Method of optimized gradient coil design

Abstract

The present invention relates to a method of discretization of the continuous current solution of a gradient coil design that allows satisfaction of the target field quality characteristics as well as other characteristics such as minimization of the energy / inductance, minimization of the residual eddy current effect, minimization of the thrust forces on the coil and cold shields, coil resistance thus the power dissipated by the coil, etc. The method of optimized gradient coil design can be applied to the design of axial or transverse gradient coils. The method of this invention includes the steps of defining at least one, and more commonly numerous performance characteristics of the desired gradient coil, concurrently varying discretization parameters to develop numerous possible hypothetical gradient coil designs, evaluating the designs to determine whether the defined performance characteristics are met by each design and selecting one design.

Description

TECHNICAL FIELD[0001]The invention relates to a new method of gradient coil design for use in magnetic resonance imaging (MRI) for spatial encoding of the MRI signal. The invention more specifically relates to a new method of optimized gradient coil design for both transverse and axial gradient coil.BACKGROUND OF THE INVENTION[0002]Magnetic resonance imaging (MRI) is a medical diagnostic imaging technique used to diagnose many types of medical conditions. An MRI system includes a main magnet for generating a main magnetic field through an examination region. The main magnet is arranged such that its geometry defines the examination region. The main magnetic field causes the magnetic moments of a small majority of the various nuclei within the body to be aligned in a parallel or anti-parallel arrangement. The aligned magnetic moments rotate around the equilibrium axis with a frequency that is characteristic for the nuclei to be imaged. An external radiofrequency (RF) field applied by...

AI Technical Summary

Benefits of technology

[0018]The present invention relates to an improved method of discretization of the continuous current solution of a gradient coil design that allows satisfaction of the target field quality characteristics as well as other characteristics such as minimization of the energy / inductance, minimization of the residual eddy current effect, minimization of the thrust forces on the coil and cold shields, coil resistance thus the power dissipated by the coil, etc. The method of optimized gradient coil design can be applied to the design of axial or transverse gradient coils. The method of this invention includes the steps of defining at least one, and more commonly numerous performance characteristics of the desired gradient coil, concurrently varying discretization parameters to develop numerous possible hypothetical gradient coil designs, evaluating the designs to determine whether the defined performance characteristics are met by each design and selecting one design. The design selected will often be the design that offers the best design solution by meeting numerous, if not all of the defined performance characteristics.

[0019]The method of gradient coil design of this invention allows designing a genuinely finite length gradient coil where the truncation of the continuous current densities is not involved. The new discretization technique allows for a gradient coil to be designed while taking into account predetermined desired characteristics of the gradient coil, without compromising the field quality characteristics, residual eddy currents, balancing the forces exerted on the gradient coil and the cold shield of the magnet.

[0020]It is an object of this invention to provide a method of gradient coil design wherein the method allows satisfaction of the target field quality characteristics as well as other characteristics such as minimization of the energy / inductance, minimization of the residual eddy current effect, minimization of the thrust forces on the coil and cold shields, coil resistance thus the power dissipated by the coil, etc.

Problems solved by technology

One of the problems in past methods of gradient coil design is the discretization procedure that approximates the continuous current densities by a number of current carrying conductors (step 4 of the above method).

It is sometimes difficult to find a continuous solution that satisfies the field quality characteristics within the FoV and where the net thrust force is nullified by constraint.

Depending on the magnet configuration the nullification of the thrust force can lead to an energy penalty or even to reversed current patterns on either the primary coil or the shield coil, which in fact leads to the energy / inductance penalty.

Another problem that arises in previous methods of gradient coil design is that eddy currents are induced in the worm bore or the cold shield of the magnet.

Eddy currents in the cold shield result in distortions of the images.

These significant forces on the cold shield may lead to the vibration of the cold shield and even quenching of the magnet.

In practice the latter is difficult to achieve (IP≠IS) because of the free parameters in the problem: the geometry of the coil is determined by the space available and usually is at premium.

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