Short-range structure and cation bonding in calcium-aluminum metaphosphate glasses.

Comprehension of short- and medium-range order of phosphate glasses is a topic of interest, due to the close relation between network structure and mechanical, thermal, and optical properties. In this work, the short-range structure of glasses (1 - x)Ca(PO(3))(2).xAl(PO(3))(3) with 0 < or = x < or = 0.47 was studied using solid-state nuclear magnetic resonance spectroscopy, Raman spectroscopy, density measurements, and differential scanning calorimetry. The bonding between a network modifier species, Al, and the network forming phosphate groups was probed using high-resolution nuclear magnetic resonance spectroscopy of (27)Al and (31)P. Changes in the compositional behavior of the density, glass transition temperature, PO(2) symmetric vibrations, and Al coordination number were verified at around x = 0.30. (31)P NMR spectra show the presence of phosphorus in Q(2) sites with nonbridging oxygens (NBOs) coordinated by Ca ions and also Q(2) sites with one NBO coordinated by Al (namely, Q(2)(1Al)). The changes in the properties as a function of x can be understood by considering the mean coordination number measured for Al and the formation of only Q(2) and Q(2)(1Al) species. It is possible to calculate that a network formed only by Q(2)(1Al) phosphates can just exist up to the upper limit of x = 0.48. Above this value, Q(2)(2Al) species should appear, imposing a major reorganization of the network. Above x = 0.30 the network undergoes a progressive reorganization to incorporate Al ions, maintaining the condition that only Q(2)(1Al) species are formed. These observations support the idea that bonding principles for cationic species inferred originally in binary phosphate glasses can also be extended to ternary systems.

Asymmetrical gradient coil for head imaging.

This work presents a novel approach to develop dedicated transverse gradient coils for head imaging. The proposed coil design is based on the stochastic optimization of an asymmetrical stream function and improves the matching between the region-of-interest and the homogeneous gradient volume. Additionally, the electric field produced by these asymmetrical coils is 30% lower than that produced by standard symmetrical designs, which minimizes the risk of magnetostimulation of nerves in fast imaging techniques. A prototype of the asymmetrical gradient coil was built to test the method and magnetic field produced by the prototype was measured. Magnetic field measurements and electrical parameters of coils are in good agreement with theoretical calculations.

Fast optimization of a biplanar gradient coil set.

This work presents an approach for fast optimization of gradient coils, using the simulated annealing method. The shielding condition derived from a target field method and the analytical evaluation of the fields produced by simple geometries were used to reduce the computing time. This method is applied to the optimization of a shielded biplanar gradient coil set. Efficiency, inductance, and homogeneity of the gradient fields produced by the optimized geometries were studied as a function of the number of wires, for the longitudinal and transverse gradient coils. A prototype of the gradient set was made to test the proposed design method. The resulting experimental values of coil efficiency, inductance, field linearity, and shielding performance exhibit good agreement between theory and experiment.

Shielded biplanar gradient coil design.

An application of the target field method to the design of shielded biplanar gradient coils for magnetic resonance imaging electromagnets is presented. Some specific cases are studied, and optimized geometries are proposed for the axial and transverse gradient coils that eliminate the third- and minimize the fifth-order terms in the magnetic field expansion.

Magnetic fields mapping with the phase reference method.

A simple method for imaging the magnetic field produced by arbitrary current distributions is presented. The method is based on the use of a phase reference image and can be easily adapted to any standard magnetic resonance imaging scanner. Examples are given to application to gradient coil characterization and quantitative field measurements.

Use of a Phase Reference for Field Mapping with Amplitude Images at Low Field

We present a method to obtain MRI amplitude images that can picture the magnetic field due to arbitrary shaped magnetized objects. The method employees the gradient recalled echo sequence and two sets of data obtained in separate experiments, one of which provides a phase reference image making it possible to eliminate the effect of the B0 field inhomogeneities. The final magnitude images have a good signal-to-noise even at low fields, and provide qualitative as well as quantitative information about the magnetic field produced by the ferromagnetic object. As an example the method is applied to study the field produced by a small metal piece in a 500-G scanner, and the experimental results are compared with numerical simulations. Copyright 1998 Academic Press.

Banco de dados relacional para tomografia por ressonância magnética nuclear / Relational data base to tomography by nuclear magnetic resonance

Este trabalho descreve um sistema de banco de dados relacional para tomografia por RMN. Desenvolvimento para computadores do tipo IBM/PC operando na plataforma operacional MS Windows 95 e NT, o banco de dados foi modelado usando diagramas EER e implementado no aplicativo MS ACCESS. O modelo conceitual é compatível com o padrão DICOM 3.0, possibilitando exportação e importação de dados.

ESR spectra of quasirandomly oriented centers: application to radiation damage centers in bone.

The angular dependence of the ESR line shape from polycrystalline samples due to incomplete randomness in the orientation of the crystallites is calculated for centers with axial symmetry. The effect of a preferential axis of orientation is considered and the results are applied to radiation produce centers in bone.