Designs for an asymmetric gradient set and a compact superconducting magnet for neural magnetic resonance imaging

Crozier, Stuart; Luescher, Kurt; Hinds, G. W.; Roffmann, Wolfgang U.; Doddrell, David M.

doi:10.1063/1.1150037

Cited by 17 publications

(28 citation statements)

References 10 publications

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“…[7] and [8] for a coil whose current distribution takes the form of Eq. [10], thus generating a perfectly uniform z-gradient within the sphere, yields performance figures The efficiency of the spherical coil design was found to be 0.51a Ϫ2 Tm Ϫ1 A Ϫ1 per turn. Figure 4a shows the wirepaths of a spherical x-gradient coil, with 5 turns per quadrant, defined as contours of the P 2,1 (cos )cos stream function, while Fig.…”

Section: Methodsmentioning

confidence: 99%

“…However, removing the axial symmetry of a transverse gradient coil means that it is no longer naturally torque-balanced, i.e., it will experience a net torque when carrying a current in the presence of a large static magnetic field. It is therefore necessary to take special steps in the design process to ensure that the wire paths are arranged so as to yield zero torque (7)(8)(9)(10)(11). Adopting this approach leads to the production of coils that have significantly higher values of 2 /L and 2 /R than the best whole-body gradient coils.…”

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confidence: 99%

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Hemispherical gradient coils for magnetic resonance imaging

Green

Leggett

Bowtell

2005

Magnetic Resonance in Med

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Hemispherical gradient coils offer an open geometry that is well suited to imaging of the human brain. The windings of a hemispherical gradient coil are on average closer to the target region than those of a comparable cylindrical coil, and consequently hemispherical coils can produce higher efficiency at fixed inductance. The mathematical formalism needed for the design of hemispherical gradient coils is described here, including expressions relating the current distribution on the hemisphere to the magnetic field generated, as well as the stored energy and power dissipation.

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Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

Hemispherical gradient coils for magnetic resonance imaging

Green

Leggett

Bowtell

2005

Magnetic Resonance in Med

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“…Smaller gradient coils tailored to local anatomical regions offer the potential of higher gradient strengths and, when appropriately combined with local RF coils, produce improved image resolution. A number of strategies have been presented for the design of local gradient coils (1)(2)(3)(4)(5)(6). In this article we present two new designs for head gradients coils and examine each in detail.…”

Section: Introductionmentioning

confidence: 99%

Toward designing asymmetric head gradient coils for high‐resolution imaging

Vegh

Sanchez

Brereton

et al. 2007

Concepts Magn Reson

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ABSTRACT:The aim of the research described here is to design head gradient coils using approaches previously developed by the authors. The wave equation method for designing gradient coils is used to design a transverse gradient coil winding on a cylindrical/spherical former. The results are compared with asymmetric cylindrical designs and other well-known head gradient coil designs to establish the quality of the gradient coil winding. Comparisons of field, inductance, torque, and other quality factors are made before conclusions are drawn about the adequacy of such a winding pattern to perform high-resolution imaging of the head. We show that it is possible to generate a robust winding pattern that has good efficiency and produces a sufficiently large imaging volume.

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“…An approach to enhance gradient capacities without the onset of PNS is to use local gradient coils. As smaller gradient coils produce less magnetic flux than the whole-body coils, therefore, local coils can be switched faster than whole-body coils without causing PNS [6][7][8]. Although local gradient coils have been demonstrated to be faster and more effective to achieve high performance and high gradient strength compared to the whole-body gradient coils [4,9,10], the applications of head coils have been restricted by the coil design and implementation [5].…”

Section: Research Problem and Motivationmentioning

confidence: 99%

“…In addition, local gradient coils dissipate less power and minimize the stored energy. designed an asymmetric gradient set for a compact superconducting MRI magnet for head and neck imaging [6]. Jean-Baptiste developed a force and torque balanced gradient coil to constrain individual harmonics of the field produced by the gradient coil and eddy currents in a specialty 3T head only magnet [74,75].…”

Section: Introductionmentioning

confidence: 99%

Gradient coil design and intra-coil eddy currents in MRI systems

Tang¹

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The work in this thesis is primarily concerned with the enhancement of gradient coil performance in magnetic resonance imaging (MRI). In MRI, gradient coils are used to produce gradient fields to spatially encode magnetic resonance signals. However, rapid switching on and off of the gradient coils induces fields and eddy currents in the human tissues and surrounding conducting structures. This thesis will provide novel solutions for the design of gradient coils and the reduction of eddy currents.Gradient switching induces electric fields in human tissue that can cause safety problems, in particular peripheral nerve stimulation (PNS), which limits the gradient performance such as the slew rate and maximum gradient strength. One approach to solve this problem is to use local insertable gradient coils to achieve high gradient performance over localised regions of interest (ROI) yet not trigger PNS. Head only coils or head and neck coils are by far the most commonly used local coils. However, due to the constraints of the human head and shoulder, the head gradient coils are usually designed in an asymmetric configuration, with the region-of-uniformity (ROU) close to the patient end of the coil. This asymmetric configuration leads to technical difficulties in maintaining a high gradient performance for the head coil inserts given the very limited space available. Therefore, in this thesis, a practical configuration of an insertable asymmetric gradient head coil that offers improved performance was proposed. In the proposed design, at the patient end, the primary and secondary coils were connected using an additional radial surface, thus allowing the coil conductors distributed on the flange to ensure an improvement in the coil performance. At the service end, the primary and shielding coils were not connected, to permit access to shim trays, cooling system piping, cabling, and so on. It was found that with a similar field quality in the ROI, the proposed coil pattern improved construction characteristics (open service end, well-distributed wire pattern) and offers a better coil performance (lower inductance, higher efficiency etc.) than conventional head coil configurations.Another obstacle to the advancement of MRI is the eddy currents induced in the surrounding conducting structures including the gradient coils themselves. The eddy currents induced in the surrounding conductors depend on the geometry of the conductor and the excitation waveform. These alternating fields caused by the switching gradient coils change the spatial profile of the current density within the coil tracks and surrounding coils with the applied frequencies of the input waveform and by their proximity to other conductors. Therefore, in this thesis, inductive coupling between coil tracks and gradient coils themselves was first investigated and then solutions for mitigation of the eddy current effects were provided. IIFirst, the inductive coupling between coil tracks was studied, considering skin and proximity effects. In this investiga...

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Designs for an asymmetric gradient set and a compact superconducting magnet for neural magnetic resonance imaging

Cited by 17 publications

References 10 publications

Hemispherical gradient coils for magnetic resonance imaging

Hemispherical gradient coils for magnetic resonance imaging

Toward designing asymmetric head gradient coils for high‐resolution imaging

Gradient coil design and intra-coil eddy currents in MRI systems

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