Design and evaluation of shielded gradient coils

Carlson, Joseph W.; Derby, Kevin; Hawryszko, K. C.; Weideman, M.

doi:10.1002/mrm.1910260202

Cited by 102 publications

(112 citation statements)

References 8 publications

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“…The variables Z i 5 2L n and Z f 5 L n define the initial and the final axial limits ofJ for symmetric gradient coils, where 2L n is the coil length of layer n. Assuming that the radial thickness of the surface is much smaller than the cylindrical radius q n , then the vector current densityJðq; /; zÞ may be described solely in terms of the azimuthal and axial components. Carlson's approach is applied to generate coils of finite length (17). The azimuthal component of the current density for instance can be expressed as a sum of orthonormal functions multiplied by the stream function coefficients a nq (18):…”

Section: Methodology Prediction Of Gradient Coil Pattern Based On Expmentioning

confidence: 99%

Reverse‐engineering of gradient coil designs based on experimentally measured magnetic fields and approximate knowledge of coil geometry‐application in exposure evaluations

Trakic

Lopez

et al. 2009

Concepts Magn Reson

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For many MRI installations, the coil pattern that generates pulsed magnetic fields produced by gradient coils is not provided by the manufacturer. This has implications for accurate assessments of MRI worker exposures, which is currently an important topic of research. To correctly model the level of exposure, a full three-dimensional distribution of the magnetic field in the locality of the magnet end is required, which can be difficult to obtain by experimental measurements. This research presents one possible approach, in which the prediction of a current distribution that generates an approximately identical magnetic field profile is constrained by a small number of experimentally measured magnetic field sample points within a plane outside the gradient set. The presented methodology may take into consideration other important descriptors such as field uniformity in the imaging volume, gradient coil geometry/dimension, driving current, central field strength, etc. To exemplify the application of the proposed approach, the current density and matching magnetic field distributions of x-and z-axis gradient coils are approximated without preknowledge of the gradient coil patterns for a MRI system, followed by exposure evaluations of a tissue-equivalent numerical worker model in the vicinity of said gradients.

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Section: Methodology Prediction Of Gradient Coil Pattern Based On Expmentioning

confidence: 99%

Reverse‐engineering of gradient coil designs based on experimentally measured magnetic fields and approximate knowledge of coil geometry‐application in exposure evaluations

Trakic

Lopez

et al. 2009

Concepts Magn Reson

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“…Previously, we designed axial magnetic field gradient coils (5) using a multilayer variant of Carlson's harmonic minimization technique (6). Here, the technique is extended to the design and construction of transverse field gradient coils.…”

Section: Introductionmentioning

confidence: 99%

Multilayer transverse gradient coil design

Leggett

Crozier

Blackband

et al. 2003

Concepts Magn Reson

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ABSTRACT:In small, cylindrical gradient coils consisting of a single layer of wires, the limiting factor in achieving large magnetic field gradients is the rapid increase in coil resistance with efficiency. This behavior results from the decrease in the maximum usable wire diameter as the number of turns is increased. By adopting a multilayer design in which the coil wires are allowed to spread out into multiple layers wound at increasing radii, a more favorable scaling of resistance with efficiency is achieved, thus allowing the design of more powerful gradient coils with acceptable resistance values. By extending the theory used to design standard cylindrical gradient coils, mathematical expressions have been developed that allow the design of multilayer coils. These expressions have previously been applied to the design of a four-layer z-gradient coil. As a further development, the equations have now been modified to allow the design of multilayer transverse gradient coils. The variation in coil performance with the number of layers employed has been investigated for coils of a size suitable for use in NMR microscopy, and the effect of constructing the coil using wires or cuts in a continuous conducting surface has also been assessed. We find that at fixed resistance a small wire-wound two-layer coil offers an increase in efficiency of a factor of about 1.5 compared with a single-layer coil. In addition, a two-layer coil of 10-mm inner diameter has been designed and built. This coil had an efficiency of 0.41 Tm Ϫ1 A Ϫ1 , a resistance of 0.96 Ϯ 0.01 ⍀, and an inductance of 22.3 Ϯ 0.2 H. The coil produces a gradient that deviates from linearity by less than 5% over a central cylindrical region of interest of height and length 6.2 mm.

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“…These current densities were intended to produce a specified magnetic field over an imaging volume surface. This method was initially to create gradient coils on an infinite coil cylinder; and for those cases with a limited length, an additional apodization technique was used to design a practical solution with a specific loss of gradient linearity [23][24][25][26][27][28][29][30]. The constraint conditions applied to the target functions made the mathematical problem ill conditioned; therefore, a smoothing function [18] or coil design parameter optimization (such as inductance minimization or power loss minimization) [23][24][25][26] had to be added to guarantee convergence.…”

Section: Gradient Coil Design Methodsmentioning

confidence: 99%

Gradient coil design and acoustic noise control in magnetic resonance imaging systems

Wang¹

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In magnetic resonance imaging (MRI), three-dimensional linearly varied magnetic field gradients are required in the imaging volume, for the encoding of the spatial information of the imaged object.The gradient magnetic field is generated by gradient coils, whose performance directly affects the final MR image quality. Modern MRI applications demand gradient coils to be designed with higher performance such as a faster slew rate for rapid imaging, and fewer system interactions to ensure distortion-free images.During MR imaging, the gradient coil current switches quickly and the alternating current under a strong magnetic field generates a large Lorentz force, thus increasing the vibrations and noise in the gradient assembly, and the vibrations can also transmit to other components in the system. Moreover, the gradient magnetic field induces eddy currents on the surrounding conductive materials, resulting in further mechanical vibration. The sound pressure level (SPL) of an MRI scanner very commonly exceeds 100 dB, which may be discomforting to patients. Therefore, an effective acoustic noise control is necessary for the MRI operation.This thesis attempts to design a novel gradient coil system in an MRI scanner with a particular focus on the investigation of an acoustic noise control scheme. A brief summary of the thesis work is as follows.(1) Gradient coil design A conventional cylindrical MRI scanner has a long patient bore, which may make some patients uneasy due to claustrophobia. In order to alleviate this, an asymmetric gradient coil was proposed to accommodate an asymmetric MRI magnet. The coil was designed with a number of features, for example, to enable the installation of the shim tray, the coil was configured with one end connected and the other end separated. The electromagnetic and acoustic performances were also improved compared with a conventional non-connected coil system.The stream function approach is commonly used in gradient coil design, but an issue with this design is a connection problem between coil loops. To eliminate the field errors due to the coil connections, this thesis proposed a novel spiral coil design scheme, including both transverse coils and a longitudinal coil. The proposed coil design method was not only able to improve the magnetic II performance of the coil, but also could integrate the cooling system into the coil design using hollow wires in the discrete wire space.Wire spacing is a major engineering problem in gradient coil design. Conventional gradient coils are designed with three primary layers and three shielding layers, and in general, primary transverse coils are located in two layers separated by a very dense wire structure. In this thesis, a novel gradient coil design strategy using a layer-sharing scheme was proposed. This design scheme mutually combined the x and y gradient coil layers, effectively utilizing the unoccupied or sparse coil-layer space. The new method was modelled in the case of an asymmetric head coil design, and enhanced coil performan...

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Design and evaluation of shielded gradient coils

Cited by 102 publications

References 8 publications

Reverse‐engineering of gradient coil designs based on experimentally measured magnetic fields and approximate knowledge of coil geometry‐application in exposure evaluations

Reverse‐engineering of gradient coil designs based on experimentally measured magnetic fields and approximate knowledge of coil geometry‐application in exposure evaluations

Multilayer transverse gradient coil design

Gradient coil design and acoustic noise control in magnetic resonance imaging systems

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