Quantitative photoacoustic tomography from boundary pressure measurements: noniterative recovery of optical absorption coefficient from the reconstructed absorbed energy map

We describe a noniterative method for recovering optical absorption coefficient distribution from the absorbed energy map reconstructed using simulated and noisy boundary pressure measurements. The source reconstruction problem is first solved for the absorbed energy map corresponding to single- and multiple-source illuminations from the side of the imaging plane. It is shown that the absorbed energy map and the absorption coefficient distribution, recovered from the single-source illumination with a large variation in photon flux distribution, have signal-to-noise ratios comparable to those of the reconstructed parameters from a more uniform photon density distribution corresponding to multiple-source illuminations. The absorbed energy map is input as absorption coefficient times photon flux in the time-independent diffusion equation (DE) governing photon transport to recover the photon flux in a single step. The recovered photon flux is used to compute the optical absorption coefficient distribution from the absorbed energy map. In the absence of experimental data, we obtain the boundary measurements through Monte Carlo simulations, and we attempt to address the possible limitations of the DE model in the overall reconstruction procedure.

The ratio of diffusion coefficient to mobility for slow electrons in dry air

Using Huxley's solution of the diffusion equation for electron-attaching gases, the ratio of diffusion coefficient D to mobility μ for electrons in dry air was measured over the range 3·06 × 10-17

Dispersion of the stress-optic coefficient in glasses

The stress-optic coefficient (n3/2)(q11-q12) has been determined for a series of 18 optical glasses of different compositions in the wavelength range 5700-3200 Å. The coefficients are negative for all the glasses except for a high-lead-content glass of density 6·7 and refractive index 1·89. The numerical value of the coefficient decreases as one proceeds to the ultraviolet. This behaviour is just the opposite of what is observed in fused silica. By applying Mueller's theory, the strain polarizability constant and its dispersion have been evaluated.

Assessment of ultrasound modulation of near infrared light on the quantification of scattering coefficient

Purpose: To assess the effect of ultrasound modulation of near infrared (NIR) light on the quantification of scattering coefficient in tissue-mimicking biological phantoms.Methods: A unique method to estimate the phase of the modulated NIR light making use of only time averaged intensity measurements using a charge coupled device camera is used in this investigation. These experimental measurements from tissue-mimicking biological phantoms are used to estimate the differential pathlength, in turn leading to estimation of optical scattering coefficient. A Monte-Carlo model base numerical estimation of phase in lieu of ultrasound modulation is performed to verify the experimental results. Results: The results indicate that the ultrasound modulation of NIR light enhances the effective scattering coefficient. The observed effective scattering coefficient enhancement in tissue-mimicking viscoelastic phantoms increases with increasing ultrasound drive voltage. The same trend is noticed as the ultrasound modulation frequency approaches the natural vibration frequency of the phantom material. The contrast enhancement is less for the stiffer (larger storage modulus) tissue, mimicking tumor necrotic core, compared to the normal tissue. Conclusions: The ultrasound modulation of the insonified region leads to an increase in the effective number of scattering events experienced by NIR light, increasing the measured phase, causing the enhancement in the effective scattering coefficient. The ultrasound modulation of NIR light could provide better estimation of scattering coefficient. The observed local enhancement of the effective scattering coefficient, in the ultrasound focal region, is validated using both experimental measurements and Monte-Carlo simulations. (C) 2010 American Association of Physicists in Medicine. [DOI: 10.1118/1.3456441]

A pulsed field gradient spin echo NMR spectrometer for diffusion coefficient measurements

A pulsed field gradient spin echo NMR spectrometer has been assembled by interfacing a programmable pulse generator and a data acquisition system designed and fabricated in our laboratory with other imported units. Calibration results of the magnetic field gradients are presented.

Varistors based on mixed‐phase ceramics containing ZnO and negative temperature coefficient spinels

High nonlinearity coefficients of 60–150 are observed in the current‐voltage (I‐V) curves of the mixed phase ceramics formed by cosintering ZnO with spinel phases having large negative temperature coefficients (NTCs) in resistivity. The region of negative slope in the I‐V curves of the NTC ceramics is progressively made positive with ZnO phase content, wherein ZnO grains function as a built‐in resistor in series to the resistance of the NTC phase. High α depends on the optimum phase content of ZnO as much as its intrinsic conductivity. The studies indicate that the predominent contribution to power dissipation is by way of joule heating from the resistive component of the current.

On some results of A. E. Livingston and coefficient problems for concave univalent functions

We consider functions that map the open unit disc conformally onto the complement of a bounded convex set. We call these functions concave univalent functions. In 1994, Livingston presented a characterization for these functions. In this paper, we observe that there is a minor flaw with this characterization. We obtain certain sharp estimates and the exact set of variability involving Laurent and Taylor coefficients for concave functions. We also present the exact set of variability of the linear combination of certain successive Taylor coefficients of concave functions.

A Novel Method for the Uniform Incorporation of Grain-Boundary Layer Modifiers in Positive Temperature Coefficient of Resistivity Barium Titanate

The presently developed two-stage process involves diping the prefired porous disks of n-BaTiO3 in nonaqueous solutions containing Al-buty rate, Ti-isopropoxide, and tetraethyl silicate and subsequent sintering. This leads to uniform distribution of the grain-boundary layer (GBL) modifiers (Al2O3+ TiO2+ SiO2) and better control of the grain size as well as the positive temperature coefficient of resistivity characteristics. The technique is particularly suited for GBL modifiers in low concentrations (< 1%).

Analysis of reflection characteristics of a normal incidence plane wave on resonant sound absorbers: A finite element approach

Resonant sound absorbers are used widely as anechoic coatings in underwater applications. In this paper a finite element scheme based on the Galerkin technique is used to analyze the reflection characteristics of the resonant absorber when insonified by a normal incidence plane wave. A waveguide theory coupled with an impedance matching condition in the fluid is used to model the problem. It is shown in this paper that the fluid medium encompassing the absorber can be modeled as an elastic medium with equivalent Lamé constants. Quarter symmetry conditions within the periodic unit cell are exploited. The finite element results are compared with analytical results, and with results published elsewhere in the literature. It is shown in the process that meshing of the fluid domain can be obviated if the transmission coefficients or reflection coefficients only are desired as is often the case. Finally, some design curves for thin resonant absorbers with water closure are presented in this paper.

Tropical Clays .1. Index Properties And Microstructural Aspects

Soils showing changes in plasticity characteristics upon driving form an important group in tropical soils. These changes are attributed to the grouping of particles into aggregates either due to mineralogy or presence of cementing agents and/or pore fluid characteristics. These changes are found to be permanent. In this paper, the effect of these changes leading to changes in index properties is discussed. The coefficient of permeability has been found to be comparable at liquid limit water content for different soils of varying liquid limit values. Permeability is an indirect reflection of microstructure and indicates the flow rate, which depends upon pore geometry. Other mechanical properties like compressibility and shear strength also depend upon pore geometry. These microstructural aspects of liquid limit as a reference state for the analysis of engineering behavior of tropical soils are examined in detail.

Hybridization of carbon-glass epoxy composites: An approach to achieve low coefficient of thermal expansion at cryogenic temperatures

The paper is based on a study to develop carbon-glass epoxy hybrid composites with desirable thermal properties for applications at cryogenic temperatures. It analyzes the coefficient of thermal expansion of carbon-epoxy and glass-epoxy composite materials and compares it with the properties of carbon-glass epoxy hybrid composites in the temperature range 300 K to 125K. Urethane modified epoxy matrix system is used to make the composite specimens suitable for use even for temperatures as low as 20K. It is noted that the lay-up with 80% of carbon fibers in the total volume fraction of fibers oriented at 30 degrees and 20% of glass fibers oriented at 0 degrees yields near to zero coefficient of thermal expansion as the temperature is lowered from ambient to 125 K. (c) 2010 Elsevier Ltd. All rights reserved.