y.o. female) and six different tissues (forearm, arm, chest, abdomen, thigh, calf). The spectra in vivo show recognizable features such as the deoxy-hemoglobin absorption peak at 758 nm, the lipid absorption peak at 925 nm, and the water absorption peak at 975 nm.

Fig. 1. Broadband absorption spectra obtained by combining dual-slope broadband CW-NIRS with multi-distance (or self-calibrated) FD-NIRS.