The effect of overlying tissue on the spatial sensitivity profile of near-infrared spectroscopy

The exact volume of brain tissue interrogated in near-infrared spectroscopy (NIRS) studies of cerebral oxygenation is unknown, the inhomogeneity of the head and resulting variation in optical paths making determination of the interrogated volume difficult. In this study, the spatial sensitivity profiles in simple two-component inhomogeneous cylindrical models are predicted by Monte Carlo simulation in order to reveal the effects of the overlying tissue on the volume of tissue interrogated by NIRS. The predictions are validated by experimental measurements on solid 'tissue equivalent' cylindrical phantoms. The phantom diameter is 30 mm and the thickness of the outer layer is 2 mm. The optical properties of the inner cylinder match those of adult brain white matter ( mu '_sl=6.4 mm^-1 and mu _al=0.02 mm ^-1), but those of the outer layer are changed ( mu _s0' from 1.7 to 8.3 mm^-1 and mu _a0 from 0.002 to 0.1 mm ^-1). Results show that the spatial sensitivity profile is largely confined to the outer layer at small optode spacing (<15 degrees ) and to the inner layer at large spacing (>120 degrees ). At intermediate angles, the sensitivity profiles are sensitive to the optical properties of the outer layer. A low mu _s0' or mu _a0 moves the profile toward the surface, at high mu _a0 it moves in towards the inner layer and at high mu _s0' it is similar to the homogeneous case.