The development of biomedical instrumentation using backscattered laser light
This thesis is concerned with the measurement of blood flow and oxygen saturation in the microcirculation using the techniques of laser Doppler flowmetry and pulse oximetry. An investigation of the responses of Doppler flowmeters using different signal processing bandwidths and laser sources revealed two major findings. Firstly, that careful choice of processing bandwidth is required in order to sample the whole range of possible Doppler frequencies present in the backscattered light. Secondly, that the choice of laser source is important in governing the output stability of a flowmeter. Another investigation focused on the evaluation of a dual channel laser Doppler flowmeter using both in vitro and in vivo models. It was demonstrated that the instrument permitted a useful method of obtaining flow information by comparing simultaneous responses at experimental and control sites. The choice of laser wavelength was investigated in a study to determine whether blood flow measurements are obtained from different depths within the skin tissue. The results indicate that some depth discrimination is obtainable using instruments operating at different wavelengths, however it is difficult to demonstrate the effect in vivo. In a separate study it was shown that pressure applied to the skin surface greatly affects the underlying blood flow. It is recommended that care has to be taken when positioning Doppler probes on the skin. A reflection pulse oximeter was developed using laser light backscattered from the skin. The instrument was evaluated in vitro and in vivo by comparing desaturation responses with a commercial transmission pulse oximeter. The reflection oximeter was demonstrated to reliably follow trends in oxygen saturation but several problems prevented instrument calibration. Finally, a device combining laser Doppler flowmetry with reflection pulse oximetry was developed and used in vivo to follow trends in blood flow and oxygen saturation from the same tissue sample.