Stream function optimization for gradient coil design.

This work presents a method applied to the design of short self-shielded gradient coils of cylindrical geometry. The method uses a hybrid technique that combines the simulated annealing and target field methods to optimize the standard stream functions. The optimized stream functions were parameterized using a few degrees of freedom to reduce the computing time. The optima stream function parameters are given for easy coil design purposes. The proposed approach is compared to the target field method. The main advantage of the present method over the target field method is its ability to enlarge the homogeneous gradient volume. In addition, the designs of short coils based on this approach have shown lower inductance than the coil design based on the target field method. The fast-simulated annealing technique presented in this work enables the gradient coil optimization in less than 3 min of computing time. Magn Reson Med 45:505-512, 2001.

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.