Simulation Environment to Predict the Effect of Eddy Currents on Image Quality in MRI

Lechner-Greite, Silke M.; Mathieu, Jean Baptiste; Amm, Bruce

doi:10.1109/tasc.2012.2186781

Cited by 3 publications

(1 citation statement)

References 13 publications

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“…The gradient coil optimization was undertaken using Matlab 2015a, and the design can be completed in 10 minutes on a computer equipped with Intel Core i7-3770CPU and 16 G RAM. Table 4.2 [108,121]. The coil designed with the long secondary coil dissipates less power on the warm bore, first cold shield and secondary cold shield (Figure 4.5).…”

Section: Methods 421 Coil Design Methodsmentioning

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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Section: Methods 421 Coil Design Methodsmentioning

confidence: 99%

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

Tang¹

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An improved asymmetric gradient coil design for high-resolution MRI head imaging

Tang

Freschi

et al. 2016

Phys. Med. Biol.

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For head magnetic resonance imaging, local gradient coils are often used to achieve high solution images. To accommodate the human head and shoulder, the head gradient coils are usually designed in an asymmetric configuration, allowing the region-of-uniformity (ROU) close to the coil's patient end. However, the asymmetric configuration leads to technical difficulties in maintaining a high gradient performance for the insertable head coil with very limited space. In this work, we present a practical design configuration of an asymmetric insertable gradient head coil offering an improved performance. In the proposed design, at the patient end, the primary and secondary coils are 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 are not connected, to permit access to shim trays, cooling system piping, cabling, and so on. The new designs are compared with conventional coil configurations and the simulation results show that, with a similar field quality in the ROU, the proposed coil pattern has 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.

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Minimizing eddy currents induced in the ground plane of a large phased-array ultrasound applicator for echo-planar imaging-based MR thermometry

Lechner-Greite¹,

Hehn

Werner

et al. 2016

J Ther Ultrasound

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BackgroundThe study aims to investigate different ground plane segmentation designs of an ultrasound transducer to reduce gradient field induced eddy currents and the associated geometric distortion and temperature map errors in echo-planar imaging (EPI)-based MR thermometry in transcranial magnetic resonance (MR)-guided focused ultrasound (tcMRgFUS).MethodsSix different ground plane segmentations were considered and the efficacy of each in suppressing eddy currents was investigated in silico and in operando. For the latter case, the segmented ground planes were implemented in a transducer mockup model for validation. Robust spoiled gradient (SPGR) echo sequences and multi-shot EPI sequences were acquired. For each sequence and pattern, geometric distortions were quantified in the magnitude images and expressed in millimeters. Phase images were used for extracting the temperature maps on the basis of the temperature-dependent proton resonance frequency shift phenomenon. The means, standard deviations, and signal-to-noise ratios (SNRs) were extracted and contrasted with the geometric distortions of all patterns.ResultsThe geometric distortion analysis and temperature map evaluations showed that more than one pattern could be considered the best-performing transducer. In the sagittal plane, the star (d) (3.46 ± 2.33 mm) and star-ring patterns (f) (2.72 ± 2.8 mm) showed smaller geometric distortions than the currently available seven-segment sheet (c) (5.54 ± 4.21 mm) and were both comparable to the reference scenario (a) (2.77 ± 2.24 mm). Contrasting these results with the temperature maps revealed that (d) performs as well as (a) in SPGR and EPI.ConclusionsWe demonstrated that segmenting the transducer ground plane into a star pattern reduces eddy currents to a level wherein multi-plane EPI for accurate MR thermometry in tcMRgFUS is feasible.

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Simulation Environment to Predict the Effect of Eddy Currents on Image Quality in MRI

Cited by 3 publications

References 13 publications

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

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

An improved asymmetric gradient coil design for high-resolution MRI head imaging

Minimizing eddy currents induced in the ground plane of a large phased-array ultrasound applicator for echo-planar imaging-based MR thermometry

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