DETERMINATION OF THE WAVELENGTH DEPENDENCE OF THE DIFFERENTIAL PATHLENGTH FACTOR FROM NEAR-INFRARED PULSE SIGNALS

For the calculation of changes in oxyhaemoglobin, deoxyhaemoglobin and the redox state of cytochrome-c-oxidase from attenuation data via a m odified Beer-Lambert equation the wavelength dependence of the differe ntial pathlength factor (DPF(lambda)) has to be taken into account. Th e DPF, i.e. the ratio of the mean optical pathlength and the physical light source-detector separation at each wavelength, determines the cr osstalk between the different concentrations and is therefore essentia l for a sensitive detection of chromophore changes. Here a simple meth od is suggested to estimate the wavelength dependence of the DPF(lambd a) from pulse-induced attenuation changes measured on the head of adul t humans. The essence is that the DPF is the ratio of the attenuation changes over absorption coefficient changes, and that the spectral for m of the pulse correlated absorption coefficient change can be assumed to be proportional to the extinction coefficient of blood. Indicators for the validity of the DPF(A) derived for wavelengths between 700 an d 970 nm are the stability of the calculated haemoglobin and cytochrom e signals with variations of the wavelength range included for their c alculation and its overall agreement with the data available from the literature.