The readings in laser Doppler perfusion monitoring are affected by the optical properties of the tissue in which the
microvasculature is embedded, through their effect on the optical path lengths. Thus for a constant perfusion, the LDF
output signal is affected by the variance in individual photon path lengths due to the changes in tissue optical properties
and probe geometry. We will present efforts to render blood flow measurements independent of the tissue optical
properties by using low coherence interferometry. We will give evidence of the improvement in quantification of our
approach. In particular we show that low coherence interferometry can measure dynamic properties of particles in
Brownian motion, independent of optical properties of the surrounding tissue matrices. Furthermore, demonstration is
given of the applicability of the method in vivo.
The influence of tissue optical properties on laser Doppler perfusion imaging (LDPI) is not well understood. We address this problem by quantifying the dependence of the signal response to tissue optical properties based on speckles or coherence areas and on photon statistics. We investigate the effect in vivo, showing the amplitude of photocurrent fluctuations in normal skin and port-wine stain with a range of beam diameters, and its relation to the speckle size variation difference between these two tissues. For the case of a low concentration of moving particles moving within a static turbid medium, a model is described and applied to predict the influence of speckles on the overall and depth sensitivity of LDPI, for a range of scattering levels and absorption levels. The results show that the speckle related effects on overall and depth sensitivity are large and that the depth sensitivity is highly likely to be misinterpreted without taking the speckle phenomenon into account.
We report the first path-length-resolved perfusion measurements on human skin measured with a phase-modulated low-coherence Mach-Zehnder interferometer with spatially separated fibers for illumination and detection. Optical path lengths of Doppler shifted and unshifted light and path-length-dependent Doppler broadening of multiply scattered light from skin are measured from the Doppler broadened interference peaks appearing in the power spectrum. Perfusion and its variations during occlusion are measured in real time for a given optical path length, and the results are compared with the perfusion signal obtained with a conventional laser Doppler perfusion monitor.
In optical Doppler measurements, the path length of the light is unknown. To facilitate quantitative measurements, we develop a phase-modulated Mach-Zehnder interferometer with separate fibers for illumination and detection. With this setup, path-length-resolved dynamic light scattering measurements of multiple scattered light in static and dynamic turbid media are performed. Optical path length distributions spanning a range from 0 to 11 mm are measured from the area under the phase modulation peak around the modulation frequency in the power spectrum. A Doppler-broadened phase modulation interference peak is observed that shows an increase in the average Doppler shift with optical path length, independent of absorption. Validation of the estimated path length distributions is done by measuring their deformation for increasing absorption and comparing these observations with predictions based on Lambert-Beer's law.
We describe path length resolved Doppler measurements of the multiply scattered light in turbid media using phase
modulated low coherence Mach-Zehnder interferometer, with separate fibers for illumination and detection. A Doppler
broadened phase modulation interference peak observed at the modulation frequency shows an increase in the average
Doppler shift with optical path length. The path length dependent Doppler broadening of scattered light due to the
detection of multiple scattered light is measured from the Lorentzian linewidth and the results are compared with the
predictions of Diffusive Wave Spectroscopy. For particles with small scattering anisotropy, the diffusion approximation
shows good agreement with our experimental results. For anisotropic scatterers, the experimental results show deviations
from the Diffusion theory. The optical path lengths are determined experimentally from the Zero order moment of the
phase modulation peak around the modulation frequency and the results are validated with the Monte Carlo technique.
We report on a theoretical/experimental model to predict the depth sensitivity of laser Doppler perfusion imager. Further
more we show the quantitative influence of speckles on laser Doppler perfusion imager response to scattering at different
depths. The model is based on Monte Carlo simulations and experiments on static and dynamic scattering phantoms
made of polystyrene microspehers. The experimental results are in good agreement with our theoretical predictions. The
results show that the depth sensitivity of the laser Doppler perfusion imager is influenced by the speckles. The effects are
big, especially when a narrow beam is used for measurement. We propose that a correction method should be developed
based on speckle size in order to have a reliable blood perfusion information independent of tissue optical properties.
We show that speckles play an important role in laser Doppler perfusion imaging. The influence of speckles on the signal amplitude and the Doppler spectrum is demonstrated experimentally on particle suspensions with different scattering levels and varying beam width. Polystyrene microsphere suspensions with known optical properties are used to make scattering samples. A theoretical model is explained to calculate the speckle size from the back scattered intensity distributions. The coherence area is calculated with Monte Carlo simulations on different scattering samples and experiments are performed to validate our theoretical model. The experimental results are in good agreement with our theoretical predictions.
We report the development of non-invasive, path length resolved Doppler measurements of the multiply scattered light in turbid media, for different absorptions using phase modulated Mach-Zehnder low coherence interferometer, with separate fibers for illumination and detection. A Doppler broadened phase modulation interference peak is observed that shows an increase in the average Doppler shift with optical path length, independent of absorption. The estimated path length distributions indicate suppression and narrowing for increasing absorption and can be related by Lambert-Beer's law.
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