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

Abstract

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...

Thermal variation in gradient response: measurement and modeling

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A cone-shaped gradient coil design for high-resolution MRI head imaging

“…The surface S is discretised to triangles with the continued flow. Then the current density can be expressed by basis functions (Tang 2016).…”

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“…The variables C p k,ni and C s k,ni are the coefficients of the primary and shielding coils which were determined by using ( 6) and computed by the built-in MATLAB ® numerical integration function integral2 (the Math Works ® , Inc. MATLAB ® ). Given the target field points on the DSV and stray-field regions, the following linear equation can be formed from (7).…”