Abstract:

In order to quantify the chromophore components from in vivo blood NIRS, a blood- equivalent phantom has been developed which consists of properly diluted intralipid and ICG dye. The reflection and transmission near infrared spectroscopy (NIRS) of the phantoms with different scattering backgrounds and ICG concentrations are measured and analyzed by the spectral multicomponent analysis (MCA) method to extract ICG concentration. The experimental results show that the MCA method can be used to quantify absolute ICG concentrations in scattering media if the average path lengths are known. Moreover, it was found by the experiments that both the water absorption peak at 970 nm and the ICG absorption peak at 800 nm show similar behavior during the change of the scattering background. Thus the ratio of the MCA-estimated concentration factor of ICG to water is independent of the blood-phantom scattering.

Abstract:

On the basis of both homogeneous and layered skin models this paper analyzes the influences of multiple scattering of skin on the measurements of NIR spectroscopy. Emphasis is laid on the wavelengths of 660, 805, and 940 nm which are used in clinical monitoring systems. The results of Monte-Carlo simulation show that the overwhelming scattering of tissue leads to a nonlinearity in the Lambert-Beer's relation between optical density and chromophore concentration. The consequences of this effect shall be discussed using as an example the non invasive measurement of Indocyanine Green (ICG) in the blood. In this case the multiple scattering of skin causes substantial non-linear relation between the optical density at 805 nm and the concentration of an injected NIR dye in the blood if the concentration of ICG exceeds 10 mg/l. This leads to a significant distortion of the ICG clearance curve and in consequence to a systematic error in the determination of physiological parameters. For multi-wavelength spectroscopy the wavelength dependency of scattering coefficients has to be noticed. The consequence of this effect is demonstrated for blood oxygen saturation (SaO2) measurements.