Acoustic analysis of a gradient coil winding in an MRI scanner

Shao, Wei; Mechefske, Chris K.

doi:10.1002/cmr.b.20023

Cited by 14 publications

(22 citation statements)

References 15 publications

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“…Currently, both 1.5-and 3.0-T MRI systems are most commonly used in clinical applications. The N2 ghost effect could also depend on the magnetic field strength [16] because the oscillating magnetic field gradients of EPI can result in considerable mechanical vibrations of the gradient coil assembly as the Lorentz forces exerted on the electric currents in the coil increase with increasing magnetic field, causing the N2 ghost [17]. Therefore, we expect that the N2 ghost effect could be less on both 1.5-and 3.0-T MRI systems than on the 4.0-T MRI system if the gradient system is identical.…”

Section: Discussionmentioning

confidence: 99%

Diffusion anisotropy indexes are sensitive to selecting the EPI readout-encoding bandwidth at high-field MRI

Jahng

Weiner

Schuff

2008

Magnetic Resonance Imaging

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Diffusion tensor magnetic resonance imaging (DT-MRI) is generally performed using an echo planar imaging (EPI) acquisition to map directional water diffusion. However, the oscillating magnetic field gradients of the EPI acquisition can result in considerable mechanical vibrations, which lead, in turn, to magnetic field fluctuations causing Nyquist ghosting in the EPI data. The objective of this study was to investigate effects of EPI readout gradient modulation frequency, which is directly associated with the EPI readout bandwidth (BW), on the accuracy of DT-MRI measurements in a high magnetic field system. A spherical water phantom was used to study the relationship between the EPI BW and the Nyquist ghost for a spin-echo EPI acquisition with a matrix size of 128×128, complemented by diffusion sensitization gradients of up to b=800 s/mm 2 along six directions for DT-MRI. Nine volunteers (four males and five females) were studied using EPI at different BW acquisitions. Analysis of variance was used to investigate the EPI BW effects. The phantom studies demonstrated a systematic relationship between BWs and the intensities of Nyquist ghosts. In the human brain studies, EPI BW variations substantially corrupted diffusion anisotropy indexes (i.e., fractional anisotropy and relative anisotropy) (F=10.5, P=.0001) but were unrelated to diffusion-encoding directions (F=0.14, P=.98). It was possible to minimize BW dependence (F=1.48, P=.25) by tuning the modulation frequency of the EPI readout gradient. In conclusion, diffusion anisotropic indexes are sensitive to the readout BW of EPI due to associated Nyquist ghosting. However, the effect can be minimized by tuning the modulation frequency of the EPI readout gradient, that is, the EPI BW, to a range outside the harmonics of mechanical gradient vibrations.

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Section: Discussionmentioning

confidence: 99%

Diffusion anisotropy indexes are sensitive to selecting the EPI readout-encoding bandwidth at high-field MRI

Jahng

Weiner

Schuff

2008

Magnetic Resonance Imaging

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“…Also, the acoustic pressure and the fluid particle speed have the following relationship [101]: 0 where is the fluid density; v is fluid particle speed; t is the time; is the gradient operator; and p is the acoustic pressure. At the interface, the structure speed and the fluid particle speed are identical The entity is the physical structure of the modelled object.…”

Section: Acoustic Modelling Of Mri Scannermentioning

confidence: 99%

“…The correlation between gradient magnetic field fluctuations and the acoustic noise has also been investigated [65]. It is commonly recognized that the most of the noise is generated by the gradient coils [8,56,65,67,69,101,115,[126][127][128][129][130][131][132][133][134]. In order to theoretically analyse the vibration and radiated acoustic noise of a MRI scanner, a number of gradient models have been built.…”

Section: Introductionmentioning

confidence: 99%

“…These models can be divided into two groups, namely analytical models and numerical models. In many cases, analytical models have proven to be more efficient [56,101,129] as the model parameters can be easily adjusted to simulate different conditions.…”

Section: Introductionmentioning

confidence: 99%

“…In addition, similar to typical MRI systems, the faster the gradient current switches, the louder the noise will be [11,52,101,[130][131][132][133]140].…”

Section: Introductionmentioning

confidence: 99%

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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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An analytical approach to the design of quiet cylindrical asymmetric gradient coils in MRI

Forbes

Brideson

Crozier

et al. 2007

Concepts Magn Reson

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ABSTRACT:We consider the design of asymmetric gradient coils in a conventional cylindrical bore magnetic resonance imaging system. The gradient coils are switched on and off during the scan, and are therefore subject to Lorentz forces that cause them to buckle during the switching process. This in turn creates a pressure wave within the coil, and that gives rise to acoustic noise. We present a simplified linearized model for the deflection of the coil due to electromagnetic forces, which is amenable to solution using analytical methods. Closed-form solutions for the coil deflection and the pressure pulse and noise level within the coil are obtained. These are used to design new coil winding patterns so as to reduce the acoustic noise. Sample results are shown both for unshielded and shielded gradient coils. Extensions of this model are indicated, although it is suggested that the advantages of the present closed-form solutions might then not be available, and fully numerical solutions may be needed instead.

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Acoustic analysis of a gradient coil winding in an MRI scanner

Cited by 14 publications

References 15 publications

Diffusion anisotropy indexes are sensitive to selecting the EPI readout-encoding bandwidth at high-field MRI

Diffusion anisotropy indexes are sensitive to selecting the EPI readout-encoding bandwidth at high-field MRI

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

An analytical approach to the design of quiet cylindrical asymmetric gradient coils in MRI

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