Calibration of near-infrared frequency-domain tissue spectroscopy for absolute absorption coefficient quantitation in neonatal head-simulating phantoms

Frequency-domain tissue spectroscopy is a method to measure the absolute ab sorption coefficient of bulk tissues, assuming that a representative model can be found to recover the optical properties from measurements. While rel iable methods exist to calculate absorption coefficients from source-detect or measurements less than a few centimeters apart along a flat tissue volum e, it is less obvious what methods can be used for transmittance through th e larger tissue volumes typically associated with neonatal cerebral monitor ing. In this study we compare the use of multiple distance frequency-domain measurements processed with (i) a modified Beer-Lambert law method, (ii) a n analytic infinite-medium diffusion theory expression, and (iii) a numeric al finite element solution of the diffusion equation, with the goal of reco vering the absolute absorption coefficient of the medium. Based upon our ob servations, the modified Beer-Lambert method provides accurate absolute cha nges in the absorption coefficient while analytic infinite-medium diffusion theory solutions or finite element-based numerical solutions can be used t o calculate the absolute absorption coefficient, assuming that the data can be measured at multiple source-detector distances. We recommend that the i nfinite-medium multi-distance method or the finite element method be used a cross large tissue regions for calculation of the absolute absorption coeff icient using frequency-domain near-infrared measurements at multiple positi ons along the head. (C) 2000 Society of Photo-Optical instrumentation Engin eers. [S1083-3668(00)00402 -0].