Laser Doppler perfusion imaging : methodology and skin applications

Abstract: Over the last decade a number of emerging technologies have become available for the non-invasive study of skin anatomy and physiology. In this thesis, a method for mapping tissue perfusion based on the laser Doppler technique is presented together with some of the associated experimental and clinical applications.The laser Doppler perfusion imager is a novel computer-controlled system for two-dimensional mapping of tissue blood flow that uses a low power He-Ne laser beam to sequentially scan a tissue surface. At each measurement site the beam illuminates the tissue volume to a depth of a few hundred micrometers. In the presence of moving blood cells, partially Doppler-broadened backscattered light is detected by a photodetector and processed to form a value scaled linearly to perfusion. A full format image includes 64x64 measurement sites corresponding to a 12 cm x 12 cm area and is captured in 4.5 minutes. When all measurement values have been recorded, processed and stored, a color-coded perfusion image showing the spatial heterogeneity of the tissue perfusion is displayed on a monitor. In addition, data analysis functions such as statistics, profile generation and image subtraction can be performed. A theory describing the influence on the output signal by the distance between the object and the detector as well as the incident angle is presented. Changes in distance and incident angle cause a variation in'the number of coherence areas on the detector surface and thereby an alteration in the system amplification factor. To correct for the distance dependence, the laser beam is made slightly divergent by the use of a lens system. The angular dependence is corrected for in the software. By the use of a flow simulator, a linear relationship between the processor output signal and red blood cells up to a concentration of 0.2% was confirmed. The spatial and temporal system-related noise was found to be less than 0.3% and 0.5% of the maximum value, respectively.Spatial and temporal variations in normal forearm skin perfusion were investigated with laser Doppler perfusion imaging and a comparison with topographic laser Doppler flowmetry mapping was performed revealing substantial heterogeneity in normal skin blood flow. By the use of electrical nerve stimulation the extent, intensity and time course of the cutaneous axon reflex response was investigated. Changes in skin circulation after microdialysis probe insertion visualized the hyperperfusion induced both at the point of insertion and at the probe tip. The perfusion in port wine stains before and after argon laser treatment was studied and compared to the clinical results.