Sampling depth in Laser Doppler flowmetry

Abstract: The influence of probe design on the measuring depth in laser Doppler flowmetry using a 632.8 nm light source has been investigated by in vivo models and Monte Carlo simulations. Five different probe configurations were used to measure perfusion in feline intestine to which a constant blood flow was applied. The results indicated an increase in measuring depth when the transmitting and receiving fibres in the probes were separated. To further investigate the discrepancy in measuring depth between different probes, a computer model of light scattering in tissue was developed. By using the computer it was possible to simulate different probe configurations applied to tissue heds with well-defined optical properties. All probes were based on fibres with a diameter of 120 μm. Simulating homogeneous skin tissue gave an increase in sampling depth from about 117 μm to 233 μm, when the centre distance between the transmitting and the receiving fibre was changed from 125 μm to 700 μm. Keeping the centre-separation of the fibres at 250 nm, resulted in estimated median sampling depth of 100 μm, 146 μm and 537 μm for liver, skin and brain tissue respectively. The median measuring depth showed a strong dependence on the spatial perfusion distribution in the tissue.A series of measurements where living skin served as a scattering medium were carried out. A glass capillary was inserted between the skin of the forearm and the underlying tissue. The capillary was perfused with blood of different red blood cell concentrations at constant velocity and the flow through the capillary was detected, through the skin, with a laser Doppler flowmeter. The results were in accordance with the results of the Monte Carlo simulations and demonstrated how the LDF-signal decreased with increasing skin thickness for different probe configurations. It also demonstrated how a single vessel at a depth of ten times the median sampling depth can give a substantial contribution to the laser Doppler output signal.