Characteristics of laser Doppler perfusion imaging in vitro and in vivo

Traditional laser Doppler flowmetry (LDF) employs continuous recording of perfusion at one point with time. In order to eliminate the large spatial and temporal fluctuations that occur in the microcirculation, laser Doppler perfusion imaging (LDI) integrates flow readings over a large area. This paper describes a number of experiments to identify some of the characteristics of the LDI, its relationship to flow and no-flow conditions and to compare it with LDF. We undertook experiments to establish the effect of scanner head height, avascular skin thickness and haematocrit on LDI output. We also investigated the contribution of the biological zero signal (the signal obtained from skin when flow is arrested) to the LDI output. LDI output increased with scanner height in vitro and in vivo. Increasing avascular skin thickness reduced the LDI output although linear output characteristics with flow were maintained over the flow range studied. Increasing the haematocrit resulted in a loss of linearity of output with flow at lower velocities. The biological zero signal contributes a similar proportion of the output signal in LDF and LDI. We have presented a series of experiments that will contribute to the understanding of the characteristics of laser Doppler perfusion imaging, its comparison to laser Doppler flowmetry and its relationship to flow and no flow situations. However, our experiments were restricted to one machine, and may not necessarily be applicable to other instruments.