Asymmetric gradient coil design for use in a short, open bore magnetic resonance imaging scanner

Advancements in Gradient System Performance for Clinical and Research MRI

In magnetic resonance imaging (MRI), spatial field gradients are applied along each axis to encode the location of the nuclear spin in the frequency domain. During recent years, the development of new gradient technologies has been focused on the generation of stronger and faster gradient fields for imaging with higher spatial and temporal resolution. This benefits imaging methods, such as brain diffusion and functional MRI, and enables human imaging at ultra-high field MRI. In addition to improving gradient performance, new technologies have been presented to minimize peripheral nerve stimulation and gradient-related acoustic noise, both generated by the rapid switching of strong gradient fields. This review will provide a general background on the gradient system and update on the state-of-the-art gradient technology. EVIDENCE LEVEL: 5 TECHNICAL EFFICACY: Stage 1.

Overview of Methods for Noise and Heat Reduction in MRI Gradient Coils

Magnetic resonance imaging (MRI) gradient coils produce acoustic noise due to coil conductor vibrations caused by large Lorentz forces. Accurate sound pressure levels and modeling of heating are essential for the assessment of gradient coil safety. This work reviews the state-of-the-art numerical methods used in accurate gradient coil modeling and prediction of sound pressure levels (SPLs) and temperature rise. We review several approaches proposed for noise level reduction of high-performance gradient coils, with a maximum noise reduction of 20 decibels (dB) demonstrated. An efficient gradient cooling technique is also presented.

A Dichotomization Winding Scheme on a Novel Asymmetric Head Gradient Coil Design With an Improved Force and Torque Balance

The head gradient coil is advantageous for brain imaging compared to the conventional whole-body gradient coil. It is usually asymmetrically designed for the accommodation of human shoulders. The asymmetric head coil has a specific issue associated with an unbalanced force/torque that requires minimization for imaging applications. This paper will improve the force and torque balance solution and propose a dichotomization winding scheme to augment the coil slew rate. A square force and torque optimization enables the available balanced asymmetric head gradient coil design, with a force and torque approaching the minimum level. Subsequently, two practical parallel connection winding schemes were quantitatively analyzed and evaluated. The results show that the proposed dichotomization winding scheme can increase the slew rate to almost twice that of the conventional winding counterpart, without obviously influencing the magnetic field performance.

Numerical Design of High-Efficiency Whole-Body Gradient Coils With a Hybrid Cylindrical-Planar Structure

In this paper, a set of novel whole-body gradient coils is designed for cylindrical magnetic resonance imaging (MRI) systems. For the sake of high coil efficiency, the design scheme focuses on the imaging volume occupied by the patient, discounting the large space under the patient bed that is unused during imaging. To further improve the coil performance, the coils are designed on an unconventional, hybrid cylindrical-planar structure, where the primary coils are arranged on a chord-truncated cylindrical former while the shielding coils remain on the cylindrical assemblies. In this new coil configuration, the primary layers make the best possible use of the space closest to the imaging volume, whilst the shielding layers are positioned the furthest from the primary layers; thus, the best gradient field/current ratio can be obtained for the body coil configuration. Using a boundary element method, a full gradient set was designed to demonstrate the effectiveness of the proposed scheme. Compared with conventional designs, the new approach provides significantly improved coil performance. For the three gradient axes, the inductance was reduced by 25%-50%, the resistance was decreased by 19%-39%, and the minimum wire distance was increased by 5.2%-45.5%. In terms of shielding effect, the maximum stray fields of the X- and Y-gradient coils are reduced by 79.5% and 38.7%, respectively. It is concluded that the new design is capable of producing high-quality gradients with less eddy currents and thermal heating concerns, being suitable for MRI applications demanding a high gradient performance.

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.

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.

A spiral, bi-planar gradient coil design for open magnetic resonance imaging

PURPOSE

To design planar gradient coil for MRI applications without discretization of continuous current density and loop-loop connection errors.

METHODS

In the new design method, the coil current is represented using a spiral curve function described by just a few control parameters. Using a proper parametric equation set, an ensemble of spiral contours is reshaped to satisfy the coil design requirements, such as gradient linearity, inductance and shielding.

RESULTS

In the given case study, by using the spiral coil design, the magnetic field errors in the imaging area were reduced from 5.19% (non-spiral design) to 4.47% (spiral design) for the transverse gradient coils, and for the longitudinal gradient coil design, the magnetic field errors were reduced to 5.02% (spiral design). The numerical evaluation shows that when compared with conventional wire loop, the inductance and resistance of spiral coil was reduced by 11.55% and 8.12% for x gradient coil, respectively.

CONCLUSION

A novel spiral gradient coil design for biplanar MRI systems, the new design offers better magnetic field gradients, smooth contours than the conventional connected counterpart, which improves manufacturability.

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.