|Parallel Transmission Methods|
Young Investigator Award Finalist: Electrodynamic
Constraints on Homogeneity and RF Power Deposition in Multiple Coil
Riccardo Lattanzi1, 2, Daniel K. Sodickson3, Aaron K. Grant2, 4, Yudong Zhu5
1Harvard-MIT, Cambridge, Massachusetts, USA; 2Beth Israel Deaconess Medical Center, Boston, Massachusetts, USA; 3New York University Medical Center, New York, New York, USA; 4Harvard Medical School, Boston, Massachusetts, USA; 5GE Global Research, Niskayuna, New York, USA
This work explores electrodynamic constraints on transmit homogeneity and SAR in the case of fully parallel transmission and RF shimming. Ultimate SAR was computed for various target excitation profiles on a transverse plane through the center of a homogeneous sphere. The behavior with respect to main magnetic field strength, object size and acceleration was investigated in the ultimate case, as well as in the case of finite coil arrays. Ideal current patterns resulting in the lowest possible SAR were calculated and, as expected, increasingly complex current distributions were observed at higher magnetic field strengths.
Increasing Bandwidth of Spatially Selective Transmit
SENSE Pulses Using Constrained Optimization
David Otto Brunner1, Klaas Paul Pruessmann1
1University and ETH Zurich, Zurich, Switzerland
Although spatially selective multichannel transmission pulses can be shortened by means of k-space undersampling with respect to single channel pulses, their bandwidth is often too limited to cope with strong off-resonances or the requirements of MR spectroscopy at ultra-high fields. In this study, we exploit the formalism of spatial-spectral pulse design to increase the bandwidth and hence the robustness of such pulses. The pulse design problem is stated in a highly underdetermined fashion as a constrained optimization problem, searching for the best performing pulse within existing SAR and power limits.
Uniform Wideband Slab Selection with B1+
Mitigation at 7T Via Parallel Spectral-Spatial Excitation
Kawin Setsompop1, Vijayanand Alagappan2, Borjan Gagoski1, Lawrence Wald3, Elfar Adalsteinsson1
1Massachusetts Institute of Technology, Cambridge, Massachusetts, USA; 2A. A. Martinos Center for Biomedical Imaging, MGH, Charlestown, Massachusetts, USA; 3A. A. Martinos Center for Biomedical Imaging, Department of Radiology, MGH, Charlestown, Massachusetts, USA
Parallel RF (pTx) designs based on small-flip-angle excitations with “spoke”-based trajectories efficiently mitigate large B1+ inhomogeneities at high field, but posess a narrow-band off-resonance response. Proton chemical shift imaging benefits from higher B0, but requires B1+ mitigation over both a specified bandwidth and a spatial FOV. This additional bandwidth constraint presents a challenge for past methods on water-only B1+ mitigations. We describe a general pTx spectral-spatial excitations, demonstrate the technique on a wideband slice-selective spoke excitation, and validated it on a water phantom using an 8-channel transmit array system on a 7T human MRI scanner.
Dual-Band RF Shimming at High-Field with Parallel
Adam Bruce Kerr1, Maryam Etezadi-Amoli1, Hans-Peter Fautz2, Mika W. Vogel2, Patrick Gross2, Yudong Zhu3, John Mark Pauly1
1Stanford University, Stanford, California , USA; 2GE, Munich, Germany; 3GE, Albany, New York, USA
A significant problem with current design approaches for RF shimming using three-dimensional selective excitation is their lack of ability to adequately shim simultaneously over multiple resonances such as water and fat. A new dual-band approach is presented that resolves this issue and is demonstrated on an eight-channel parallel transmit whole-body 3T system.
Optimal Control Design of Phase-Relaxed Parallel
Transmission RF Pulses for Arbitrary Flip Angles
Dan Xu1, Kevin F. King1, Zhi-Pei Liang2
1General Electric Healthcare, Waukesha, Wisconsin, USA; 2University of Illinois at Urbana-Champaign, Urbana, Illinois, USA
Conventional parallel transmission RF pulse designs are limited in two aspects: 1) an overly restrictive flat phase profile of the desired magnetization is enforced, which may sacrifice the quality of the magnitude profile, and 2) only small-tip-angle pulses can be accurately designed. More elaborate designs have been proposed to address either one of the issues, but none of them addresses both. In this paper, we propose a new, spinor-based, optimal control method which is capable of designing parallel transmission pulses with optimal phase profile and arbitrary tip angle simultaneously. Bloch simulation results of dual-channel transmission of 90 degree RF excitation pulses for B1 inhomogeneity correction show significantly more homogeneous magnitude profile of the transverse magnetization than existing designs.
Mode Compression of Transmit and Receive Arrays for
Parallel Imaging at 7T
Vijayanand Alagappan1, 2, Kawin Setsompop3, Jonathan R. Polimeni1, Andreas Potthast4, Adam C. Zelinski3, Graham C. Wiggins1, Ulrich J. Fontius5, Franz Schmitt5, Elfar Adalsteinsson3, Lawrence L. Wald1
1Athinoula A. Martinos Center for Biomedical Imaging, Charlestown, Massachusetts, USA; 2Tufts University, Medford, Massachusetts, USA; 3MIT, Cambridge, Massachusetts, USA; 4Siemens Medical Solutions, Charlestown, USA; 5Siemens Medical Solutions, Erlangen, Germany
While increasing the number of array elements appears beneficial for both parallel transmission and reception, this approach is limited in practice, especially in the parallel transmit case. Forming linear combinations of array elements can transform the spatial modes of the array into a different basis set potentially capturing a majority of the sensitivity and acceleration capabilities in a subset of the channels. We develop a mode transformation of a 16 element 7T T/R strip-line array using a 16x16 Butler matrix and analyze both the Stripline and orthogonal Birdcage mode basis sets when truncated from 16 modes to 8. The birdcage basis set were found to be spatially orthogonal, gave a lower excitation error and had significant SNR benefits and acceleration capabilities compared to the stripline basis set.
In Vivo B1+ Inhomogeneity Mitigation at 7
Tesla Using Sparsity-Enforced Spatially-Tailored Slice-Selective
Adam Charles Zelinski1, Kawin Setsompop1, Vijayanand Alagappan2, Vivek K. Goyal1, Lawrence L. Wald3, 4, Elfar Adalsteinsson1, 4
1Massachusetts Institute of Technology, Cambridge, Massachusetts, USA; 2A. A. Martinos Center for Biomedical Imaging, Charlestown, Massachusetts, USA; 3Harvard Medical School, Longwood, Massachusetts, USA; 4Harvard-MIT Division of Health Sciences & Technology, Cambridge, Massachusetts, USA
We design & demonstrate a 7-ms slice-selective pulse that mitigates B1+ inhomogeneity in the human brain at 7T without the use of a parallel transmission system. First, a magnetization reset pulse is used to rapidly acquire a series of images. These images are then fit to an intensity equation to estimate B1+. A sparsity-enforced spoke placement algorithm is then used to find a small set of spoke locations & weights that lead to a short, slice-selective mitigation pulse. Unlike a shimming approach, the B1+ field is not flattened; rather, the gradient modulation of the excitation process produces a uniform magnetization.
A Minimum SAR RF Pulse Design Approach for Parallel
Tx with Local Hot Spot Suppression and Exact Fidelity Constraint
Ingmar Graesslin1, Ferdinand Schweser1, Bjoern Annighoefer1, Sven Biederer2, Ulrich Katscher1, Kay Nehrke1, Henry Stahl1, Henk Dingemans3, Giel Mens3, Peter Börnert1
1Philips Research Europe, Hamburg, Germany; 2Lübeck University, Lübeck, Germany; 3Philips Medical Systems, Best, Netherlands
SAR is a limiting factor in high-field MRI. Parallel transmit systems are able to tailor E-fields via SAR optimized RF pulse design. Certain SAR optimal algorithms constrain a specific local region for SAR, which is generally insufficient for whole body imaging. In this paper, a SAR optimal RF pulse design approach is presented using a predefined excitation error as trade-off for minimizing SAR. Additionally, based on this approach, also a method for local hot spot reduction is presented. The concept was successfully validated by simulations and experiments via a graphic card-based (almost) real-time whole body local SAR calculation.
Single-Shot Z-Shim Technique Using Parallel
Transmitters for Reduced Suscpetibility Artifacts
Weiran Deng1, Fernando E. Boada
2, Victor Andrew Stenger 1
Susceptibility artifacts are major limitation in T2* weighted MRI such as BOLD fMRI. Z-shim techniques and 3D RF pulses have been proposed to mitigate the through-plane susceptibility artifact. However, z-shim techniques require multiple shots and 3D RF methods are complex with long pulse lengths. Parallel transmission methods have been proposed to reduce 3D RF pulse lengths, however, the current implementation of these techniques is computationally very challenging. We present a potentially simple parallel transmission method using time-shifted 1D sinc pulses for performing a single-shot z-shim. The method is shown to reduce susceptibility artifacts in T2* weighted images at 3T.