Frequency-domain multichannel optical detector for noninvasive tissue spectroscopy and oximetry

We have designed a multisource frequency-domain spectrometer for the optical study of biological tissues. Eight multiplexed, intensity-modulated LEDs are employed as the light sources. Four of them emit light at a peak wavelength of 715 nm (λ₁); the other four, 850 nm (λ₂). The frequency of intensity modulation is 120 MHz. This instrument measures the frequency-domain parameters phase, dc intensity, and ac amplitude at the two wavelengths λ₁ and λ₂ and for different distances between light source and detector. From these frequency-domain raw data, the absolute values of the absorption and reduced scattering coefficients of tissue at λ₁ and λ₂ are obtained. The oxy- and deoxyhemoglobin concentrations, and hence the hemoglobin saturation, are then analytically derived from the molar extinction coefficients. Acquisition times as short as hundreds of milliseconds provide real-time monitoring of the measured parameters. We performed a systematic test in vitro to quantify the precision and accuracy of the instrument reading. We also report in vivo measurements. This spectrometer can be packaged as a compact portable unit.