Gel-free experiments of reaction-diffusion front kinetics.

We present a gel-free experimental system to study the kinetics of the reaction front in the A+B-->C reaction-diffusion system with initially-separated reactants. The experimental setup consists of a CCD camera monitoring the kinetics of the front formed in the reaction-diffusion process Cu(2+) + tetra [disodium ethyl bis(5-tetrazolylazo) acetate trihydrate] -->1:1 complex, in aqueous, gel-free solution, taking place inside a 150 microm gap between two flat microscope slides. The experimental results agree with the theoretical predictions for the time dependence of the front's width, height, and location, as well as the global reaction rate.

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.

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.

Optimization of magnetization transfer experiments to measure first-order rate constants and spin-lattice relaxation times.

A recent analysis of the optimization of magnetization transfer experiments to estimate first-order rate constants is extended to the design of experiments for measuring these constants and T1 when both parameters are initially unknown. When the number of measurement times is fixed, an optimal design will consist of a set of measurement times which ensures that the precision of the estimates of the combination of k and T1 is as small as possible in a worst-case scenario. Simple expressions are given for the parameters that characterize the optimal design.

Optimization of magnetization transfer experiments for kinetic rate measurements.

A framework is developed for optimizing magnetization transfer experiments for measuring first-order rate constants of a two-component exchange network. In the analysis the spin-lattice relaxation time of the nonsaturated resonance is assumed to be known. However, we demonstrate that our work can also be applied to cases in which the spin-lattice relaxation time falls within a physiologically reasonable range for this parameter. We provide an algorithm for the parameters permitting the experimenter to optimize the selection of measurement times in order to control the error in a worst-case scenario. In addition, we compare the performance of the time-dependent saturation experiment with an inversion experiment in this framework, finding that the former always leads to a more effective design.

Approximate theory of photon migration in a two-layer medium.

We recently examined properties of the surface emission profiles of particles (photons) injected into a turbid medium consisting of two layers [R. Nossal, J. Kiefer, G. H. Weiss, R. F. Bonner, H. Taitelbaum, and S. Havlin, "Photon Migration in Layered Media," Appl. Opt. 27, 3382-3391 (1988)]. The two layers differ in the coefficient that appears when internal absorption is modeled in terms of Beer's law. The model relates to the injection of laser radiation into tissue for diagnostic or therapeutic purposes. Results of our earlier work were derived from extensive computer simulations. In the present paper we discuss a simple analytical approximation to the surface intensity profile valid when the absorptivity of the upper layer is greater than that of the lower.

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.