Quantitation of near infrared spectroscopic data in a scattering medium such as tissue requires knowledge of the optical pathlength in the medium. This can now be estimated directly from the time of flight of picosecond length light pulses. Monte Carlo modelling of light pulses in tissue has shown that the mean value of the time dispersed light pulse correlates with the pathlength used in quantitative spectroscopic calculations. This result has been verified in a phantom material. Time of flight measurements of pathlength across the rat head give a pathlength of 5.3 +/- 0.3 times the head diameter.

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In order to quantify near-infrared spectroscopic (NIRS) data on an inhomogeneous medium, knowledge of the contribution of the various parts of the medium to the total NIRS signal is required. This is particularly true in the monitoring of cerebral oxygenation by NIRS, where the contribution of the overlying tissues must be known. The concept of the time point spread function (TPSF), which is used extensively in NIRS to determine the effective optical pathlength, is expanded to the more general inhomogeneous case. This is achieved through the introduction of the partial differential pathlength, which is the effective optical pathlength in the inhomogeneous medium, and an analytical proof of the applicability of the modified Beer-Lambert law in an inhomogeneous medium is shown. To demonstrate the use of partial differential pathlength, a Monte Carlo simulation of a two-concentric-sphere medium representing a simplified structure of the head is presented, and the possible contribution of the overlying medium to the total NIRS signal is discussed.

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We have determined the spectral dependence of the temporal point spread functions of human tissues experimentally between 740 and 840 nm in transmittance measurements on the adult head, forearm, and calf (in vivo) and the infant head (post mortem) by using picosecond laser pulses and a streak camera detector. Two parameters are extracted from the temporal point spread function; the differential path-length factor (DPF), calculated from the mean time, and the slope of the logarithmic intensity decay. In all tissues the DPF and the logarithmic slope show a reciprocal relationship and exhibit characteristics of the absorption spectra of hemoglobin. The DPF falls with increasing wavelength, the variation being typically 12%, while the logarithmic slope increases with wavelength. A quantitative analysis of the logarithmic slope spectrum significantly underestimated expected tissue chromophore concentrations. The absolute magnitudes of the DPF showed considerable intersubject variation, but the variation with wavelength was consistent and thus may be used in the correction of tissue attenuation spectra.