Parallel RF Transmission
Monday 3 May 2010
Room A1 16:30-18:30 Moderators: Adam B. Kerr and Kawin Setsompop

16:30 99.

Joint Design of Dual-Band Large-Tip-Angle RF and Gradient Waveforms in Parallel Excitation
William A. Grissom1, Adam B. Kerr, Pascal P. Stang2, Greig C. Scott2, Ileana Hancu3, Mika W. Vogel4, John M. Pauly2
1Electrical Engineering and Radiology, Stanford University, Stanford, CA, United States; 2Electrical Engineering, Stanford University, Stanford, CA, United States; 3GE Global Research, Niskayuna, NY, United States; 4Advanced Medical Applications Laboratory, GE Global Research, Munich, Bavaria, Germany

We introduce a new framework for optimizing the phase encoding locations of a 2D or 3D parallel excitation pulse in the large-tip-angle regime. The framework is analogous to the hard pulse approximation, and yields a straightforward analytical relationship between the pulses' spin-domain rotations and the phase encoding locations. This relationship can be exploited to optimize locations using gradient descent, or using optimization transfer for monotonic, parameter-free optimization. We apply our method to the design of dual-band (fat + water) spin echo parallel excitation pulses along 3D rungs trajectories.

16:42 100.

Fast and Accurate Large-Tip-Angle RF Pulse Design for Parallel Excitation Using a Perturbation Analysis of the Bloch Equation
Hai Zheng1,2, Tiejun Zhao3, Tamer Ibrahim1, Fernando Emilio Boada1
1MR Research Center, University of Pittsburgh, Pittsburgh, PA, United States; 2Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA, United States; 3Siemens Medical Systems, Malvern, PA, United States

The design of RF pulses in parallel excitation (PTX) commonly relies on the small-tip-angle approximation, which, although efficient, leads to distorted excitation patterns at large tip angles because of the intrinsic nonlinear nature of the Bloch equation. In this work, we introduce a fast and accurate method for large-tip-angle PTX RF pulse design based on a perturbation analysis (PTA) to the Bloch equation. Experimental data at 7T as well as computer simulations demonstrate the improvements produced by the proposed techniques without the need of prohibitively long calculation times.

16:54 101. 

Fast High-Flip PTx Pulse Design to Mitigate B1+ Inhomogeneity Using Composite Pulses at 7T
Rene Gumbrecht1,2, Joonsung Lee1, Hans-Peter Fautz3, Dirk Diehl4, Elfar Adalsteinsson1,5
1Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States; 2Department of Physics, Friedrich-Alexander-University, Erlangen, Germany; 3Siemens Healthcare, Erlangen, Germany; 4Siemens Corporate Technology, Erlangen, Germany; 5Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, United States

Parallel RF transmission offers flexible control of ma magnetization generation and has been successfully applied at 7T for spatially tailored excitations and mitigation of in-plane B1+ inhomogeneity for slice-selection. Composite Pulses are known to have favorable robustness properties for large-flip-angle excitations in the presence of B1+ variations, but they have not yet been demonstrated on pTx systems. We propose a composite RF pulse design for pTx systems and demonstrate the method for B1+ mitigation in a 90º excitation pulse design.

17:06 102

Kt Points: Fast Three-Dimensional Tailored RF Pulses for Flip-Angle Homogenization Over an Extended Volume
Martijn Anton Cloos1, Nicolas Boulant1, Michel Luong2, Guillaume Ferrand2, Christopher J. Wiggins1, Eric Giacomini1, Alain France2, Dennis Le Bihan1, Alexis Amadon1

1CEA, DSV, I2BM, NeuroSpin, LRMN, Gif-sur-Yvette, France; 2CEA, DSM, IRFU, SACM, Gif-sur-Yvette, France

Transmit-SENSE gives the opportunity to implement short excitation pulses with good flip-angle homogeneity at high field. For slice-selective pulses, this was previously demonstrated using a spoke k-space trajectory. Here we present a novel pulse design returning sub-millisecond pulses with excellent flip-angle homogenization over an extended volume. Experimental results are shown at 7T, demonstrating a 950-μs excitation pulse producing a 15±1.1° flip-angle distribution over a 16-cm spherical phantom having the same electrical properties as a human head.

17:18 103.  

Inner-Volume-Imaging Using Three-Dimensional Parallel Excitation
Johannes Thomas Schneider1,2, Raffi Kalayciyan1,3, Martin Haas2, Wolfgang Ruhm1, Olaf Doessel3, Juergen Hennig2, Peter Ullmann1
1Bruker BioSpin MRI GmbH, Ettlingen, Germany; 2Dept. of Diagnostic Radiology, Medical Physics, University Hospital Freiburg, Freiburg, Germany; 3Institute of Biomedical Engineering, Karlsruhe Institute of Technology, Karlsruhe, Germany

This study presents the first experimental realization of inner-volume-imaging using three-dimensional parallel excitation of arbitrarily shaped regions of interest. By using a temporally optimized 4-fold undersampled 3D k-space trajectory consisting of concentrical shells in combination with an 8-channel transceive RF-array, 3D selective excitation of an arbitrary volume could be achieved in only 5 ms. Featuring such short durations 3D-selective pulses are now on the verge of being used in common imaging sequences and have been successfully applied in first experiments of inner-volume-imaging in phantoms and fruits during this study.

17:30 104

SAR Reduction by K-Space Adaptive RF Shimming
Hanno Homann1, Kay Nehrke2, Ingmar Graesslin2, Olaf Dössel1, Peter Börnert2
1Karlsruhe University, Karlsruhe, Germany; 2Philips Research, Hamburg, Germany

Parallel transmission allows compensating for RF transmit field inhomogeneities and simultaneous SAR reduction by RF shimming. This study demonstrates that the trade-off between these two objectives can be overcome by using several different, adapted RF pulses: When sampling the center of the k-space, a highly uniform but relatively SAR-intensive excitation is performed to achieve optimal contrast. In the outer k-space, the homogeneity requirement is relaxed to reduce the average SAR. The concept is discussed theoretically; proof-of-principle is given based on phantom and in vivo images.

17:42 105. 

Parallel Transmit RF Design with Local SAR Constraints
Joonsung Lee1, Matthias Gebhardt2, Lawrence L. Wald3,4, Elfar Adalsteinsson1,4

1Electrical engineering and computer science, Massachusetts Institute of Technology, Cambridge, MA, United States; 2Siemens Healthcare, Erlangen, Germany; 3Department of Radiology, A. A. Martinos Center for Biomedical Imaging, Cambridge, MA, United States; 4Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, United States

The model compression method for local SAR esitmation dramatically decreases the complexity of the prediction of the local SAR calculation and enables the incorporation of local SAR constaints in pTX MLS RF design.

17:54 106

RFuGE –an Accelerated Imaging Method Combining Parallel Transmit RF Encoding Plus Gradient Encoding with Compressed Sensing Reconstruction
Muhammad Usman1, Shaihan J. Malik2, Ulrich Katscher3, Philip G. Batchelor1, Joseph V Hajnal2
1Imaging Sciences, King's College London, London, United Kingdom; 2Robert Steiner MRI Unit, Imaging Sciences Department, MRC Clinical Sciences Centre, Hammersmith Hospital, Imperial College London, London, United Kingdom; 3Sector Medical Imaging Systems Philips Research Europe, Hamburg, Germany

We describe a combination of Parallel Transmit generated radiofrequency encoding and undersampled gradient encoding that can be reconstructed using compressed sensing to achieve accelerated imaging with a non-linear encoding basis. The method, RF plus Gradient Encoding, (RFuGE) has been tested in simulation and successful reconstructions were achieved.

18:06 107.  

16-Channel Parallel Transmission in the Human Brain at 9.4 Tesla: Initial Results
Xiaoping Wu1, J. Thomas Vaughan1, Kamil Ugurbil1, Pierre-Francois Van de Moortele1

1CMRR, University of Minnesota, Minneapolis, MN, United States

It has been shown that parallel transmission (pTx), which consists of playing different RF pulses through independent transmit (Tx) channels, can be used to mitigate Tx B1 (B1+) nonuniformity and to achieve more homogeneous spatially selective RF excitation at high magnetic field. We have previously reported a successful implementation of Transmit SENSE in the human brain at 9.4 T with an 8 Tx channel system, which required addressing methodological issues such as k-space trajectory inaccuracies and large susceptibility induced δB0. Recently, our 9.4T system has been upgraded with a 16 Tx channel console. Here we report preliminary results of 2D (Transmit SENSE) and 3D (Spoke trajectories) pTx in the human brain at 9.4 T using a 16-channel RF coil.

18:18 108

Self-Refocused Adiabatic Pulse for Spin Echo Imaging at 7T
Priti Balchandani1, John Pauly2, Daniel Spielman1
1Radiology, Stanford University, Stanford, CA, United States; 2Electrical Engineering, Stanford University, Stanford, CA, United States

Adiabatic 180° pulses may be used to replace conventional 180° pulses in spin echo sequences to provide greater immunity to the inhomogeneous B1-field at 7T.  However, because the spectral profile of an adiabatic 180° pulse has non-linear phase, pairs of these pulses are used for refocusing, resulting in increased SAR and longer minimum echo times.  We have used the adiabatic SLR method to generate a matched-phase 90° for an adiabatic 180° pulse to obviate the need for a second 180° pulse.  The pulse pair was combined into a single self-refocused pulse to achieve the minimum echo time, and phantom and in vivo experiments were performed to validate pulse performance.



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