Spatial tissue oxygenation measurement with near infra-red spectroscopy using a virtual instrument

We describe near infrared spectroscopy (NIRS) to study spatial variations in tissue oxygenation: oxyhaemoglobin (HbO₂); deoxy-haemoglobin (HHb); and total haemoglobin (Hb_tot). Apparatus was constructed incorporating light-emitting diodes (LED) as the light sources, sensitive photodiodes (PD) as the detectors, a virtual instrument to control the opto devices and collect and process the signals. The MATLAB script was embedded into LABVIEW to process and display the data. Changes in chromophore concentration were calculated using the modified Lambert-Beer Law together with the diffusion theory. We attached an LED array on the volar surface of the adult forearm with a PD detector placed at various distances of 12 to 40 mm from the sources. Further studies were performed in adults with forearm blood volume and oxygenation manipulated by venous and arterial occlusion respectively. Results indicated that source-detector spacing can achieve spatial sensitivity of blood volume and blood oxygenation measurement. The in vivo results demonstrated that the methodology and equipment achieved good performance as expected. In addition, we used dye-scatterer phantom to analyze the optical properties of near-infrared spectroscopy, where a spectrophotometer was used to calibrate the performance of the instrument. The in vitro results showed good correlation between the new instrument and the laboratory standard. The virtual instrument was a flexible and convenient means to develop apparatus for in vivo NIRS measurement. Spatial changes in tissue blood volume and oxygenation could be useful for clinical and research applications.