Jeroen van Gemert¹, Wyger Brink², Andrew Webb², and Rob Remis^1
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. 

Sahar Nassirpour^1,2, Paul Chang^1,2, Ariane Fillmer^3,4, and Anke Henning^1,3
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.

Laetitia Vionnet¹, Yolanda Duerst¹, Signe Johanna Vannesjo^1,2, Simon Gross¹, and Klaas Paul Pruessmann^1
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.

Wolfram Mattar¹, Christoph Juchem², Maxim Terekhov³, and Laura Schreiber^3
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.

Irena Zivkovic¹, Christian Mirkes^1,2, and Klaus Scheffler^1,2
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.

Jason P Stockmann¹, Bastien Guerin^1,2, and Lawrence L Wald^1,2
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.

Grégoire Germain¹, Jason Stockmann², Ryan Topfer¹, Lawrence L Wald^2,3, Nikola Stikov^1,4, and Julien Cohen-Adad^1,5
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 3^rd order spherical harmonic shimming in the ROI. Simulations also revealed that precise configurations for the coils can improve shimming performance without SNR loss.

Richard Bowtell^1
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.

Yuehui Tao¹, Aaron T. Hess¹, and Matthew D. Robson^1
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.

Nick Arango¹, Jason P Stockmann², Thomas Witzel^2,3, Lawrence Wald^2,3, and Jacob White^1
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.