Gradients & Shims
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Friday May 13th
Room 511D-F  10:30 - 12:30 Moderators: Dennis Parker and Florian Wiesinger

10:30 714.   First In-vivo Results with a PatLoc Gradient Insert Coil for Human Head Imaging 
Chris A. Cocosco1, Daniel Gallichan1, Andrew J. Dewdney2, Gerrit Schultz1, Anna M. Welz1, Walter R.T. Witschey1, Hans Weber1, Juergen Hennig1, and Maxim Zaitsev1
1Dept. of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg (i.Br.), B.W., Germany, 2Siemens Healthcare Imaging, Erlangen, Germany

We report on the first in-vivo results with a prototype PatLoc gradient insert coil designed for human head imaging on a clinical 3T scanner. We found imaging volunteers with this PatLoc non-linear spatial encoding system to be safe, feasible, and to deliver locally improved image resolution. We also report on new evaluation measurements regarding the safety of this insert coil for human head imaging, including the simultaneous use of four in-plane gradient encoding fields, which is the object of current investigation.

10:42 715.   B0 Shimming in 3 T Bilateral Breast Imaging with Local Shim Coils  -permission withheld
Seung-Kyun Lee1, Lorne Hofstetter1, and Ileana Hancu1
1GE Global Research, Niskayuna, NY, United States

We show that bilateral breast shimming can be significantly improved by employing local shim coils. A pair of Helmholtz-like coils compensate for the left-right B0 field difference, on the order of 100 Hz, caused by the inherently asymmetric positioning of heart and lungs.

10:54 716.   Dynamic Multi-Coil Shimming of the Human Brain at 7 Tesla 
Christoph Juchem1, Terence W Nixon1, Scott McIntyre1, Vincent O Boer2, Douglas L Rothman1, and Robin A de Graaf1
1MR Research Center, Yale University, New Haven, CT, United States, 2Department of Radiology, UMC, Utrecht, Netherlands

A novel technique for the magnetic field homogenization of the human brain is presented in which shim fields are synthesized by a combination of non-orthogonal basis fields from a set of individually driven, generic coils. First experimental results of dynamic multi-coil (DMC) shimming at 7 Tesla are shown to outperform state-of-the-art zero-to-third order spherical harmonic dynamic shim updating (SH-DSU)

11:06 717.   First O-Space images using a high-power, actively-shielded, 12-cm Z2 gradient insert on a human 3T scanner 
Jason Peter Stockmann1, Gigi Galiana2, Leo Tam1, Terry Nixon3, and Robert Todd Constable1,2
1Biomedical Engineering, Yale University, New Haven, CT, United States, 2Diagnostic Radiology, Yale University, New Haven, CT, United States, 3Diagnostic Radiology, Yale University, New Have, CT, United States

We show the first single-channel, axial-plane O-Space images made using a custom, actively-shielded 12-cm Z2 quadratic gradient insert on a Siemens 3T human scanner. Gradient calibration via field mapping is described. Eddy currents are measured using two different methods and are found in each case to be negligible. Image reconstruction is performed using the entire encoding matrix with the Kaczmarz algorithm (also known as the algebraic reconstruction technique). As compared with images acquired using radial k-space trajectories, O-Space images show comparable resolution but reduced artifact levels owing to the less coherent point spread function of the quadratic field's curvilinear frequency isocontours.

11:18 718.   Advances in Software Compensation of Eddy Current Fields in Multislice Higher Order Dynamic Shimming. 
Saikat Sengupta1,2, Malcolm Avison2,3, John Gore2,3, and Edward Brian Welch2,3
1Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, United States, 2Vanderbilt University Institute of Imaging Science, Nashville, United States,3Radiology and Radiological Sciences, Vanderbilt University, Nashville, United States

We present advances in a prospective software compensation method of higher order shim induced eddy currents in multislice Dynamic Bo Shimming (DS). The method based on steady state generation of eddy fields requires no hardware eddy current compensation, subject specific prescanning or shim shielding. Results of application to 2nd and 3rd order DS in single shot EPI are presented demonstrating correction of distortions, ghosting and signal loss. The model of correction is modified to include static interaction between shims and invariability with time between shim switches is demonstrated.

11:30 719.   Fast characterization of higher-order shim dynamics by impulse response measurements with a dynamic field camera 
Johanna Vannesjö1, Ariane Fillmer1, Christoph Barmet1, Peter Boesiger1, Anke Henning1, and Klaas Paul Pruessmann1
1Institute for Biomedical Technology, University and ETH Zurich, Zurich, Switzerland

With the use of dynamic shimming higher demands are put on shim systems, regarding switching speed and dynamic characteristics. This calls for a fast and reliable method to comprehensively characterize the dynamic properties of the shims. Here we have investigated the use of shim impulse response functions (SIRF), as probed with a suitable set of input pulses and measurements of the field responses with a dynamic field camera. It was shown that the SIRF measurements could reveal different dynamic features of the shims, which may prove useful for system diagnostics and pre-emphasis setting.

11:42 720.   Fourier Series Network Method for 3D Simulations of Eddy Currents Induced in Multilayer Cryostats by Arbitrary Coils 
Michael Stephen Poole1, Hector Sanchez Lopez1, Shin-ichi Urayama2, Hitohsi Kitaguchi3, Osamu Ozaki4, and Stuart Crozier1
1ITEE, University of Queensland, Brisbane, QLD, Australia, 2Human Brain Research Center, Kyoto University Graduate School of Medicine, Kyoto, Japan, 3National Institute for Materials Science, Tsukuba, Japan, 4Kobe Steel Ltd., Kobe, Japan

A new method is presented for simulating the eddy currents induced in magnet cryostat structures by switching gradient coils. Multiple thick cryostat layers are modelled accurately using the network method. Fourier decomposed basis-functions in z and lower case Greek phi allow the 3D simulation of eddy currents from X, Y, Z or any other coils. Simulations are applied to the cryostat of a HTS magnet currently under construction.

11:54 721.   Magnetic Gradient Shape Optimization for Highly Accelerated Null Space Imaging 
Leo K Tam1, Jason P Stockmann1, Gigi Galiana2, and Robert Todd Constable1
1Biomedical Engineering, Yale University, New Haven, Connecticut, United States, 2Diagnostic Radiology & Neurosurgery, Yale University, New Haven, Connecticut

Previous research has demonstrated the utility of non-linear magnetic gradients such as the Z2 or C2 and S2 spherical harmonics for highly accelerated parallel imaging. In the present work, a systematic study of gradients shapes that are used in higher order gradients is performed via the Null Space Imaging (NSI) method, a technique based on designing gradients complementary to receiver coils. All combinations of spherical harmonics through the fourth order and additional combinations such as ones focused on orthogonal gradients (eg. C2 and S2 together with linears) were considered, in total 21 combinations. Simulations demonstrate that an optimal second order gradient set is composed of linear gradients and the second order in-plane gradients (C2, S2, and Z2) for NSI imaging at fixed sampling bandwidth.

12:06 722.   Distortion of Gradient Coils Performances in Presence of Iron 
Alice Borceto1, andrea Viale1, Franco Bertora1, Leonardo Bertora2, and Richard Bowtell3
1Robotics,Brain and Cognitive Science, Italian Institute of Technology, Genoa, GE, Italy, 2Paramed Medical system, Genoa, (GE), Italy, 3Sir Peter Mansfield Magnetic Resonace Center, University of Nottingham, Nottingham, United Kingdom

The performances of an unshielded gradient system can be heavily affected by the presence of an iron yoke, increasing the non-linearity and non-uniformity of the field in the region of interest. This departure from ideal performance causes image distortions. It is therefore important to study the interaction between unshielded gradient coils and the iron yoke. Here, three different techniques for modelling the effect of iron (mirror coils, boundary element method and full finite element simulation) are presented and the results compared. Among the three the boundary element method offers the best compromise between accuracy and calculation time.

12:18 723.   A Target Field Approach to the Design of RF Phase-Gradient Coils 
Jesse Bellec1, Chen-Yi Liu1, Scott B King2, and Christopher Paul Bidinosti1,3
1Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba, Canada, 2MR Technology, NRC Institute for Biodiagnostics, Winnipeg, Manitoba, Canada, 3Physics, University of Winnipeg, Winnipeg, Manitoba, Canada

Transmit array spatial encoding (TRASE) [1] is a novel gradient-free imaging technique relying on Tx RF phase gradients to spatially encode the transverse magnetization. Ideal phase gradients have a strong-uniform magnitude and strong-linear phase gradient over a large volume. To optimize TRASE MRI, a target field approach has been used to determine current distributions that produce linear phase gradients and uniform magnitude, but over limited volumes. A horizontal-B0 z-phase gradient design resembles initial spiral birdcage designs, suggesting that the approach is suitable for TRASE optimization.