A method for the design of MRI radiofrequency coils based on triangular and pulse basis functions.

This paper presents a numerical technique for the design of an RF coil for asymmetric magnetic resonance imaging (MRI) systems. The formulation is based on an inverse approach where the cylindrical surface currents are expressed in terms of a combination of sub-domain basis functions: triangular and pulse functions. With the homogeneous transverse magnetic field specified in a spherical region, a functional method is applied to obtain the unknown current coefficients. The current distribution is then transformed to a conductor pattern by use of a stream function technique. Preliminary MR images acquired using a prototype RF coil are presented and validate the design method.     

The chlamydial EUO gene encodes a histone H1-specific protease

Chlamydia trachomatis is an obligate intracellular pathogen, long recognized as an agent of blinding eye disease and more recently as a common sexually transmitted infection. Recently, two eukaryotic histone H1-like proteins, designated Hc1 and Hc2, have been identified in Chlamydia. Expression of Hc1 in recombinant Escherichia coli produces chromatin condensation similar to nucleoid condensation observed late in the parasite's own life cycle. In contrast, chromatin decondensation, observed during the early life cycle, accompanies down-regulation and nondetection of Hc1 and Hc2 among internalized organisms. We reasoned that the early upstream open reading frame (EUO) gene product might play a role in Hc1 degradation and nucleoid decondensation since it is expressed very early in the chlamydial life cycle. To explore this possibility, we fused the EUO coding region between amino acids 4 and 177 from C. trachomatis serovar Lz with glutathione S-transferase (GST) and examined the effects of fusion protein on Hc1 in vitro. The purified fusion protein was able to digest Hc1 completely within 1 h at 37 degrees C. However, GST alone exhibited no Hc1-specific proteolytic activity. The chlamydial EUO-GST gene product also cleaves very-lysine-rich calf thymus histone H1 and chicken erythrocyte histone H5 but displays no measurable activity towards core histones H2A, H2B, H3, and H4 or chlamydial RNA polymerase alpha-subunit. This proteolytic activity appears sensitive to the serine protease inhibitor 4-(2-aminoethyl)-benzenesulfonyl fluoride hydrochloride (AEBSF) and aspartic protease inhibitor pepstatin but resistant to high temperature and other broad-spectrum protease inhibitors. The proteolytic activity specified by the EUO-GST fusion product selectively digested the C-terminal portion of chlamydial Hc1, the domain involved in DNA binding, while leaving the N terminus intact. At a molar equivalent ratio of 1:1 between Hc1 and DNA, the EUO gene product cleaves Hc1 complexed to DNA and this cleavage appears sufficient to initiate dissociation of DNA-Hc1 complexes. However, at a higher molar equivalent ratio of Hc1/DNA (10:1), there is partial protection conferred upon Hc1 to an extent that prevents dissociation of DNA-Hc1 complexes.     

Finite-dimensional filters with nonlinear drift,VI: Linear structure of Ω

Ever since the concept of estimation algebra was first introduced by Brockett and Mitter independently, it has been playing a crucial role in the investigation of finite-dimensional nonlinear filters. Researchers have classified all finite-dimensional estimation algebras of maximal rank with state space less than or equal to three. In this paper we study the structure of quadratic forms in a finite-dimensional estimation algebra. In particular, we prove that if the estimation algebra is finite dimensional and of maximal rank, then the Ω=(∂f _j /∂x _i −∂f _i /∂x _j )matrix, wheref denotes the drift term, is a linear matrix in the sense that all the entries in Ω are degree one polynomials. This theorem plays a fundamental role in the classification of finite-dimensional estimation algebra of maximal rank. This research was supported by Army Research Office Grants DAAH 04-93-0006 and DAAH 04-1-0530.     

Quenched-in centers in silicon p+n junctions

Silicon p⁺n junctions heat treated at high temperatures (1200°C) for a long time (2–20 hr) and then quenched to room temperatures or below shows two deep donor levels (E_C?264meV andE_C?542meV) in the n-type side of the junction depletion layer which appear to originate from the same imperfection center. The concentration of these levels ranges from 10¹³ to 10¹⁴ cm^?3. The junction leakage current comes from carrier generation at the deeper level in the depletion region. Phosphorus gettering was found ineffective in reducing the concentration of these quenched-in levels, but they are annealed out by very slow cooling (25°C/hr to 650°C then quench to room temperatures). The thermal emission and capture rates of electrons and holes at these levels are measured as a function of temperature and electric field by the junction high frequency capacitance and d.c. leakage current transient techniques. It is demonstrated that the detailed balance relationship does not hold. The origin of this double donor center is yet to be identified.     

On the robustness of parameter estimation of superimposed signalsby dynamic programming

We analyze a recently proposed dynamic programming algorithm (REDP) for maximum likelihood (ML) parameter estimation of superimposed signals in noise. We show that it degrades gracefully with deviations from the key assumption of a limited interaction signal model (LISMO), providing exact estimates when the LISMO assumption holds exactly. In particular, we show that the deviations of the REDP estimates from the exact ML are continuous in the deviation of the signal model from the LISMO assumption. These deviations of the REDP estimates from the MLE are further quantified by a comparison to an ML algorithm with an exhaustive multidimensional search on a lattice in parameter space. We derive an explicit expression for the lattice spacing for which the two algorithms have equivalent optimization performance, which can be used to assess the robustness of REDP to deviations from the LISMO assumption. The values of this equivalent lattice spacing are found to be small for a classical example of superimposed complex exponentials in noise, confirming the robustness of REDP for this application     

A mixed-mode crack analysis of rectilinear anisotropic solids using conservation laws of elasticity

A very simple and convenient method of analysis for studying two-dimensional mixed-mode crack problems in rectilinear anisotropic solids is presented. The analysis is formulated on the basis of conservation laws of anisotropic elasticity and of fundamental relationships in anisotropic fracture mechanics. The problem is reduced to a system of linear algebraic equations in mixed-mode stress intensity factors. One of the salient features of the present approach is that it can determine directly the mixed-mode stress intensity solutions from the conservation integrals evaluated along a path removed from the crack-tip region without the need of solving the corresponding complex near-field boundary value problem. Several examples with solutions available in the literature are solved to ensure the accuracy of the current analysis. This method is further demonstrated to be superior to other approaches in its numerical simplicity and computational efficiency. Solutions of more complicated and practical engineering problems dealing with the crack emanating from a circular hole in composites are presented also to illustrate the capacity of this method.

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Résumé On présente une méthode d'analyse simple et bien adaptée à l'étude de problèmes de fissuration bidimensionnelle de mode mixte dans des solides rectilinéaires anisotropes. L'analyse est formulée sur la base des lois de conservation de l'élasticité anisotrope et sur les relations fondamentales dans la mécanique de rupture anisotrope. Le problème est ramené à un système d'équations algébriques linéaires exprimant les facteurs d'intensité d'entaille pour un mode mixte. L'un des faits saillants de l'approche envisagée est qu'elle peut déterminer directement les solutions d'intensité de contrainte suivant un mode mixte à partir des intégrales de conservation évaluées le long d'un parcours éloigné par rapport à l'extrémité de la fissure, sans qu'il soit nécessaire de résoudre le problème correspondant de la valeur du champ proche complexe. Divers exemples, ainsi que des solutions disponibles dans la littérature, sont résolus et confrontés en vue de tester la fiabilité de l'analyse réalisée. La méthode s'avère supérieure aux autres approches de par sa simplicité numérique et son efficacité de calcul. Des solutions à des problèmes plus compliqués et pratiques rencontrés en ingénièrie où une fissure émane d'un trou circulaire dans des matières composites sont également présentées, en vue d'illustrer les capacités de cette méthode.

     

Derivation of optimal processing parameters of polypropylene foam thermoforming by an artificial neural network

The effects of processing parameters on the thermoforming of polymeric foam sheets are highly nonlinear and fully coupled. The complex interconnection of these dominant processing parameters makes the process design a difficult task. In this study, the optimal processing parameters of polypropylene foam thermoforming are obtained by the use of an artificial neural network. Data from tests carried out on a lab‐scale thermoforming machine were used to train an artificial neural network, which serves as an inverse model of the process. The inverse model has the desired product dimensions as inputs and the corresponding processing parameters as outputs. The structure, together with the training methods, of the artificial neural network is also investigated. The feasibility of the proposed method is demonstrated by experimental manufacturing of cups with optimal geometry derived from the finite element method. Except the dimension deviation at one location, which amounts to 17.14%, deviations of the other locations are all below 3.5%. POLYM. ENG. SCI., 45:375–384, 2005. © 2005 Society of Plastics Engineers     

Thermomagnetic nernst effect in an electrolytic system

A phenomenological theory for thermomagnetic effect in an electrolyte system is worked out. A sinusoidal variation of the induced emf is observed perpendicular to the heat flow when an external magnetic field is applied to the system, across the flow with varying angles. The experimental results are found in good agreement with the presented theory in HNO₃ (900 × 10^?4 M). Planar Nernst effect is therefore measured and the coefficient γ is estimated.