Frequency-domain optical absorption spectroscopy of finite tissue volumes using diffusion theory
Physics in Medicine and Biology
The goal of frequency-domain optical absorption spectroscopy is the non-invasive determination of the absorption coefficient of a specific tissue volume. Since this allows the concentration 6f endogenous and erogenous chromophores to be calculated, there is considerable potential for clinical application. The technique relies on the measurement of the phase and modulation oflight, which is diffusely reflected or transmitted by the tissue when it is illuminated by an intensity-modulated source.
... -modulated source. A model of light propagation must then be used to deduce the absorption coefficient. For simplicity, it is usual to assume the tissue is either infinite in extent (for transmission measurements) or semi-infinite (for reflectance measurements). The goal of this paper is to examine the errors introduced by these assumptions when measurements are actually performed on finite volumes. Diffusion-theory calculatians and experimental measurements were performed for slabs, cylinders and spheres with optical properties characteristic of soft tissues in the near infmed. The error in absorption coefficient is presented as a function of abject size as a guideline to when the simple models may be used. For transmission measurements, the error is almost independent of the Vue absorption coefficient, which allows absolute changes in absorption to be measured accurately. The implications of these mors in absorption coefficient for two clinical prablems-quantitation of an exogenous photosensitizer and meaurement of haemoglobin oxygenation-are presented and discussed. ~ ~~ 1990 When does the diffusion approxjmaiian fail to describe photon trimsport B W Pogue and M S Patterson path length for cerebra! near-inked spectmrcopy in newborn infanfs Dev. Neurosci. 12 14011 in random media? Phys. Rev. Lett. 64 2647-50