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.