Quantitation of Pathlength in Optical Spectroscopy

The relative transparency of tissues to near infrared light means that it is possible to transilluminate intact organs. In the infrared region, oxygen dependent absorptions due to haemoglobin and cytochrome aa3 can be observed, and it is therefore possible to monitor changes in both the blood and tissue oxygenation of the organ.1 This monitoring technique is particularly applicable to the study of the brain since there is no interfering absorption from myoglobin, and recent technical developments of the instrumentation have made it possible to transilluminate 8–9 cm of brain tissue.2 However, once measurements of absorption change at several wavelengths are available, there are still considerable problems in converting this data into quantitative changes in the concentration of oxy and deoxy haemoglobin and of oxidised cytochrome aa3.