Comparison between a time-domain and a frequency-domain system for optical tomography

Optical tomography is a noninvasive functional medical imaging modality, the purpose of which is to obtain three-dimensional (3D) images of the optical properties of tissue at near-infrared (NIR) or visible red wavelengths. The propagation of light in tissue is stochastic in nature, and the optical properties of tissue are defined accordingly. The absorption and scattering coefficients ( ${\mu }_a$ and ${\mu }_s$ ) describe the frequency of absorption and scatter events per distance traversed by the photon. The absorption at different wavelengths is of particular interest, as it depends strongly on the concentrations of oxygenated and deoxygenated hemoglobin ( ${\mathrm{HbO}}_2$ and Hb). Changes in ${\mathrm{HbO}}_2$ and Hb with respect to an arbitrary baseline can be determined using a time series of the intensity or amplitude data. If the absolute optical properties of the tissue can be determined, it is possible to estimate the oxygen saturation and blood volume of the tissue.