A numerical study of the acoustic radiation due to eddy current-cryostat interactions

Wang, Y.; F, Liu; Zhou, Xiaorong; Yu, Li; Crozier, Stuart

doi:10.1002/mp.12261

Cited by 12 publications

(4 citation statements)

References 37 publications

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“…In addition to blocking the noise propagation, attenuating noise level from the noise source is another scheme of noise elimination. One method of this category is active noise control [10][11][12][13][14], which generates active acoustic noise with equal amplitude and opposite phase to counteract the original noise. However, the results showed that this method was mainly effective at low frequency due to its low modal density and long wavelength.…”

Section: Introductionmentioning

confidence: 99%

Acoustic Control through Gradient Coil Design Using a Finite-Difference-Based Method for MRI

Kang

Xia

2021

Discrete Dynamics in Nature and Society

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An acoustic control scheme is proposed in this paper through the process of gradient coil design for magnetic resonance imaging (MRI). With a finite-difference-based method, the stream function and coil displacement caused by fast gradient switching can be unified by a simplified momentum equation, which can be incorporated into the conventional gradient coil design. A three-dimensional transverse gradient coil with an edge-connected cylindrical structure is used as a design example to verify the proposed design method. In addition, an acoustic model is established to simulate the sound pressure level (SPL).In the model, two hemispherical air volumes are added flush with the ends of the cylindrical main magnet to mimic the free propagation of sound waves on the boundaries. The simulation results show that by optimizing coil displacement, the overall SPL can be attenuated by 4 dB over the frequency range from 0 to 3000 Hz with the displacement reduced by about 50%, at the cost of a figure of merit (FOM) loss by about 8%. Therefore, the proposed acoustic control scheme can be used as a complement to conventional acoustic control methods for further noise reduction.

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Section: Introductionmentioning

confidence: 99%

Acoustic Control through Gradient Coil Design Using a Finite-Difference-Based Method for MRI

Kang

Xia

2021

Discrete Dynamics in Nature and Society

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“…Gradient coils, as a vital component of an MRI system, are usually designed to provide a linear and orthogonal gradient field along the three axes to spatially encode the information of tissue [2,3]. Gradient coil that using passive shield technology cannot prevent eddy currents on the surrounding conductors when the current switching quickly, which may lead to serious image distortion [4,5], energy loss [6], mechanical vibration [7], and eddy artifact [8]. Comparatively, active masking technology has a better image and lower noise caused by eddy currents than passive shielded coils, which is widely used in the MRI systems.…”

Section: Introductionmentioning

confidence: 99%

A theoretical framework of gradient coil designed to mitigate eddy currents for a permanent magnet MRI system

Zhang

Wang

Yan

2022

THC

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BACKGROUND: High installation and operating cost have limited applications for many circumstances. In practice, primary and shielding coils cannot insert into the magnet pole simultaneously owing to deficient workspace for the planar permanent MRI systems OBJECTIVE: To minimize eddy currents induced in the resist-eddy current plates and pole piece when the gradient coil current switches on and off rapidly. METHODS: A theoretical framework that have minimum power dispassion and magnetic energy with eddy plate is proposed for a planar gradient coil. The mirror image of the magnetostatic model is substituted into the stream function for designing a minimum power dispassion planar gradient coil. A finite-difference is used to formulate the coil distribution that makes magnetic field similar to the required magnetic field for gradient coil design. RESULTS: A coil designed with actively shielded was simulated and compared with the designed gradient coils using mirror image theory and piece pole effect. According to the numerical evaluation of the x and z coils, the operating currents in the cases were reduced to 34.4% using magnetostatic mirror-image method to replay the active shielding. Moreover, there was a significant improvement on the shielding effect when added to resistive eddy current plate. CONCLUSIONS: Using the magnetostatic mirror image theory and mirror-image model, the current density function that could not only gives the minimum power dissipation and magnetic energy with the presence of the eddy plate and pole piece effect, but also provides excellent coil performance compared with active shielding solution.

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“…In practice, the continuous-current density-based method is usually combined with other numerical algorithms such as the boundary element method (BEM) [6,[31][32][33] or the finite difference method (FDM) [34][35][36]. These numerical methods produce continuous current density profiles that can be used to obtain a coil-winding pattern with the aid of a stream function.…”

Section: Gradient Coil Design Methodsmentioning

confidence: 99%

“…Some scanners use a vacuum device to block the airborne noise propagating to the patients' ears [66,70]. Mounting the gradient assembly independently on the floor [9,66] or reducing the eddy currents [10,35] can restrain the vibration of some structures, such as the cryostat or RF coils, thus reducing the SPL. Rib stiffeners can help reinforce the gradient assembly, so the response amplitude will be reduced [71].…”

Section: Acoustic Noise In Mri -Sources and Reduction Techniquesmentioning

confidence: 99%

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

Wang¹

Self Cite

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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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A numerical study of the acoustic radiation due to eddy current-cryostat interactions

Abstract: Combining eddy current control and suggested damping scheme, the noise level in a MRI system can be effectively reduced.

Cited by 12 publications

References 37 publications

Acoustic Control through Gradient Coil Design Using a Finite-Difference-Based Method for MRI

Acoustic Control through Gradient Coil Design Using a Finite-Difference-Based Method for MRI

A theoretical framework of gradient coil designed to mitigate eddy currents for a permanent magnet MRI system

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

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