ISMRM 21st Annual Meeting & Exhibition ○ 20-26 April 2013 ○ Salt Lake City, Utah, USA
SCIENTIFIC SESSION
Gradients, Shims & Field Monitoring Thursday 25 April 2013
Room 151 AG | 13:30 - 15:30 | Moderators: | Richard W. Bowtell, Sebastian Littin
13:30 | 0662. | Micro Magnetic Resonance x,y-Gradient System for Microscopic Samples Markus V. Meissner1, Robert Ch. Meier1, Peter T. While1, Ali Moazenzadeh2, and Jan G. Korvink1,3 1Lab. for Simulation, IMTEK - University of Freiburg, Freiburg, Germany, 2Lab. for Microactuators, IMTEK - University of Freiburg, Freiburg, Germany, 3Freiburg Institute for Advanced Studies – FRIAS, University of Freiburg, Freiburg, Germany We present a micro-integrated gradient chip-design manufactured by using multiple, wire-bonded conductors. By placing the gradient conductors close to a microscopic sample, a particular strong magnetic field gradient can be achieved. The miniaturized gradient coils are based on a customized coil design, manufactured by using micro fabrication technology. A chip design was studied, based on a sandwich structure, consisting of straight, low profile wire-bonded gradient conductors, embedded and aligned by using permanent dry film photo-resist. The gradient conductors are integrated on a 2cmx2cm large glass substrate and the layer-based conductor alignment method enables statically determined bond-wire fixation.
13:42 | 0663. | Genuine Minimum Power Gradient Coil Design Accounting for Gaps Between Tracks or Wires Peter T. While1, Michael S. Poole2, and Jan G. Korvink1,3 1Department of Microsystems Engineering (IMTEK), Laboratory for Simulation, University of Freiburg, Freiburg im Breisgau, Baden-Württemberg, Germany, 2Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich GmbH, Jülich, Nordrhein-Westfalen, Germany, 3Freiburg Institute for Advanced Studies (FRIAS), University of Freiburg, Freiburg im Breisgau, Baden-Württemberg, Germany Gradient coils are often designed in terms of a continuous minimum power current density that is discretized to obtain the locations of coil tracks or wires. However, the existence of finite gaps between these conductors and a maximum conductor width leads to an underestimation of coil resistance. In this work, a current density mapping is proposed that accounts for these effects and is optimized to generate genuine minimum power coils post-discretization (i.e. accounting for build method). Many common coil types are surveyed and it is demonstrated that up to a 30% reduction in power dissipation is possible in certain cases.
13:54 | 0664. | Integrated Parallel Reception, Excitation, and Shimming (IPRES) Hui Han1, Allen W. Song1, and Trong-Kha Truong1 1Brain Imaging and Analysis Center, Duke University, Durham, NC, United States A new general concept is proposed for integrated parallel reception, excitation, and shimming (iPRES). It uses a novel coil design allowing an RF current and a DC current to flow in the same physical coil to perform parallel excitation/reception and B0 shimming with a unified coil array. To demonstrate its feasibility, proof-of-concept phantom experiments were performed with a two-coil array. The shim field induced by individually optimized DC currents applied in both coils was able to significantly reduce nonlinear B0 inhomogeneities introduced in the phantom, demonstrating that parallel excitation/reception and B0 shimming can be performed with a unified coil system.
14:06 | 0665. | Combined Shim-RF Array for Highly Efficient Shimming of the Brain at 7 Tesla Jason P. Stockmann1, Thomas Witzel1, James N. Blau1, Jonathan R. Polimeni1, Wei Zhao1, Boris Keil1, and Lawrence L. Wald1 1Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, Massachusetts, United States Multi-coil shimming shows great potential for canceling B0 inhomogeneity at ultra-high field, but its performance has been limited by the need to leave space for RF receive coils, pushing the shim coils further away from the body. We overcome this problem by combining the shim and receive RF currents through a single conductor thus optimizing the performance of both systems by bringing them as close to the body as possible. Experimental results show that shim-RF coils provide equivalent SNR to conventional receive coils and single loop shim structures can effectively shim the brain with <5A current per loop.
14:18 | 0666. | Matrix Gradient System: Concept and Performance Evaluation Feng Jia1, Gerrit Schultz1, Anna M. Welz1, Sebastian Littin1, Hans Weber1, Jürgen Hennig1, and Maxim Zaitsev1 1Dept. of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany Encoding with nonlinear encoding fields (SEMs) has raised increasing interest in the past few years. Matrix coils appear to offer a high flexibility in generating customized SEMs and are particularly promising for localized high resolution imaging applications such as PatLoc or ExLoc. However, up to now, it is still an open question of how to assess the performance of such matrix coils. In this work we propose a new performance measure that is oriented towards optimal local encoding. An optimization problem is formulated that results in high-performance cylindrical matrix coil designs with a high number of elements. The results are tested and the analysis reveals novel features of matrix coil designs.
14:30 | 0667. | Dynamic Multi-Coil Shimming of the Rat Brain at 11.7 Tesla Christoph Juchem1, Basavaraju G. Sanganahalli1, Peter Herman1, Peter B. Brown1, Scott McIntyre1, Terence W. Nixon1, and Robin A. de Graaf1 1MR Research Center (MRRC), Yale University, New Haven, CT, United States The in vivo rat model is a work horse in neuroscientific and preclinical MR research, however, excellent magnetic field homogeneity is required for meaningful results. Dynamic multi-coil (DMC) shimming is shown to successfully minimize magnetic field distortions encountered in the rat brain at 11.7 Tesla. While EPI with static, third order spherical harmonic shimming suffers from signal loss and image deformation, the brain outline is preserved with DMC-shimmed EPI. The minimization of signal loss and the improvement of the spatial accuracy of EPI with DMC shimming are expected to critically benefit a wide range of preclinical and neuroscientific MR research.
14:42 | 0668. | Dynamic Field Monitoring by 20 Channel Field Probes Integrated with 12 Channel Head Coil Ying-Hua Chu1, Yi-Cheng Hsu2, Wen-Jui Kuo3, and Fa-Hsuan Lin1 1Institute of Biomedical Engineering, National Taiwan University, Taipei, Taiwan, 2Department of Mathematics, Nnational Taiwan University, Taipei, Taiwan, 3Institute of Neuroscience, National Yang-Ming University, Taipei, Taiwan We develop a 20-channel field probe system integrated with a 12-channel head coil array to dynamically detect the frequency fluctuations caused by the subject, eddy current, or other systemic changes during the 3T MRI scan. The frequency shift was continuously estimated by the phase difference monitored by the field probes. This information was found useful in dynamic tracking of the magnetic field distribution and the k-space trajectory. Our results suggest that, under the limit of a fixed number of RF channels, an integrated RF field probe and coil array system holds promise of high quality parallel MRI.
14:54 | 0669. | Full 3rd Order Real-Time Shim Feedback for Field Stabilization and Its Application in Brain MRI at 7T Yolanda Duerst1, Bertram J. Wilm1, Benjamin E. Dietrich2, David Otto Brunner2, Christoph Barmet2,3, Thomas Schmid1, Signe Johanna Vannesjo1, and Klaas P. Pruessmann1 1Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland, 2Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland, 3Skope Magnetic Resonance Technologies, Zurich, Switzerland Spatio-temporal field fluctuations caused by hardware imperfections or physiological motion can lead to signal loss through increased T2* decay or off-resonant RF pulses and severely degrade image quality. We present a first implementation of a concurrent real-time feedback system correcting for full 3rd order spherical harmonics field changes. A proportional-integral controller was used to address gradient and shim coils of a whole-body scanner to actively compensate field fluctuations. The control loop achieved robust field stabilization of breathing induced field perturbations and significantly enhanced image quality of a commonly used T2*-weighted gradient-echo scan which otherwise strongly suffers from breathing artifacts.
15:06 | 0670. | A Low-Cost, Mechanically Simple Apparatus for Measuring Eddy Currents Kyle M. Gilbert1, L. Martyn Klassen1, and Ravi S. Menon1 1Robarts Research Institute, The University of Western Ontario, London, Ontario, Canada A transmit/receive array is presented that allows for the measurement of eddy currents of the zeroth and first orders, simultaneously. The coil system is low-cost and mechanically simple. The performance of the coil system is evaluated based on metrics required to accurately record the spatial and temporal variation of eddy currents.
15:18 | 0671. | Digital Cross-Term Pre-Emphasis for Higher-Order Dynamic Shimming Signe Johanna Vannesjo1, Benjamin E. Dietrich1, Matteo Pavan1, Christoph Barmet1,2, and Klaas P. Pruessmann1 1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland, 2Skope Magnetic Resonance Technologies, Zurich, Switzerland Dynamic shimming holds promise to improve B0 homogeneity for a range of applications, however requires fast updating of the shim fields. Eddy currents limit the shim settling times and also produce strong cross-term field components. Here we show the feasibility of digital cross-term pre-emphasis applied to the input shim waveforms. The pre-emphasis is calculated based on shim impulse response functions (SIRFs) measured with a dynamic field camera. It is shown that settling times can be shortened and cross-term responses much reduced with this approach. The digital pre-emphasis allows for full flexibility in shaping the response, within hardware limitations.