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Tissue Perfusion Measurements Using A Mini Diode Laser Doppler Perfusion Sensor

A perfusion sensor was constructed by integrating a diode laser, two photodiodes and two current-voltage convertors in a small (diameter 17 mm and height 13 mm) and light (6 grams) probe. The perfusion is determined from intensity fluctuations on the detectors. These are induced by interference of light scattered in tissue which is partly frequency shifted by scattering at moving blood cells. This causes via heterodyne detection intensity fluctuations with frequencies equal to the frequency shift due to the Doppler effect. In a signal processor, which is connected with flexible electrical leads to the probe, the first moment and the weighted first moment of the spectral density of the intensity on the photodiodes are calculated. These signals are proportional to respectively the perfusion and the mean velocity of blood cells. In order to determine the measurement volume of the probe a Monte Carlo simulation program for the measurements was developed. The simulations show that the depth and the width of the probe volume is proportional with the distance between the diode laser and the photodiodes. Absorption of tissue decreases the depth of the probe volume. With these calculations we have demonstrated to be able to distinguish between blood flow at different depths in tissue, which is important for skin perfusion measurements in order to distinguish capillary blood flow from flow in the plexus beneath.