Eddy current effects in MRI superconducting magnets

Badea, E.A.; Craiu, Ovidiu

doi:10.1109/20.582501

Cited by 17 publications

(13 citation statements)

References 3 publications

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“…When pulsing magnetic field gradients during MRI spatial encoding, multi-exponentially decaying eddy currents are always induced within the conducting system components [1][2]. These secondary magnetic fields are known to cause spatial and temporal degradation in the gradient uniformity within the imaging volume, which often result in undesired misregistration and intensity-phase variations in both images and spectra.…”

Section: Introductionmentioning

confidence: 99%

Tailoring magnetic field gradient design to magnet cryostat geometry

Trakic¹,

Liu²,

Lopez³

et al. 2006

2006 International Conference of the IEEE Engineering in Medicine and Biology Society

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Eddy currents induced within a MagneticResonance Imaging (MRI) cryostat bore during pulsing of gradient coils can be applied constructively together with the gradient currents that generate them, to obtain good quality gradient uniformities within a specified imaging volume over time. This can be achieved by simultaneously optimizing the spatial distribution and temporal pre-emphasis of the gradient coil current, to account for the spatial and temporal variation of the secondary magnetic fields due to the induced eddy currents. This method allows the tailored design of gradient coil/magnet configurations and consequent engineering trade-offs. To compute the transient eddy currents within a realistic cryostat vessel, a low-frequency Finite-Difference Time-Domain (FDTD) method using Total-Field Scattered-Field (TFSF) scheme has been performed and validated.

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Section: Introductionmentioning

confidence: 99%

Tailoring magnetic field gradient design to magnet cryostat geometry

Trakic¹,

Liu²,

Lopez³

et al. 2006

2006 International Conference of the IEEE Engineering in Medicine and Biology Society

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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%

“…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.…”

mentioning

confidence: 99%

“…), 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.…”

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confidence: 99%

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Longitudinal gradient coil optimization in the presence of transient eddy currents

Trakic

Lopez

et al. 2007

Magnetic Resonance in Med

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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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“…Because of the degradation effects, eddy currents are considered one of the major bottlenecks in the design and construction of MRI scanners [11,15], and therefore need to be controlled as much as possible.…”

Section: Research Problem and Motivationmentioning

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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Eddy current effects in MRI superconducting magnets

Cited by 17 publications

References 3 publications

Tailoring magnetic field gradient design to magnet cryostat geometry

Tailoring magnetic field gradient design to magnet cryostat geometry

Longitudinal gradient coil optimization in the presence of transient eddy currents

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

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