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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.

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.

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.

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.

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.

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.

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.

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.

Weiran Deng¹, Fernando E. Boada ², Victor Andrew Stenger ¹
University of Hawaii, Honolulu, Hawaii, USA; ²University of Pittsburgh, Pittsburgh, Pennsylvania, USA

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.