H. Sanchez

Three-Dimensional Gradient Coil Structures for Magnetic Resonance Imaging Designed Using Fuzzy Membership Functions

The design of high-performance gradient coils is essential for modern magnetic resonance imaging (MRI) applications. This work presents a new and alternative design methodology that explores three-dimensional (3-D) solution space by combining fuzzy membership functions to model and reshape the coil structure, given the magnetic field and electrical and mechanical constraints. The approach includes a stream function generator, a 3-D coil structure generator, and the evaluation of an objective function. An unconventional stream function for asymmetric transverse gradient coils is defined in terms of fuzzy sets. The method was applied to design a short, unshielded asymmetric gradient coil for breast imaging. The resulting dome-shape coil has superior gradient performance compared to a standard fingerprint coil. New quadrupolar gradient coil designs for breast imaging can also be obtained with the proposed method. The paper concludes with a study of the influence of a number of design parameters on the coil performance.

A magnetization mapping approach for passive shim design in MRI

A new passive shim design method is presented which is based on a magnetization mapping approach. Well defined regions with similar magnetization values define the optimal number of passive shims, their shape and position. The new design method is applied in a shimming process without prior-axial shim localization; this reduces the possibility of introducing new errors. The new shim design methodology reduces the number of iterations and the quantity of material required to shim a magnet. Only a few iterations (1-5) are required to shim a whole body horizontal bore magnet with a manufacturing error tolerance larger than 0.1 mm and smaller than 0.5 mm. One numerical example is presented

Modal Analysis of Currents Induced by Magnetic Resonance Imaging Gradient Coils

In magnetic resonance imaging (MRI), gradient coils are switched during fast current pulse sequences. These time-varying fields interact with the conducting structures of the scanner, producing deleterious effects such as image distortions and Joule heating. Using a multi-layer integral method, the spatiotemporal nature of the eddy currents induced by the gradient coils is investigated. The existence of the eigenmode is experimentally demonstrated by measuring the magnetic field and the time decay constant of a typical unshielded z-gradient coil and its interaction with a conductive cylinder. An effective current tailoring is achieved using the characteristic eigenvalues of the conducting domain-exciting coil system. The method can be used to understand and mitigate undesired effects of eddy currents in MRI.

3D-Gradient Coil Structures for MRI Designed using Fuzzy Membership Functions

The design of high-performance gradient coils is essential for modern magnetic resonance imaging (MRI) applications. This work presents a new and alternative design methodology that explores three-dimensional (3D) solution space by combining fuzzy membership functions to model and re-shape the coil structure, given the magnetic field, electrical and mechanical constraints. The method was applied to design a short, unshielded asymmetric gradient coil for breast imaging. The resulting dome-shape coil has superior gradient performance compared to a standard fingerprint coil. New quadrupolar gradient coil designs for breast imaging are also obtained with the proposed method.