Haim Taitelbaum

Photon migration in layered media

Surface emission profiles and related functions are computed for particles (photons) migrating within a semiinfinite medium containing a surface layer whose absorbance differs from that of the underlying layer. Photons are assumed to be inserted at a single point on the surface. In certain cases distinct features appear in the emission profiles which enable determination of the thickness of the top layer and of the absorption coefficients of both layers. Computations are performed to provide estimates of parameter ranges for which the presence of one layer distorts photon emission profiles from the other. Several ancillary functions are calculated, including the absorbance profile as a function of depth, the expected path length of photons that are reemitted at a distance rho from the point of insertion, and the average depth probed by those reemitted photons.

Measurement of oxygen saturation in venous blood by dynamic near infrared spectroscopy

A method for the measurement of oxygen saturation in the venous blood, SvO2, based on optical measurements of light absorption in the infrared region is presented. The method consists of applying relatively low external pressure of 25 mm Hg on the forearm, thereby increasing the venous blood volume in the tissue, and comparing the light absorption before and after the external pressure application. SvO2 has been determined from light absorption measurements in two wavelengths, before and after the pressure application, using a formula derived for two adjacent wavelengths. The method has been applied to the hands and fingers of 17 healthy male subjects, using wavelengths of 767 and 811 nm. SaO2, the oxygen saturation for arterial blood, was also obtained from photoplethysmographic measurements in these two wavelengths (pulse oximetry) using the same formula. The mean (+/- SD) value of SaO2 was 94.5% (+/- 3.0). The mean value of SvO2 was 86.2% (+/- 4.1) for the finger and 80.0% (+/- 8.2) for the hand. These SvO2 values are reasonable for the finger and the hand where arterio-venous anastomoses exist. The method enables the measurement of SvO2 in the limbs, a parameter which is related to tissue blood flow and oxygen consumption.

Some Properties of the A + B C Reaction-Diffusion System with Initially Separated Components

We study some properties of the A+B→C reaction-diffusion system with initially separated components, first analyzed by means of an asymptotic scaling argument by Gàlfi and Ràcz. We show that, in contrast to the asymptotic result that predicts that the rate of production of C goes liket−1, at early times it is shown to increase ast1/2. Deviations from this behavior appear at times inversely proportional to the reaction constant. Analogous crossover properties appear in the kinetic behavior of the reaction front. A second part of the study is concerned with the same chemical reaction on a fractal surface. When the substrate is a percolation cluster at criticality, both the maximum production rate and the width of the reaction zone differ considerably from those for the homogeneous space.

Reflected light intensity profile of two-layer tissues: phantom experiments

Experimental measurements of the reflected light intensity from two-layer phantoms are presented. We report, for the first time, an experimental observation of a typical reflected light intensity behavior for the two-layer structure characterized by two different slopes in the reflected light profile of the irradiated tissue. The point in which the first slope changes to the second slope, named as the crossover point, depends on the upper layer thickness as well as on the ratio between the absorption coefficients of the two layers. Since similar experiments from one-layer phantoms present a monotonic decay behavior, the existence and the location of the crossover point can be used as a diagnostic fingerprint for two-layer tissue structures. This pertains to two layers with greater absorptivity in the upper layer, which is the typical biological case in tissues like skin.

Estimation of the Optimal Wavelengths for Laser-Induced Wound Healing

According to earlier in vitro low level laser therapy (LLLT) studies, wavelengths in the red and near infrared range, that are absorbed by cytochrome oxidase, stimulate cell growth and hence wound healing. Wavelengths in the blue region that are absorbed by flavins were found to exert a bactericidal effect that is very important for treating infected wounds. However, as far as therapeutic application of light is concerned, penetration into the tissue must be considered. For this purpose we estimated the penetration depth as a function of the relevant wavelengths, using the formulae of the photon migration model for skin tissue.

Detection of inhomogeneities with ultrasound tagging of light

Ultrasound modulated light for optical tomography is very useful, since it can provide three-dimensional data with minimal mathematical processing. Although several experimental studies have shown the potential of this method, the link between the ultrasound location and the modulated signal intensity at the detector is not yet fully understood. We derive an analytical formula relating the position of the ultrasound transducer and the optical signal at the detector. We also derive an expression for the signal-to-shot-noise ratio as a function of the transducer position. We show that in certain conditions this ratio is only slowly decreasing as a function of the light penetration depth, which makes this technique attractive for optical tomography.

The measurement of oxygen saturation in arterial and venous blood

This article describes about the non-invasive optical measurement techniques to assess oxygen saturation in arterial and venous blood. The pulse oximeters for arterial oxygen saturation measurement use light in two wavelengths, in the red and infrared regions, and calibration is used to account for the difference in optical path-length between them. The accuracy of the commercial pulse oximeters is adequate for monitoring patients during surgical operation, where the clinical information that is needed is the absence of dramatic deterioration in the respiration efficiency. However, the error in pulse oximetry is too high for the clinical assessment of lung function, probably because of the need for calibration in the available technique. The use of two wavelengths in the infrared, instead of one wavelength in the red and one in the infrared region, enables the use of pulse oximetry without calibration. The technique can also be used for determining venous blood saturation, which is important for assessment of the adequacy of tissue blood supply.

Reactive wetting in metal–metal systems

Wetting and spreading in high temperature reactive metal-metal systems is of significant importance in many joining processes. An overview of reactive wetting is presented outlining the principal differences between inert and reactive wetting. New experimental evidence is presented that identifies an early time regime in reactive wetting in which spreading occurs without macroscopic morphological change of the solid-liquid interface. This regime precedes the heavily studied reactive wetting regime. Additional new experimental evidence is presented of kinetic roughening in a high temperature reactive system. Quantitative characterization of this roughening reveals similarities with room temperature systems. These new data provide evidence that supports the existence of several sequential time regimes in the reactive wetting process in which different physicochemical phenomena are dominant.

On Phantom Experiments of the Photon Migration Model in Tissues

Light-tissue interaction is common in clinical treatments and medical research, therefore investigation of light path in irradiated tissues is of high importance. In this research, reflected light intensity measurements from different homogeneous phantoms, combined with numerical simulations, have been used for the investigation of phantoms absorption properties. Our results suggest a good fitting between the theoretical model and the random walk simulations, enabling the extraction of the lattice absorption parameter. Yet, as long as low scattering phantom experiments are concerned, the photon migration model does not provide an adequate description for the phantom absorption coefficient extraction.

Spreading of a mercury droplet on thin gold films

The spreading of a small mercury droplet (150μm) on thin gold films is studied, using an optical microscope enhanced with a differential interference contrast system. The growing interfaces are analyzed in order to determine the roughness (α) and growth (β) exponents. For gold film thickness of 1500A we find that α=0.88±0.03 and β=0.76±0.03, while for gold thickness of 3000A, α=0.96±0.04 and β=1.00±0.04. Both sets of exponents satisfy the scaling relation α+α/β=2. In both systems the roughness exponent α crosses over to a value close to 0.5 in the final stages of the experiment and for relatively long length scales (order of a few microns).