Laser Doppler Flowmetry

The aim of this thesis was to develop laser Doppler instrumentation capable of distinguishing nutritional and sub-capillary blood flow in human skin, by using 543 nm and 633 nm laser light. Existing theoretical models were reviewed and a revised model proposed which fully accounted for non correlated, multiple Doppler shifting events and the effect of red blood cell absorption. Signal processing techniques proposed for laser Doppler instrumentation were reviewed and a modified technique proposed. The applicability of theoretical models and of the signal processing techniques with both red and green laser wavelengths, was investigated using three in vitro models perfused with fresh human blood. The revised theoretical model was found to better describe the trends observed in the power spectra recorded from in vitro models compared to previously proposed models. A comprehensive comparison of the signal processing techniques from all the in vitro models showed that a modified frequency squared weighted processing technique out performed the commercially employed signal processing technique. It was found that the usual, integrating signal processing techniques could not be employed with the green laser in order to obtain a signal proportional to blood flow. Examination of the theoretical model showed that this was the combined effect of multiple, Doppler shifting scattering and photon absorption by red blood cells. The effect is a fundamental limitation to the use of strongly absorbed green laser light in the determination of nutritional skin blood flow.