Spectroscopy and Tomography of Tissues in the Frequency-Domain

Abstract

Substantial progress in the field of light spectroscopy and imaging of tissues was achieved when the group of Chance, Patterson and Wilson showed that the optical parameters of a turbid medium can be obtained from time resolved measurements of short light pulses propagating in the medium (Patterson et al, 1991a). Essentially, a fit of the intensity as a function of time, measured at some distance from the source, can provide separately the values of the absorption and of the reduced scattering coefficients. This demonstration was important because the focus was shifted from attempts to separate the scattering from absorption, using empirical corrections to the Beer-Lambert law, to a rigorous application of a physical model. During the same period, our lab proposed employing the Fourier transform equivalent concept using an intensity modulated light source (Gratton et al, 1990). Since frequency domain methods have better resolution and sensitivity and are much faster than the time domain methods, our proposal was followed by many others including Chance, Patterson and Wilson (Boas et al, 1993, 1994; Cui and Ostrander, 1993; Duncan et al, 1993; Kaltenbach and Kaschke, 1993; O’Leary et al, 1992; Patterson et al, 1991b; Tromberg et al, 1993). It is well known that time domain and frequency domain measurements are mathematically equivalent, when the frequency domain measurement is carried out using a wide range of modulation frequencies (Gratton et al, 1983; Alcala et al, 1984). However, frequency domain measurements can be made at a single modulation frequency, thereby sacrificing some of the information. The advantage of measurements at a single frequency is that they can be very fast, accurate and have an excellent signal-to-noise ratio.