ISMRM 24th Annual Meeting & Exhibition • 07-13 May 2016 • Singapore

Scientific Session: B0 Shimming

Friday, May 13, 2016
Nicoll 3
08:00 - 10:00
Moderators: Jurgen Hennig, Hoby Hetherington

An Efficient 3D RF Simulation Tool for Dielectric Shimming Optimization
Jeroen van Gemert1, Wyger Brink2, Andrew Webb2, and Rob Remis1
1Circuits & Systems, University of Technology, Delft, Netherlands, 2Radiology, Leiden University Medical Center, Leiden, Netherlands
High permittivity materials, in the form of “dielectric pads” are used in neuroimaging and body applications to improve B1+ homogeneity and intensity or to reduce corresponding SAR measures. In 3D, systematic pad design is computationally intensive with very long associated simulation times. We propose a hybrid solution to this problem by combining the flexibility of FDTD to model complex background configurations (coil/shield/subject) with an integral equation approach that takes the presence of a dielectric pad into account. This solution leads to speed up factors of 30 – 40 compared with conventional FDTD approaches and enables effective 3D dielectric pad design. 

A Comparison of Optimization Algorithms for Localized in-vivo B0 Shimming
Sahar Nassirpour1,2, Paul Chang1,2, Ariane Fillmer3,4, and Anke Henning1,3
1Max Planck Institute For Biological Cybernetics, Tübingen, Germany, 2IMPRS for Cognitive and Systems Neuroscience, Eberhard Karls University of Tübingen, Tübingen, Germany, 3Institute for Biomedical Engineering, UZH and ETH Zürich, Zürich, Switzerland, 4Physikalisch-Technische Bundesanstalt, Berlin, Germany
This work presents a study on the performance of several least-squares optimization algorithms used for localized in-vivo B0 shimming. Seven different algorithms were tested in 4 different shim volumes in the brain: global shimming region, single slice, and single voxels in two different positions with 3rd order shimming at 7T. Each algorithm's robustness and convergence were tested against noisy inputs and different starting values. The results give an interesting overview of the properties of each algorithm and their applicability. The regularized iterative inversion algorithm proves to be the best algorithmic approach suited to this problem.

Full matrix pre-emphasis for higher-order dynamic shimming with 1 kHz bandwidth
Laetitia Vionnet1, Yolanda Duerst1, Signe Johanna Vannesjo1,2, Simon Gross1, and Klaas Paul Pruessmann1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland, 2FMRIB centre, Nuffield Department of Clinical Neuroscience, University of Oxford, Oxford, United Kingdom
Full matrix pre-emphasis was used for slice-wise dynamic shimming.

Multi-Coil B0 shimming of the Human Heart: A Theoretical Assessment - Permission Withheld
Wolfram Mattar1, Christoph Juchem2, Maxim Terekhov3, and Laura Schreiber3
1Department of Radiology, Section of Medical Physics, Johannes Gutenberg University Medical Center, Mainz, Germany, 2Departments of Radiology and Imaging Sciences, and Neurology, Yale University School of Medicine, New Haven, CT, United States, 3Department of Cellular and Molecular Imaging, Comprehensive Heart Failure Center, Wuerzburg, Germany
This study entails a comprehensive, theoretical analysis of B0 shimming capabilities in the human heart. Three-dimensional B0 distributions over the in vivo human heart are addressed with various spherical harmonic and multi-coil shimming (shimming with individual placed magnetic coils to modify the B0 field) approach in a static, dynamic and a hybrid fashion. The results of the study show that, as expected, the global standard static spherical harmonic shimming (clinical standard) are generally inferior in comparison with the results of specifically tailored and customized shimming methods based on dynamic and multi-coil approaches.

B0 shimming at 9.4T using a multicoil approach – coil design with genetic algorithm
Irena Zivkovic1, Christian Mirkes1,2, and Klaus Scheffler1,2
1High Field MRI Department, Max Planck Institute for Biological Cybernetics, Tuebingen, Germany, 2Dept. for Biomedical Magnetic Resonance, University of Tuebingen, Tuebingen, Germany
It is a big challenge to produce as homogeneous as possible B0 static magnetic field. Susceptibility differences between the tissue and air introduce inhomogeneities especially pronounced at high fields. Recently proposed close fitting array of circular loops provide improvement in B0 shimming. Based on the same concept, we proposed coil elements with irregular shape. The shape of the coils was designed by using of genetic algorithm. Theoretical investigation showed that performance of the 16 channel array of irregular elements was comparable or better than 48 channel array consisting loop elements.

Improving the efficiency of integrated RF-shim arrays using hybrid coil designs and channel placement and compression via a genetic algorithm
Jason P Stockmann1, Bastien Guerin1,2, and Lawrence L Wald1,2
1A. A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, 2Harvard Medical School, Boston, MA, United States
Integrated RF-shim coils combine RF receive arrays and matrix shim arrays into a single set of close-fitting loops, provide a promising alternative to spherical harmonic shim coils for compensating dynamic high-order B0 offsets in the brain.  However, the potentially large design space for optimizing these arrays remains little explored.  In this work, we investigate ways to improve the efficiency of RF-shim coils by (a.) creating “hybrid” RF-shim arrays that use additional shim-only loops over the face for targeted shimming of the frontal lobes and (b.) using a genetic algorithm to choose optimal subarrays of coils for shimming, thus reducing hardware complexity.

Optimization of geometry for combined RF/shim coil arrays for the spinal cord
Grégoire Germain1, Jason Stockmann2, Ryan Topfer1, Lawrence L Wald2,3, Nikola Stikov1,4, and Julien Cohen-Adad1,5
1Institute of Biomedical Engineering, École Polytechnique de Montréal, Montréal, QC, Canada, 2Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Athinoula A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States, 3Harvard Medical School, Boston, MA, United States, 4Montreal Heart Institute, Université de Montréal, Montréal, QC, Canada, 5Functional Neuroimaging Unit, CRIUGM, Université de Montréal, Montréal, QC, Canada
Spatial variations of B0 in the region of the spinal cord are known to cause many artifacts. Local combined RF/shim coil array could provide an alternative to spherical harmonic shim coil. Here, we simulated several realistic coil array geometries for spinal cord imaging and demonstrated that arrays of 16 coils could outperform 3rd order spherical harmonic shimming in the ROI. Simulations also revealed that precise configurations for the coils can improve shimming performance without SNR loss.

Generating unilateral field modulation for MRI using a pyrolytic-graphite-based Halbach array
Richard Bowtell1
1University of Nottingham, Nottingham, United Kingdom
A flat Halbach array consisting of an array of long, thin permanent magnets whose magnetization orientation varies linearly with position, has the interesting property of generating a unilateral field perturbation.  Such a pattern of field variation could be usefully employed in MRI, for example for attenuating signals from surface structures. Here we show that a Halbach array can be formed by exposing  appropriately oriented strips of material with anisotropic magnetic susceptibility to a strong static field, and also validate the predicted behaviour in experiments carried at 3T using a 40-element structure formed from  pieces of pyrolytic graphite sheet.

A two-stage RF shimming method for 7T human first-pass myocardial perfusion
Yuehui Tao1, Aaron T. Hess1, and Matthew D. Robson1
1OCMR, Radcliffe Department of Medicine, University of Oxford, Oxford, United Kingdom
RF shimming usually aims at uniform transmit field. For 7T human first-pass myocardial perfusion, maximizing the lowest transmit field strength is beneficial. The shimming optimization cost function corresponding to the lowest strength is not smooth, leading to impractically long shimming calculation if all transmit magnitudes and phases are optimized simultaneously. We evaluate several optimization strategies for static RF shimming for maximizing the lowest transmit field strength within a practical duration, and propose a two-stage method to accelerate in situ shimming calculation. In our experiments, this proposed method consistently found near optimal solutions in less than 10 seconds.

Open-source, low-cost, flexible, current feedback-controlled driver circuit for local B0 shim coils and other applications
Nick Arango1, Jason P Stockmann2, Thomas Witzel2,3, Lawrence Wald2,3, and Jacob White1
1Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, 2A. A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, 3Harvard Medical School, Boston, MA, United States
We demonstrate a low-cost (<$75/channel), open source, scalable, multi-channel current supply board that can provide up to 8 amps per channel for driving inductive loads such as local B0 shim coils. The design shows excellent stability while retaining sufficient gain in the audio frequency range to reject disturbances (e.g. gradient switching) and maintain stable output current.

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