Assessment of a laser-powered multiwavelength near-infrared spectrometer

Near infrared spectroscopy is a non-invasive technique for measuring relative blood volume and oxygen saturation in tissue. We have designed and built a research NIR-spectrometer which offers the flexibility to study changes in blood oxygen saturation (SO2) and in blood volume (BV) during skeletal muscle pacing. The instrument consists of five 1 watt solid state lasers (780, 800, 830, 850 and 980 nm) fired sequentially at 5 mu s pulses for a 1 ms cycle, and a 5 mm(2) photodiode receiver. Features of the spectrometer include, rapid realtime data acquisition (1000 samples/s), receiver protection against ambient light, large dynamic optical power output adjustable for each wavelength, and portability. In vitro photon scattering experiments and linear response to blood oxygen saturation changes for differential absorption (780 - 850 nm) provide an accurate measure of changes in SO2, while the 800 nm signal can be used as a measure of blood volume change independently of SO2 (+/-2%-SO2 error). In addition, the 980 nm signal level is explored as an index of mean pathlength which may provide crucial information for determining absolute SO2.