Ling Xia¹, Minhua Zhu¹, Guofa Shou¹, Feng Liu², and Stuart Crozier^2
1Department of Biomedical Engineering, Zhejiang University, Hangzhou, China, People's Republic of, ²School of Information Technology & Electrical Engineering, University of Queensland, Brisbane, Australia

This paper presents a finite difference (FD) based method for the design of gradient coils in MRI. The design method firstly uses the FD approximation to describe the current density of the coil space and then employs the stream function method to extract the coil pattern. During the numerical implementation, a linear equation has been constructed and solved using a regularization scheme. This abstract briefly describes the algorithm details through X-gradient coil design examples. The proposed gradient coil design scheme can be integrated into a finite-difference based EM framework, which can handle multi-physics including RF and gradient fields presented in the MR engineering.

Feng Jia¹, Zhenyu Liu², and Jan G Korvink^1,3
1Freiburg Institute of Advanced Studies (FRIAS), University of Freiburg, Freiburg, Germany, ²Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP), Chinese Academy of Sciences, Changchun, China, People's Republic of, ³Department of Microsystems Engineering (IMTEK), University of Freiburg, Freiburg, Germany

Based on an alternative differentiable objective function which is related with the maximum of current density, an iterative optimization method is proposed to design a biplanar micro-coil in this abstract. The main goal of this method is to increase the minimal distance between adjacent conductors of the coil. The planar surface can be discretized using a high-order triangular mesh. The magnetic field is calculated using the Biot-Savart law where the surface current density is expressed using a stream function and the surface integration is implemented using surface numerical integration based on the shape functions of the finite element.

Michael Stephen Poole¹, Peter While², Hector Sanchez Lopez¹, Larry Forbes², and Stuart Crozier^1
1ITEE, University of Queensland, Brisbane, QLD, Australia, ²Mathematics, University of Tasmania, Hobart, Tasmania, Australia

A new method to design gradient and shim coils was recently presented that spreads out close wires. This has the effect of being able to increase the coils efficiency when limited by minimum wire size. Also, it can be used to reduce the peak temperature in a coil. Here we investigate the behaviour of such coils on the interval between standard minimum power and the new minimax current density coils. Performance properties and heating experiments and simulations were performed and the results analysed.

Zhen Yao¹, Aaron Shojinaga¹, Yong Wu¹, Timothy Eagan², Shmaryu Shvartsman², Thomas Chmielewski², and Robert Brown^1
1Department of Physics, Case Western Reserve University, Cleveland, OH, United States, ²ViewRay Inc., Oakwood Village, OH, United States

A design goal for MRI is to limit the eddy currents generated on the RF coils and shields due to the gradient coils and in particular to suppress the associated eddy current heating. It is important, for example, to avoid hot spots in the proximity of the patient as well as in the vicinity of soldering joints. We simulate the generation of eddy current and corresponding heat distributions resulting from gradient magnetic flux, and make successful comparisons to experimental measurements. Specific patterns of cuts for reducing troublesome heating locations are studied.

Peter Klauer^1,2, Martin Andreas Rückert^1,2, Patrick Vogel^1,2, Walter H. Kullmann¹, Peter M. Jakob^2,3, and Volker Christian Behr^2
1Electrical Engineering, University of Applied Sciences Würzburg-Schweinfurt, Schweinfurt, Germany, ²Department of Experimental Physics 5 (Biophysics), University of Würzburg, Würzburg, Germany, ³Research Center for Magnetic Resonance Bavaria e.V (MRB), University of Würzburg, Würzburg, Germany

This work presents a new gradient design for magnetic particle imaging that will allow performing dynamic imaging in a linear sampling scheme rather than a Lissajous trajectory by generating a traveling wave. Aside from the simplified trajectory, this approach provides the possibility of increasing the field of view arbitrarily in one dimension without increasing the sampling time.

Feng Liu¹, Jianfeng Zhu², Ran Zhang³, Ling Xia², and Stuart Crozier^1
1School of Information Technology and Electrical Engineering, University of Queensland, Brisbane, Queensland, Australia, ²Department of Biomedical Engineering, Zhejiang University, Hangzhou, Zhejiang, China, People's Republic of, ³School of Electrical Engineering, Shandong University, Jinan, Shandong, China, People's Republic of

This paper presents a new passive shimming (PS) design scheme for the correction of static magnetic field inhomogeneities in MRI systems. The PS procedure usually employs field-based (a) or harmonics-based (b) methods to find an optimum iron piece configuration to improve the field uniformity in the imaging region. For the PS technique (a), the peak-to-peak field inhomogeneity is minimized and the harmonic components are inherently unconstrained; in the technique (b), selected unwanted harmonics are minimized and the overall field uniformity is consequently reduced. The approach (a) usually provides good field homogeneity but lacks flexibility in managing all terms of spherical harmonic field expansion; the approach (b) is capable of controlling targeted low-order harmonic terms but can have difficulty in producing optimal overall field homogeneity and in controlling high-order harmonics. The proposed algorithm attempts to combine the strengths of these two methods for a better PS solution. During the PS implementation, an explicit expression of the system matrix with both field and harmonics sensitivities is generated, and then an optimization procedure is performed for the determination of shim piece thicknesses and locations. An experimental study showed that the hybrid method provided good quality, flexible solutions for controlling individual harmonics impurities and also overall field uniformity.

Mohan Lal Jayatilake^1,2, Judd Storrs^1,3, Jeff Osterhage¹, and Jing-Huei Lee^1,4
1Center for Imaging Research, University of Cincinnati, Cincinnati, OH, United States, ²Department of Physics, University of Cincinnati, Cincinnati, OH, United States,³Department of Psychiatry and Behavioural Neuroscience, University of Cincinnati, Cincinnati, OH, United States, ⁴School of Energy, Environmental, Biological, and Medical Engineering, University of Cincinnati, Cincinnati, OH, United States

Magnetic susceptibility variation can lead to B0 field inhomogeneity and cause artifacts including signal dropout and image distortions. We introduce a method that the addition of the third order passive shim system along with the first and second active shimming significantly improves the homogeneity of the static magnetic field within the human brain.

Chad Tyler Harris¹, William B Handler¹, and Blaine A Chronik^1
1Physics and Astronomy, University of Western Ontario, London, Ontario, Canada

The Boundary Element (BE) method is proving to be an extremely powerful and versatile tool in the electromagnetic design of gradient, shim, and shielding coils. We have been strongly motivated by its many strengths to optimize the computational implementation of the math since its first use. We have found that a significant decrease in computation time is achievable relatively easily. In this study, we describe the most important steps in the optimization of this algorithm, and show that it is able to produce detailed wire patterns in a matter of seconds.

Rahul Dewal¹, Zhipeng Cao¹, Christopher Sica², Christopher Collins², and Qing Yang^1,2
1Bioengineering, The Pennsylvania State University, Hershey, PA, United States, ²Radiology, The Pennsylvania State University, Hershey, PA, United States

Passive shimming can be used to correct for the static B₀ field inhomogeneities caused by air-tissue boundaries in the sinus cavities and ear canals. By placing pieces of shim material with high susceptibility values into the magnet at precise locations, the magnetic field can be perturbed such that higher-order inhomogeneities are corrected. This method uses a numerical optimization algorithm in conjunction with a Fourier transform-based fast magnetic field calculation method to determine the optimal configuration of shim materials for an agar gel phantom. Optimized active shims are calculated alongside passive shim optimization to form a synergistic shimming approach.

Hector Sanchez-Lopez¹, Michael Poole¹, Limei Liu¹, and Stuart Crozier^1
1School of Information Technology & electrical Engineering, The University of Queensland, Brisbane, QLD, Australia

In this paper we presented a new and fast eddy current simulation method. The approach is valid for currents induced in thick and split cylinders of finite length induced by coils of arbitrary geometry. The method divides thick conducting axially split/continues cylinders into thin layers (thinner than the skin depth) and expresses the current density on each as a normalized Fourier series. The coupling between each mode with every other is modeled with an inductive network method calculated in Fourier space. In this way, the eddy currents induced in realistic cryostat surfaces by coils of arbitrary geometry can be simulated.

Paul Freitag^1
1Bruker BioSpin MRI GmbH, Ettlingen, Germany

Classic Cartesian phase encoding schemes cause high short temporal power dissipation in the gradient system in the K-space edges. Optimizing the phase encoding scheme allows to achieve an approximately constant averaged power dissipation. Applying this to fast 3D imaging sequences, a smaller field of view or shorter repetition times can be selected without damaging the gradient system. The proposed optimization algorithm can easily be integrated into routine sequences allowing performance gains of up to 20% regarding minimum repetition time.

Michael Stephen Poole¹, Hector Sanchez Lopez¹, Stuart Crozier¹, Iwao Nakajima², and Shin-ichi Urayama^3
1ITEE, University of Queensland, Brisbane, QLD, Australia, ²Takashima Seisakusho Co., Ltd., Tokyo, Japan, ³Human Brain Research Center, Kyoto University Graduate School of Medicine, Kyoto, Japan

Gradient and shim coils were designed for a high-temperature superconductor magnet system. Particular attention was paid to minimisation of the eddy currents generated in the cryostat of the experimental magnet. The constrained space and requirement for wide wire spacing in the design dictated the use of minimax current density technique which more than doubled the efficiency of the transverse gradient coils.

Dustin W Haw¹, Chad T Harris¹, William Bradfield Handler¹, and Blaine A Chronik^1
1Physics and Astronomy, University of Western Ontario, London, Ontario, Canada

A simple method for calculating shielding coils on arbitrary geometries is presented. This new method not a time-consuming iterative method. It is a direct calculation of an energy minimizing total current density.

Peter T. While¹, Larry K. Forbes¹, and Stuart Crozier^2
1School of Mathematics and Physics, University of Tasmania, Hobart, TAS, Australia, ²ITEE, University of Queensland, Brisbane, QLD, Australia

Gradient heating and coil hot spots can result in system failure or image distortion. A previously reported method is extended to the design of asymmetric gradient coils with improved temperature distributions and reduced hot spot temperatures. The method combines an optimisation constraint derived from a spatial temperature distribution model with a relaxed fixed point iteration routine. In comparison to minimum power coils, the new coil windings are more spread out with lower maximum temperatures, at little or no cost to coil performance. For the design of coils with different thermal material properties, considerably different winding pattern solutions are obtained.

Daiki Tamada¹, Yasuhiko Terada¹, and Katsumi Kose^1
1Institute of applied physics, University of Tsukuba, Tsukuba, Ibaraki, Japan

A new method using stream function was proposed to correct inhomogeneity of the magnetic field with single-channel shim coils. The inhomogeneity achieved by the stream function method was comparable to that achieved by the target field method. Moreover, the inhomogeneity along z-axis was also corrected, which could not be done by the target field method. The method presented here provides large three dimensional homogeneity without multi-channel shim coils, which leads to the enlargement of available space for samples.

Mitsushi Abe¹, Yukinobu Imamura¹, and Hiroyuki Takeuchi^2
1Energy and Environmental Syustems Lab., Hitachi, Ltd., Hitachi, Ibaraki, Japan, ²Hitachi Medical Corp., Kashiwa, Chiba, Japan

Design of a compact planar GC for high field open MRI using the computational tool DUCAS was done. The DUCAS can treat arbitral sheet currents and was applied on the GC design. The designed GC is a ASGC. It has umbrella shaped shield coil and plane main coil with direct connection between them. These two characteristics made the GC size compact and the leakage field weak. Then we conclude this ASGC can be used as planar ASGC for high-field open MRI.