RF Pulses: Novel Design & Applications
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Thursday 10 May 2012
Plenary Hall  16:00 - 18:00 Moderators: Fa-Hsuan Lin, Peter Ullmann

16:00 0634.   kT-Points-Based Inversion Pulse Design for Transmit-SENSE enabled MP-RAGE Brain Imaging at 7T
M.A. Cloos1,2, N. Boulant1, M. Luong2, G. Ferrand2, E. Giacomini1, M-F. Hang1, C.J. Wiggins1, D. Le Bihan1, and A. Amadon1
1CEA, DSV, I2BM, Neurospin, LRMN, Gif-Sur-Yvette, Ile de france, France, 2CEA, DSM, IRFU, Gif-Sur-Yvette, Ile de france, France

Among the advantages of the Transmit-SENSE method is the ability to facilitate short low SAR excitation pulses with excellent flip-angle homogeneity at high field. In this framework, large tip angle pulses are of particular interest as they could provide a viable alternative to the SAR-demanding adiabatic solutions. In this abstract, we demonstrate the kT-points method in the MP-RAGE sequence for high-resolution T1-weighted brain imaging at 7 Tesla, omitting adiabatic pulses by introducing the kT-points-based inversion pulse design.

16:12 0635.   Tailored Excitation Using Non-Linear B0-Shims
Qi Duan1, Peter van Gelderen1, and Jeff H. Duyn1
1Advanced MRI section, LFMI, NINDS, National Institutes of Health, Bethesda, MD, United States

This abstract proposes an alternative approach to shorten multi-dimensional excitation by utilizing non-linear spatial variations in the stationary (B0) magnetic field during a B0-sensitive excitation pulse. As initial demonstration, the method was applied to 2D gradient echo (GE) MRI of human brain at 7T. Using B0 shims with up to second order spatial dependence, it is demonstrated that root-mean-squared flip angle variation can be reduced from 20% to 11% with RF pulse lengths that are practical for general GE imaging applications without requiring parallel excitation.

16:24 0636.   
Four-Dimensional Spectral-Spatial Pulse for Fat Saturation with Parallel Excitation: Preliminary Results on 3T Scanners
Feng Zhao1, Jon-Fredrik Nielsen1, and Douglas C. Noll1
1Biomedical Engineering, University of Michigan, Ann Arbor, MI, United States

Fat saturation techniques that use spectrally selective pulses suffer from inaccurate excitation due to inhomogeneous B0 or B1 maps. To solve this problem, wee propose a 4D spectral-spatial pulse with parallel excitation, which is demonstrated by preliminary 3T data.

16:36 0637.   
Direct method for parallel transmit pulse design by time-reversal of the small-tip angle excitation
Kaveh Vahedipour1, N. Jon Shah1,2, and Tony Jon Stöcker1
1Institue of Neuroscience and Medicine - 4, Forschungszentrum Jülich, Jülich, Germany, 2JARA - Faculty of Medicine, RWTH Aachen University, JARA, Aachen, Germany

This abstract demonstrates that the spatio-temporal symmetries fundamental to the Bloch equation and the principle of reciprocity allow one to utilise MR signal acquisition or simulation for small tip-angle pulse design. Furthermore, it is demonstrated how the method is intrinsically capable of parallel transmit as well as how mitigation of static and high frequency field inhomogeneities is intuitively performed. The system-encoding matrix is never considered in theory or algorithmically. A theoretical derivation is formulated and proven in principle. It is further demonstrated that the MRI experiment can be used directly as an analogue computer for small tip-angle pulses.

16:48 0638.   
Enhanced whole brain excitation performance of parallel transmission with a Z-encoding RF coil array at 7T
Xiaoping Wu1, Sebastian Schmitter1, Gregor Adriany1, Edward J. Auerbach1, Kamil Ugurbil1, and Pierre-Francois Van de Moortele1
1CMRR, Radiology, University of Minnesota, Minneapolis, MN, United States

Multi-channel transmit B1 (B1+) manipulation techniques, such as static B1 shimming and parallel transmission (pTX), can be used to reduce the severe B1+ inhomogeneities that arise at 7T and above. One of the fundamental limits of multi-channel B1 manipulation is determined by the spatial encoding capabilities of the utilized multi-element RF coil, although different B1 methods, e.g. static B1 shim vs. spoke trajectories, will not be affected similarly by a given RF coil geometry. In a previous study it has been shown that distributing individual elements of a 7T head array coil in two rings along Z direction provided better RF efficiency than a standard single ring array when applying static B1 shimming over a large brain area. The goal of the current study was to determine, by comparing two transceiver arrays, whether, and to what extent, the use of kT points, a promising approach recently proposed to achieve homogeneous excitation over the whole brain at 7T, would also benefit from RF coil elements distributed along the Z axis.

17:00 0639.   Revisiting RF Feedback Pulses: Encoding Image Contrast during Excitation
Tony Stöcker1, Kaveh Vahedipour1, and N. Jon Shah1,2
1Institute of Neuroscience and Medicine - 4, Forschungszentrum Jülich, Jülich, Germany, 2JARA - Faculty of Medicine, RWTH Aachen University, Aachen, Germany

This work discusses multidimensional RF feedback pulses (FBP), which copy the received MRI signal(s) back to the RF transmit channel(s), taking advantage of transmit and receive reciprocity. Spatially selective FBP, which are accompanied by suitable gradient waveforms, have the property to recursively enhance image contrast, which is promising in several applications. The paper presents a theoretical description of FBP in the context of the small tip angle approximation, first experimental results, and numerical simulations of multiple transceivers FBP (parallel transmit), which are promising to shorten the pulse length.

17:12 0640.   
Selective Labeling of Moving Spins Using Parallel 3D Spatially Selective Excitation
Johannes T. Schneider1,2, Wolfgang Ruhm1, Sarah R. Herrmann1, Martin Haas2, Jürgen Hennig2, and Peter Ullmann1
1Bruker BioSpin MRI GmbH, Ettlingen, Germany, 2Department of Radiology, Medical Physics, University Medical Center Freiburg, Freiburg, Germany

This study presents the use of parallel excitation (PEX) for selective labeling of moving spins. In a multi-tube phantom the spins of one particular tube were labeled by 3D spatially-selective saturation in a short tube-segment. The difference to a control image acquired without labeling depicts the labeled tube’s moving spins exclusively thus demonstrating the largely sufficient PEX performance for this type of applications. With the same technique, the course and mixing properties of different flow components were observed in a “Y”-shaped tube-junction. Compared to other labeling techniques, 3D-PEX provides an unprecedented grade of flexibility and specificity regarding the labeling volume.

17:24 0641.   
High B1 efficiency with uniform image contrast in 3D FFE and TSE at 7T.
Irene Maria Louise van Kalleveen1, Vincent O. Boer1, Peter Luijten1, and Dennis W.J. Klomp1
1Radiology, UMC Utrecht, Utrecht, Utrecht, Netherlands

In high field clinical MRI the B1 field is limited and inhomogeneous, while RF power deposition is high. Surface coils can improve the B1 efficiency, however the B1 field becomes even more inhomogeneous. As the inhomogeneity is mainly dominant in one dimensional, we designed a 1D compensating RF pulse derived from a B1 map. In combination with slab selective gradients the RF pulse will provide a uniform flip angle as demonstrated by 3D FFE and 3D TSE in the human breast.

17:36 0642.   Local SAR reduction in multi-slice pTx via “SAR hopping” between excitations
Bastien Guerin1, Elar Adalsteinsson2,3, and Lawrence L. Wald1,3
1Martinos Center for Biomedical Imaging, Dept. of Radiology, Massachusetts General Hospital, Charlestown, MA, United States, 2Dept of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, 3Harvard-MIT Division of Health Sciences Technology, Cambridge, MA, United States

The time scale of RF transmission being much smaller than the time scale of temperature changes, only SAR averaged over several pulses needs to be constrained, not the SAR of individual pulses. In this work, we propose a multislice excitation strategy using pulses with minimally overlapping SAR distributions and jointly optimized so as to minimize their average local SAR. This strategy is not affected by ghosting since a given slice is always excited by the same pulse. It allows reduction of pulse-averaged local SAR by 35% for RF shimming and pTx arrays with two rows compared to pulses designed independently.

17:48 0643.   (RATE) Rapid MRI Acquisition using Tailored Signal Excitation modules: A k-space aliasing method to accelerate MRI scans
Arjun Arunachalam1, and Kamlesh Pawar1
1Electrical Engineering, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India

A new MRI acceleration technique that relies on the concept of k-space aliasing is introduced. K-space signal can either have a narrow spectrum because they come from features that are non-dynamic or because they come from structures that have weak signal magnitude. In the latter case, the temporal spectrum may not be narrow but only a portion of it is relevant as the rest remains buried under noise. The proposed method exploited these properties by using tailored signal excitation modules consisting of RF pulses and gradients to overlap distinct k-space points that are then resolved through Fourier transformation in time.