H.W. Jentink

Small laser Doppler velocimeter based on the self-mixing effect in a diode laser

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.

Monte Carlo simulations of laser Doppler blood flow measurements in tissue.

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.

Light propagation parameters for anisotropically scattering media based on a rigorous solution of the transport equation

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.

Skin blood flow changes, measured by laser Doppler flowmetry, in the first week after birth

Changes 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 +/- 7.4 h, 58.9 +/- 6.2 h and 121.5 +/- 14.2 h (mean +/- S.D.), 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 = 0.01), 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 = 0.01), 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 second measurements and between the second and third measurements did not reach the level of statistical significance.

Forehead Skin Blood Flow in Normal Neonates during Active and Quiet Sleep, Measured with a Diode Laser Doppler Instrument

ABSTRACT. Changes in forehead skin blood flow during active and quiet sleep were determined in 16 healthy neonates using a recently developed semi‐conductor laser Doppler flow meter without light conducting fibres. Measurements were carried out at a postnatal age varying from 5 hours to 7 days. The two sleep states could be distinguished in 17 recordings. The mean skin blood flow values during active sleep were significantly higher (p<0.01) than those during quiet sleep, the mean increase being 28.1%. The variability of the flow signal, expressed as the coefficient of variation, changed significantly from 23.1% during active sleep to 18.2% during quiet sleep.

Signal processing for a laser-Doppler blood perfusion meter

Two signal processing methods for laser-Dopper perfusion velocimetry are presented. The methods are based on the calculation of the moments of the frequency power spectrum. The first uses Vω-filtering (ω is the frequency) with analogous electronics, the second uses signal autocorrelation with digital electronics. Comparison is made with a third instrument: a spectrum analyzer coupled to a computer, using Fourier transform tecniques. The performance of these setups (sensitivity, limit sensitivity and accuracy) are investigated. We propose a calibration standard for signal processors to be used for blood perfusion measurements. The analogous instrument proved to be the cheapest but the digital instrument had the best performance.

Postocclusive Reactive Hyperaemia of Cutaneous Blood Flow in Premature Newborn Infants

Beaufort‐Krol, G. C. M., Suichies, H. E., Aarnoudse, J. G., Okken, A., Jentink, H. W. and Greve, J. (Departments of Obstetrics and Paediatrics, University Hospital, Groningen, and Department of Applied Physics, Enschede, The Netherlands). Postocclusive reactive hyperaemia of cutaneous blood flow in premature newborn infants. Acta Paediatr Scand Suppl 360: 20, 1989.

Digital Blood Flow in Cooled and Contralateral Finger in Patients with Raynaud's Phenomenon. Comparative Measurements Between Photoelectrical Plethysmography and Laser Doppler Flowmetry

The effects of cooling of a hand on lateral and contralateral digital skin blood flow were investigated in 18 patients with primary or secondary Raynauds phenomenon. The aim of the study was to compare photoelectrical plethsmog raphy (PhEP) and laser Doppler flowmetry (LDF). PhEP and LDF were used simultaneously for skin blood flow measurements of the third finger of both hands. One hand was cooled in water from 33 ° to 3 ° C in steps of 3 ° C, each step lasting four minutes. It was followed by a recovery period of ten minutes in room air of 24 ° C. During stepwise cooling from 33 ° to 9 ° C the relative PhEP and LDF values, measured on the cooled hand, decreased to 6.2% ± 3.2% and 10% ± 12% respectively. The correlation coefficients between LDF and PhEP varied between 0.79 and 0.99. In the contralateral hand the relative PhEP and LDF values decreased to 38% ± 30.% and 64% ± 7.9% respec tively. The correlation coefficients between LDF and PhEP values were lower on the contralateral hand and ranged from 0.26 to 0.95. By calculating the LDF/PhEP ratios during cooling and recovery, more specific changes in red blood cell velocities during cooling were studied. Increasing LDF/PhEP ratios suggest increasing red blood cell velocities during cooling at 9 ° C and a difference in the measuring methods. For testing the severity of Raynauds phenomenon and the effect of treatment the results of both methods show consistent and quite comparable results when measured on the cooled hand. The effect of indirect cooling on the contralateral hand is, however, less consistent.

Laser Doppler flowmetry: measurements in a layered perfusion model and Monte Carlo simulations of measurements.

Fluid flow in a flow model was measured with a laser Doppler flowmeter normally used for tissue perfusion measurements. The flow parameter is deduced from the first moment of the spectral power density of intensity fluctuations at a detector viewing the model, which is illuminated by a laser. The model consisted of layers, filled with liquid, containing scattering particles, at different distances to the optics of the flowmeter. To each of the layers a flow can be applied. The configuration of the optical components could be varied, which resulted in an adjustable relative sensitivity for the flow in the different layers. Here, the feasibility of depth-dependent flowmetry with the laser Doppler flowmeter is shown experimentally. The measurements in the flow model were simulated with Monte Carlo calculations. The results indicate that these calculations may be of help to interpret experimentally obtained flow results.

A compact differential laser Doppler velocimeter using a semiconductor laser

A small differential laser Doppler velocimeter which uses a semiconductor laser and a small number of optical components is described. In this device the light from the laser diode is split into coherent beams by means of a diffraction grating. The two first-order beams are crossed in a probe volume with a lens. In a test experiment the velocity of water containing 0.9 mu m polystyrene spheres was determined from the fluctuation of the scattered light. This velocity agreed with the velocity calculated from flux measurements.