Ultrashort shielded gradient coil design with 3D geometry

The switching of magnetic field gradient coils in magnetic resonance imaging (MRI) inevitably induces transient eddy currents in conducting system components, such as the cryostat vessel. These secondary currents degrade the spatial and temporal performance of the gradient coils, and compensation methods are commonly employed to correct for these distortions. This theoretical study shows that by incorporating the eddy currents into the coil optimization process, it is possible to modify a gradient coil design so that the fields created by the coil and the eddy currents combine together to generate a spatially homogeneous gradient that follows the input pulse. During the pulsing of magnetic field gradients in MRI, multiexponentially decaying eddy currents are always induced within the conducting materials of the MR imager. Eddy currents in cold, highly conductive radiation shields of the superconducting magnet produce particularly longacting effects relative to the image acquisition period (1-2). These secondary magnetic fields are known to cause spatial and temporal degradation of the gradient uniformity within the imaging volume, which often results in undesired misregistration and intensity-phase variations in both images and spectra.With the recent push of MRI towards high signal-tonoise ratios (SNRs) and improved image resolution, tremendous efforts have been made to prevent and minimize the eddy-current fields. For instance, active screening is often engaged to minimize leakage fields and hence spatially and temporally complex residual eddy currents induced in the cryostat vessel (3-8). Unfortunately, the use of active shielding layer(s) occupies vital space inside the bore of the magnet, increases system cost, and reduces gradient efficiency. Furthermore, residual eddy currents are never completely removed through active shielding, and experimental compensation methods are also required for optimal results (9,10).During the design of conventional gradient coils (cylindrical, planar, etc.), the current distributions in one or more gradient layers are commonly optimized to obtain a target gradient uniformity in the imaging volume while satisfying other design constraints such as minimum inductance, resistance, leakage fields, force, torque, and maximum gradient efficiency (1). Traditionally, these design approaches do not take eddy currents into direct consideration when optimizing the gradient coil. Consequently, during the pulsing of the gradient current, eddy currents are induced in the cryostat vessel and other conducting materials that lead to degradation of the target field uniformity and the field stability.In 1986, Turner and Bowley (13) were among the first to introduce an analytical technique for passive magnetic screening. Their study considered spatial eddy-current variations in a thick, highly conductive, infinitely long aluminum shield as the secondary source contributing to target gradient fields. By varying the single-layer gradient coil positions to accommodate for the spatial presence of ed...

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“…[4]- [7] are the generic time-stepping operations as expressed in Eqs. [1]- [3]. The second terms on the RHS of Eqs.…”

Section: Computation Of the Primary And Secondary Magnetic Fieldsmentioning

confidence: 99%

“…For instance, active screening is often engaged to minimize leakage fields and hence spatially and temporally complex residual eddy currents induced in the cryostat vessel (3)(4)(5)(6)(7)(8). Unfortunately, the use of active shielding layer(s) occupies vital space inside the bore of the magnet, increases system cost, and reduces gradient efficiency.…”

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confidence: 99%

Longitudinal gradient coil optimization in the presence of transient eddy currents

Trakic

Lopez

et al. 2007

Magnetic Resonance in Med

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“…For a given coil efficiency, this method can produce a smaller resistance compared with an actively-shielded two-layer coil, resulting in a potentially improved thermal performance. Three-dimensional (3D) gradient coils have been proposed that connect the primary coil layer and the shielding coil layer, allowing the current flow directly from the primary coil to the shielding coil [3][4][5][6][7]. Under the same constraints as a layered coil, a 3D coil is able to increase the wire spacing and reduce the coil inductance and local heating problems [4,8,9].…”

Section: Introductionmentioning

confidence: 99%

Design of transverse head gradient coils using a layer-sharing scheme

Wang

Zhou

et al. 2017

Journal of Magnetic Resonance

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In this paper, a new design for transverse asymmetric head gradient coils is proposed for magnetic resonance imaging (MRI). Unlike the conventional coil designs where the x and y coils are placed onto separate radial layers, the new design has windings for both the x and y coils in each transverse coil layer. The coil performance using the new design was compared with the conventional coils with the same dimensions and constraints. The results showed that the new design can improve coil performance in terms of a lower inductance, lower resistance and a higher figure of merit.

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“…For a given coil efficiency, this method can produce a smaller resistance compared with an actively-shielded two-layer coil, resulting in potentially improved thermal performance. 3D gradient coils have been proposed and connect the primary coil layer and the shielding coil layer, allowing the current flow directly from the primary coil to the shielding coil [72,102,110,112,119]. Under the same constraints as a layered coil, a 3D…”

Section: Design Of Transverse Head Gradient Coils Using a Layer-sharimentioning

confidence: 99%

“…For example, the ultra-short gradient coil was designed with three-dimensional (3D) geometry for ultra-short cylindrical MRI systems, where the length of the gradient coil can be controlled throughout the design process [110]. However, compared with standard long gradient coil sets, the short, layered gradient coils (both having the same ROIs, design methods, border conditions and similar coil patterns, and so on) tend to have a dense coil pattern.…”

Section: Introductionmentioning

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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Ultrashort shielded gradient coil design with 3D geometry

Cited by 44 publications

References 20 publications

Longitudinal gradient coil optimization in the presence of transient eddy currents

Longitudinal gradient coil optimization in the presence of transient eddy currents

Design of transverse head gradient coils using a layer-sharing scheme

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

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