The accuracy of near infrared spectroscopy and imaging during focal changes in cerebral hemodynamics

Near infrared spectroscopy (NIRS) can detect changes in the concentrations of oxy-hemoglobin ([HbO]) and deoxy-hemoglobin ([Hb]) in tissue based upon differential absorption at multiple wavelengths. The common analysis of; NI RS data rises the modified Beer-Lambert law, which is an empirical formulat ion that assumes global concentration changes. We used simulations to exami ne the errors that result when this analysis is applied to focal hemodynami c changes, and we performed simultaneous NIRS measurements during a motor t ask in adult humans and a neonate to evaluate the dependence of the measure d changes on detector-probe geometry. For both simulations and in vivo meas urements, the wide range of NIRS results was compared to an imaging analysi s, diffuse optical tomography (DOT). The results demonstrate that relative changes in [HbO] and [Hb] cannot, in general, be quantified with NIRS. In c ontrast to that method, DOT analysis was shown to accurately quantify simul ated changes in chromophore concentrations. These results and the general p rinciples suggest that DOT can accurately measure changes in [Hb] and [HbO] , but NIRS cannot accurately determine even relative focal changes in these chromophore concentrations. For the standard NIRS analysis to become more accurate for focal changes, it must account for the position of the focal c hange relative to the source and detector as web as the wavelength dependen t optical properties of the medium. (C) 2001 Academic Press.