Measuring absolute heme concentrations

 Whilst all mNIRS devices can in principle measure changes in oxy and deoxy heme concentrations, more expensive equipment is needed to measure absolute concentrations.

NIR light is multiply scattered in tissue. Light reaching the detector is therefore a function of both the tissue absorption and scattering properties.

A time-domain system measures the time at which photons arrive at the detector. A short laser pulse is detected as a spread of photons, the so-called temporal point spread function (TPSF). The beauty of the TPSF is that its shape contains information that can be used to separate absorption and scattering. Essentially, the earliest photons have travelled shorter paths through the tissue, whilst later photons have travelled progressively longer paths. The shape of the TPSF can therefore be fitted to a model of light diffusion to obtain an absorption coefficient (μa) and reduced scattering coefficient (μs′) at each wavelength.

Whilst scattering information may be interesting in its own right, the absorption coefficient is what is needed to measure absolute heme concentrations. The assumption is made that the vast majority of the relevant NIR absorption is due to oxy and deoxy heme, so these two chromophores are fitted to the absorption (μa) data. Unlike continuous-wave systems, a differential pathlength factor (DPF) is unnecessary because the time-domain measurement provides the information needed to determine the tissue optical properties. The Beer-Lambert Law can therefore be used to obtain absolute oxy and deoxy heme concentrations.

Re R, et al. “Time Domain Near Infrared Spectroscopy Device for Monitoring Muscle Oxidative Metabolism: Custom Probe and In Vivo Applications.” Sensors 18 (2018): 264.

A frequency-domain system can also separate tissue absorption and scattering from measurements of the intensity and phase of modulated NIR light, and hence measure absolute oxy and deoxy heme concentrations.

Maier JS, et al. “In vivo study of human tissues with a portable near-infrared tissue spectrometer.” Proceedings of SPIE2387 (1995): 240.