Diffusion theory is an approximation ofthe equation of radiative transport, that is used to describe light propagation in turbid media. This approximation is very popular because ofits simplicity, possibilities to describe time-resolved light propagation, and for its appeal to physical intuition. However, it has also its restrictions. It is the aim ofthis contribution to discuss this method, and to evaluate what can be undertaken to avoid the deviations caused by its restrictions, based on results obtained with the equation of radiative transport.
A setup to measure skin autofluorescence was developed to assess accumulation of advanced glycation endproducts (AGE) in patients noninvasively. The method applies direct blacklight tube illumination of the skin of the lower arm, and spectrometry. The setup displays skin autofluorescence (AF) as a ratio of mean intensities detected from the skin between 420-600 nm and 300-420 nm, respectively. In an early clinical application in 46 and control subjects matched for age and gender, AF was significantly increased in the patients (p = 0.015), and highly correlated with skin AGE's that were determined from skin biopsies in both groups. A large follow-up study on type 2 diabetes mellitus, ongoing since 2001 with more than 1000 subjects, aims to assess the value of the instrument in predicting chronic complications of diabetes. At baseline, a relation with age, glycemic status and with complications present was found. In a study in patients with end stage renal disease on dialysis AF was a strong and independent predictor of total and cardiovascular mortality. A commercial version of this AGE-reader is now under development and becomes available early 2005 (DiagnOptics B.V., Groningen, The Netherlands).
One of the remaining questions, that will be answered by measuring so-called Exciation-Emission Matrices (EEM's) of the skin tissue in vivo, is whether a more selective choice of wavelengths is more strongly related to clinical characteristics. An experimental instrument to measure these EEM's was, therefore, developed as well. Clinical measurements are underway of EEM's in patient groups with diabetes mellitus and in healthy volunteers.
In pulse oximetry, the red and infrared intensity fluctuations are often assumed to have the same form. However, we observed strong phase delays between these signals during measurements on the forehead of some of our subjects when no pressure was applied onto the probe, which practically disappeared when pressure was applied on the probe. The signals obtained at different distances from the light source were transformed into fluctuations in the absorption and reduced scattering coefficient by means of results from Monte Carlo simulations. The changes in the reduced scattering coefficient appeared to be inverted when no pressure was applied onto the probe. Although the calculated relation between the red and infrared fluctuations in the absorption coefficient was sometimes free of hysteresis, the ratio between the fluctuations still depend on pressure on the probe, and on the chosen optical properties of the medium.
Laser Doppler flowmetry (LDF) is a method that can be used for measuring blood flow changes in the microcirculation. We have contributed to the development of a new device for LDF, based on digital signal processing. A method for correcting the disregarding of frequency components was developed, by approximating the noise-free Doppler spectrum with an exponential shape. The frequency components from 40 kHz to 50 kHz can be used to correct for white noise. We introduced variable resistors for the case common mode components from both detectors have different magnitudes. However, after adjustment we found that noise may still be present. We have observed, that cutting off at 150 Hz suppresses many noise contributions and still provides sufficient Doppler information. For the transfer of a moment calculated from 150 Hz - 20 kHz into 0 - infinity Hz, the correction method mentioned above can be applied.
A tissue-optical model is presented in which changes in the blood volume fraction, f<SUB>v</SUB>, and tissue saturation, SO<SUB>2</SUB>, are calculated from non-invasively measured intensity changes at two wavelengths during ischemia. Measurements were performed during occlusion and during muscle contraction at the human forearm with a sensor containing two LEDs, (lambda) equals 660 nm and (lambda) equals 940 nm, and photodiodes at 7.0 mm, 9.5 mm, and 20 mm from the LEDs. We used diffusion theory for a homogeneous semi-infinite medium to obtain registrations of (Delta) f<SUB>v</SUB> and (Delta) SO<SUB>2</SUB> from measured changes in the photon fraction, (Delta) I, during the experiment for each detector separately. As expected, f<SUB>v</SUB> stays nearly constant during occlusion, whereas SO<SUB>2</SUB> decreases, for each detector. During muscle contraction we observed that the intensity changes at each detector are much smaller than during occlusion. As expected, both f<SUB>v</SUB> and SO<SUB>2</SUB> decrease at the beginning of the contraction period, but increase before the end of the contraction period. (Delta) SO<SUB>2</SUB> depends more strongly than (Delta) f<SUB>v</SUB> on the assumed myoglobin concentration, the scattering coefficients, the blood volume fraction and the saturation at the beginning of the session. The success of the homogeneous model in the occlusion experiment is probably caused by simultaneous deoxygenation in the muscle and in the skin. However, during muscle contraction the changes in SO<SUB>2</SUB> and f<SUB>v</SUB> were different at each detector. The failure of the homogeneous model in that case may be explained by the deoxygenation which is expected to be larger in muscle tissue than in skin tissue.
In order to study the behavior of laser-Doppler based tissue blood perfusion meters an experimental flow model has been developed consisting of a set of layers with dispersed scatterers and/or absorbing material which are moveable with respect to each other and to the laser-Doppler probe. As the material for the layers gelatin was used, and for the scatterers polystyrene spheres were chosen. Light (from a diode laser) scattering in the sample was measured in reflection using a photodiode array. The intensity and the Doppler spectrum were recorded as a function of the source-detector distance and the angle of laser light incidence. For comparison a number of Monte Carlo simulations of the dynamic light scattering in the sample were performed. The simulations included data regarding the Doppler spectrum, the number of scatter events, paths lengths, positions and angles of emergence and penetration depths. It is seen that in the heterodyne detection case good agreement between measurements and simulations is obtained, while in the homodyne the simulations have to be downscaled in frequency (factor 3). This may be caused by coherence effects due to the finite aperture of the detector. In the simulations the averaged Doppler frequency and averaged absolute Doppler frequency turn out to be quadratic and linear dependent on the numerical aperture. This effect was verified with an independent calculation.
In reflectance pulse oximetry the ratio R/IR between the red and infrared intensity fluctuations, as measured at the skin surface, is used to estimate the arterial oxygen saturation. This ratio is influenced by light propagation in tissue, as measurements at several distances between light sources and detectors simultaneously show that R/IR depends on this distance. In the present study the influence of the estimated tissue properties on R/IR and its distance dependence are investigated by means of Monte Carlo simulations, a method to vary the optical properties without the need for a new Monte Carlo simulation. A three wavelength model has been introduced, because of secondary emission of the red LED. The influence of water absorption has been taken into account. The simulation results depend on the chosen optical properties. Results of R/IR for S<SUB>a</SUB>O<SUB>2</SUB> equals 98% with properties from in vivo experiments agree much better with the measured values than the predictions based on in vitro data available in literature. The results show that the condensed Monte Carlo simulation is a valuable tool to gain insight in the principles of reflectance pulse oximetry: The model, assuming a homogeneous distribution of pulsations, is able to describe the experimental results for pulse sizes, R/IR and its distance dependence very well.
In order to investigate the applicability of Monte-Carlo simulations for (Doppler) light scattering in tissue, two upscaled experimental models were constructed. The models consisted of thin layers, either water or gelatin, with scatterers, which can be moved relative to each other. Measurements and simulations of the scattered intensity and the Doppler frequency moments are in rather good agreement.
A velocimeter, consisting of a semiconductor laser, coupled to a glass fiber, to be inserted in the flow, and applying self-mixing as the detection technique, is described. A special application is the measurement of blood velocity in veins and arteries. Technical aspects, including flow profile calculations and measurements, and in-vivo and in-vitro velocity measurements are described and discussed.
A novel velocimeter for the measurement of blood velocity in veins and arteries is described. It consists of a semiconductor laser, coupled to a glass fiber, to be inserted in the flow, and applying self-mixing as the detection technique. Theoretical aspects and in-vivo and in-vitro measurements are described and discussed.
The validity of the similarity parameter ∑<sup>'</sup><sub><i>s</i></sub> ≡ ∑<sub><i>s</i></sub>(1 - <i>g</i>), the reduced scattering coefficient, where <i>g</i> is the average cosine of the scattering phase function is investigated. Attenuation coefficients α and diffusion patterns are obtained from solutions of the transport equation for isotropic scattering and Rayleigh-Gans scattering, applied to infinite media. Similarity is studied for the attenuation coefficient α, as well as for the Kubelka-Munk absorption and backscattering coefficients in the positive and negative directions, and for predictions of the internal reflection at interfaces. Similarity between solutions of the Boltzmann equation for highly forward scattering and isotropic scattering (<i>g</i> = 0) exist only when ∑<sub><i>a</i></sub> << ∑<sub><i>s</i></sub>(l - g). However, because similarity between results, both with g > 0.9, is independent of the value of the absorption coefficient, it is advantageous to simulate highly forward scattering media like biological tissues with g > 0.9, e.g., by Monte Carlo simulations, instead of using isotropic scattering or diffusion theory. Monte Carlo simulations on slabs confirm the deviations from the diffusion approximation and show the behavior near boundaries. Application of similarity may save calculation time in Monte Carlo simulations, because simulation with a lower value for g will increase the mean free path.
A laser Doppler velocimeter has been developed to measure blood flow velocity in vivo. It consists of a semiconductor laser coupled to a fiber. Laser light is guided into a blood vessel and backscattered light (by red blood cells) is guided back into the laser. The backscattered Doppler shifted light produces an intensity modulation of the laser (self-mixing effect). The beat-frequency of the intensity modulation is related to the Doppler shift of the backscattered light. A model is presented to calculate modulation signals, and results of measurements in vitro and in vivo are shown.
Laser Doppler velocimetry provides a method for non invasive measurements of the perfusion of tissue. Therefore the tissue is illuminated with a monochromatic light source and back scattered light from the tissue is collected at a detector at an adjacent site. Some of the back scattered photons have had interaction with moving red blood cells and are frequency shifted. Due to interference of frequency shifted and non-frequency shifted photons the intensity at the detector fluctuates. These fluctuations provide the information from which a rate for the perfusion can be derived. In this paper we present perfusion measurements and Monte Carlo (MC) simulations on both a scale model and human skin tissue. The Monte Carlo results are used to quantify the size and position of the probe volume. Three different ways are presented to vary the size and position of the probe volume.
Laser Doppler velocimetry provides a method for non invasive measurements of the
perfusion of tissue. Therefore the tissue is illuminated with a monochromatic light
source and back scattered light from the tissue is collected at a detector at an
adjacent site. Some of the back scattered photons have had interaction with moving
red blood cells and are frequency shifted. Due to interference of frequency shifted
and non-frequency shifted photons, the intensity at the detector fluctuates. These
fluctuations provide the information from which a rate for the perfusion can be
derived. ' Recently, the possibility of performing depth dependent measurements on
the skin by3 4vrying the position an size of the sample volume has been
investigated. In this paper we demonstrate that there are at least three
different ways to vary the position and size of the probe volume:
1) Using a different wavelength of the laser.
2) Varying the distance from the laser to the detector.
3) Varying the angle of penetration of the laser light into the tissue.