Error assessment of a wavelength tunable frequency domain system for noninvasive tissue spectroscopy

Abstract

A system is described and evaluated for noninvasive measurement of the scattering and absorption coefficients of tissue in the 600 to 850-nm range. An arc lamp/monochromator combination provides a tunable source which is intensity modulated at 135 MHz by a Pockel's cell and coupled by fiber optics to the tissue surface. Diffusely reflected light is collected by optical fiber bundles at distances of 1 and 2 cm from the source and the intensity ratio and phase difference between these two signals are measured. These data are analyzed by means of a diffusion model of light to provide estimates of the scattering and absorption coefficients. The accuracy of these estimates is assessed by calculation of the influence of instrumental noise, tissue optical properties, and modulation frequency. Because the analysis is based on the assumption of a semi-infinite tissue geometry, systematic errors caused by idealized finite volumes (slabs, cylinders, and spheres) are also considered. The biomedical utility of the system is illustrated by noninvasive measurement of the uptake of an intravenously injected dye in rabbit leg muscle. © 1996 society of Photo-Optical instrumentation Engineers.