Spatial Sensitivity of NIRS Tissue Oxygenation Measurement using a Simplified Instrument

N

ear infra red spectroscopy (NIRS) is of interest for chemical and structural analysis in cells, tissues and organs. We describe simplified instrumentation aimed at clinical applications. This is a four-wavelength system (735nm, 760nm, 810nm and 850nm) for tissue oxygenation measurement, using light-emitting diodes (LED) as light sources and a sensitive photodiode (OPT101) detector.

In vitro

testing was done using ink solutions of varying concentrations and the NIRS instrument results were compared with those from a spectrophotometer. Changes in chromophore concentration were calculated using the modified Lambert-Beer Law together with the application of Diffusion Theory. The

in vitro

results showed good correlation between the new instrument and the laboratory standard at all four wavelengths. We examined the

in vivo

performance in the diffuse reflectance mode on the adult forearm. In arterial occlusion experiments and saw the expected changes in the values of HHb and HbO

2

. We investigated the spatial sensitivity of the instrument by varying the source-detector spacing from 20mm to 35mm also with arterial occlusion. There were progressive changes in HHb concentration, with return to baseline after occlusion. For spacings of 20mm and 22mm the HHb changes were much smaller than those seen at spacings of 25mm, 29mm, 30mm and 35mm. This is because at the smaller spacings a more superficial layer of skin is interrogated and this has a lower blood volume than the deeper sections seen for higher spacings. The simplified instrument has performance that is adequate for diffuse reflectance measurements and could be useful in some clinical applications.