Light propagation in a model for blood perfusion in tissue was simulated with Monte Carlo calculations to investigate the dependence of the output of laser Doppler perfusion meters on the configuration of the optical probe and on the multiple scattering of photons by moving particles in the tissue. Laser Doppler perfusion meters registrating the first moment ?v? and the first weighted moment ?v?(s) of the spectral power density S(v) of intensity fluctuations on a detector viewing tissue illuminated by a laser are considered. The model was scaled up about a factor of 10 compared with real tissue, to make experimental tests possible. From the simulations of the Doppler scattering, it will be shown that the location of the effective probe volume of the perfusion meter can be extended to deeper layers in tissue by increasing the distance between the illuminating light beam and the detector. This opens the possibility to measure perfusion in skin layers as a function of the distance to the surface. Other calculations show how the degree of multiple scattering of individual photons by moving cells determines which flow parameter is measured with the perfusion meter. If the degree is low, the output of the meter depends linearly on the mean velocity of cells. For high degrees, a dependence on the root mean square value of this distribution is found. At a high moving particle concentration, multiple scattering by moving particles also results in deviations from the linear dependence of ?v? on the concentration of moving particles and in deviations from the concentration independence of ?v?(s). Intensity distributions of light inside the tissue model were obtained from the simulations.
New expressions are presented for light propagation in media for the whole range of absorption and for isotropic as well as for anisotropic scattering with an average cosine of the scattering angle between 0 and 0.9995. The method is based on the rigorous solution of the transport equation for Rayleigh-Gans scattering. The calculated angular intensity distribution was used to determine the absorption parameter K. Expressions for K and the backscattering parameter S are given that can be used to improve existing photon diffusion and two- or four-flux models.
A very small and simple velocimeter is presented consisting of a diode laser with a gradient-index lens in front of it. The basis of the velocity measurement is the mixing that occurs when light, scattered back by the moving object into the laser cavity, interferes with light inside the laser. This mixing induces large fluctuations of the laser intensity with the Doppler frequency. These fluctuations can be detected either with a photodiode or by measuring the voltage across the diode laser. As an illustration of the performance of the velocimeter, velocity measurements of a rotating disk covered with white paper are described. The differences arising because of using a single-mode or a multilongitudinal mode laser were calculated and verified in experiments. The advantage of the use of a multimode laser is that differential measurements of the distance between laser and moving object are also possible.
SummaryChanges in forehead skin blood flow were determined in 17 healthy, term newborns, using a fiberless diode laser Doppler flow meter (Diodopp). Measurements were carried out three times on each infant, at postnatal ages of 16.8 f 7.4 h, 58.9 f 6.2 h and 121.5 -C 14.2 h (mean + SD.), respectively. Skin blood flow, respiration, heart rate and skin temperature were recorded simultaneously, while the newborns were asleep. During the recordings, the behavioural state of the newborns was observed and environmental temperature and humidity were kept constant. Postocclusive hyperaemia of the skin blood flow was obtained by pressing the laser Doppler probe against the skull for 30 or 60 s. The following parameters changed significantly between the first and third measurements (t-test for paired samples): the basal skin blood flow during active and quiet sleep decreased, the average decrease being 29.4% (P = 0.002) and 25.9% (P = O.Ol), respectively; skin blood flow during postocclusive hyperaemia also changed: the time taken to reach maximum hyperaemia increased from 17.3 to 22.7 s (P = O.Ol), while the halftime recovery increased from 46.1 to 57.1 s (P = 0.02). The changes in skin blood flow between the first and gecond measurements and between the second and third measurements did not reach the level of statistical significance. skin blood flow; laser Doppler; reactive hyperaemia; newborn.Correspondence to: J.G Aarnoudse. 0378-3782/90/%03.50 0
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