Yaohui Wang

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

Purpose To investigate the acoustic radiation due to eddy current‐cryostat interactions and perform a qualitative analysis on noise reduction methods. Methods In order to evaluate the sound pressure level (SPL) of the eddy current induced warm bore wall vibration, a Finite Element (FE) model was created to simulate the noises from both the warm bore wall vibration and the gradient coil assembly. For the SPL reduction of the warm bore wall vibration, we first improved the active shielding of the gradient coil, thus reducing the eddy current on the warm bore wall. A damping treatment was then applied to the warm bore wall to control the acoustic radiation. Results Initial simulations show that the SPL of the warm bore wall is higher than that of the gradient assembly with typical design shielding ratios at many frequencies. Subsequent simulation results of eddy current control and damping treatment application show that the average SPL reduction of the warm bore wall can be as high as 9.6 dB, and even higher in some frequency bands. Conclusions Combining eddy current control and suggested damping scheme, the noise level in a MRI system can be effectively reduced.

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

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 coils 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.

Spiral Gradient Coil Design for Use in Cylindrical MRI Systems

In magnetic resonance imaging, the stream function based method is commonly used in the design of gradient coils. However, this method can be prone to errors associated with the discretization of continuous current density and wire connections. In this paper, we propose a novel gradient coil design scheme that works directly in the wire space, avoiding the system errors that may appear in the stream function approaches. Specifically, the gradient coil pattern is described with dedicated spiral functions adjusted to allow the coil to produce the required field gradients in the imaging area, minimal stray field, and other engineering terms. The performance of a designed spiral gradient coil was compared with its stream-function counterpart. The numerical evaluation shows that when compared with the conventional solution, the inductance and resistance was reduced by 20.9 and 10.5%, respectively. The overall coil performance (evaluated by the figure of merit (FoM)) was improved up to 26.5% for the x -gradient coil design; for the z-gradient coil design, the inductance and resistance were reduced by 15.1 and 6.7% respectively, and the FoM was increased by 17.7%. In addition, by directly controlling the wire distributions, the spiral gradient coil design was much sparser than conventional coils.

Design of transverse head gradient coils using a layer-sharing scheme

In this paper, a new design for transverse asymmetric head gradient coils is proposed for Magnetic Resonance Imaging (MRI). Unlike the conventional coil designs where the x and y coils are placed onto separate radial layers, the new design has windings for both the x and y coils in each transverse coil layer. The coil performance using the new design was compared with the conventional coils with the same dimensions and constraints. The results showed that the new design can improve coil performance in terms of a lower inductance, lower resistance and a higher figure of merit.

Numerical simulations on active shielding methods comparison and wrapped angle optimization for gradient coil design in MRI with enhanced shielding effect

The switching of a gradient coil current in magnetic resonance imaging will induce an eddy current in the surrounding conducting structures while the secondary magnetic field produced by the eddy current is harmful for the imaging. To minimize the eddy current effects, the stray field shielding in the gradient coil design is usually realized by minimizing the magnetic fields on the cryostat surface or the secondary magnetic fields over the imaging region. In this work, we explicitly compared these two active shielding design methods. Both the stray field and eddy current on the cryostat inner surface were quantitatively discussed by setting the stray field constraint with an ultra-low maximum intensity of 2 G and setting the secondary field constraint with an extreme small shielding ratio of 0.000 001. The investigation revealed that the secondary magnetic field control strategy can produce coils with a better performance. However, the former (minimizing the magnetic fields) is preferable when designing a gradient coil with an ultra-low eddy current that can also strictly control the stray field leakage at the edge of the cryostat inner surface. A wrapped-edge gradient coil design scheme was then optimized for a more effective control of the stray fields. The numerical simulation on the wrapped-edge coil design shows that the optimized wrapping angles for the x and z coils in terms of our coil dimensions are 40° and 90°, respectively.

An actively shielded gradient coil design for use in planar MRI systems with limited space

In planar magnetic resonance imaging (MRI) systems, gradient coils are usually placed within a very limited space owing to the physical constraints of the small gap size (pole-pole) distance of the permanent magnet. Typically, the unshielded or partially shielded design scheme is adopted to generate required magnetic fields with reduced system costs. However, non-fully shielded coils can induce large eddy currents on the surrounding metal structures, including magnet poles, that significantly impact the imaging performance. This paper elaborates a new design strategy to resolve the limited space problem. Using the peripheral sections of the MRI system, a set of actively shielded gradient coils are purposefully designed. Between the two magnet poles, the actively shielded gradient coils occupy merely four coil layers (six coil layers are usually required), which offers an excellent shielding effect, thus reducing the image distortions. The saved space can be used to integrate a high-efficient cooling system. Moreover, the design scheme does not significantly increase the fabricating complexity.

Optimization magnetic resonance imaging shim coil using second derivative discretized stream function

In Magnetic Resonance Imaging (MRI) equipment, a set of shim coils are designed to generate specific magnetic fields, thus eliminating harmonic components of magnetic field to obtain a high level homogeneous magnetic field within the region of interesting (ROI). In the electromagnetic design process, in order to produce the desired magnetic field, the deviation between the calculated magnetic field of shim coil and the theoretical magnetic field is treated as a kind of traditional objective functions to optimize the distribution of current density on the surface of shim coil skeleton. However, such function is ill-posed because of the overdetermined or underdetermined system of equations. The regularization method is commonly used to solve such problem by constructing the regularization term. This article proposes a new iterative optimization method for the design of shim coils in MRI. Based on the boundary element method (BEM), the discretized stream functions can be obtained by discretizing the surface of coil skeleton using a set of triangular elements. As the regularization term, the second derivative stream function is included in the minimization of the deviation between calculated magnetic fields and target magnetic fields. The distribution of coil which meets the design requirements can be obtained by using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm. At last, the cubic spline interpolation is used to make lines as smooth as possible to be processed. In this article, the proposed method was employed to design two kinds of room temperature shim coils for cylindrical and/or biplanar MRI shim coil system. The simulation results demonstrate that the proposed method is effective and practical.