ISMRM 23rd Annual Meeting & Exhibition • 30 May - 05 June 2015 • Toronto, Ontario, Canada

Scientific Session • RF Pulse Design

Thursday 4 June 2015

Room 701 A

16:00 - 18:00


Charles H. Cunningham, Ph.D., T.B.A.

16:00 0912.   Hyperbolic Secant RF Pulses for Simultaneous Multi-Slice Excitation with Reduced Susceptibility Artifacts
Mehran Baboli1, Bastien Guerin2, Lawrence Wald2, and V. Andrew Stenger1
1Medicine, University of Hawaii, Honolulu, Hawaii, United States, 2Radiology, Massachusetts General Hospital, Massachusetts, United States

Hyperbolic Secant (HS) RF pulses can be used to generate a quadratic phase in the slice-select direction to reduce susceptibility induced signal dropout in regions including the frontal lobe of brain and are useful for applications such as fMRI. Recently simultaneous multi-slice (SMS) imaging techniques have been shown to provide speed increases by factors up to 12 in fMRI applications. Typically SMS excitation is accomplished by using a modulated sinc RF pulse. Additionally the Power Independent Number of Slices (PINS) pulse design has been proposed for low power SMS excitation using a series of non-selective pulses separated by z-gradient blips. This study demonstrates both modulated sinc and PINS SMS excitations weighted by an HS envelope to reduce susceptibility artifacts in the brain at 3T.

16:12 0913.   
Root-Flipped Multiband Radiofrequency Pulses
Anuj Sharma1, Michael Lustig2, and William A. Grissom1
1Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, United States, 2EECS, University of California, Berkeley, California, United States

In simultaneous multislice imaging, the peak power of the multiband pulse increases as square of the number of slices. This is a major concern for spin echo refocusing pulses. We present a method to design low peak-power refocusing and phase-matched excitation pulses for use in spin echo simultaneous multislice imaging. The pulses are designed based on the Shinnar Le-Roux algorithm and the peak power is reduced by finding an optimal root configuration of the pulse filter. Simulations and experiments demonstrate that the proposed pulses are of shorter duration compared to phase-optimized and time-shifted pulses for the same peak RF power.

16:24 0914.   
A wavelet-based optimization for RF pulse design applied to multiband imaging at 7T
Andrew M. Huettner1, Nikolai J. Mickevicius1, Ali Ersoz1, Kevin M. Koch2, L.Tugan Muftuler3, and Andrew S. Nencka1
1Biophysics, The Medical College of Wisconsin, Milwaukee, Wisconsin, United States, 2Biophysics and Radiology, The Medical College of Wisconsin, Milwaukee, Wisconsin, United States, 3Neurosurgery, The Medical College of Wisconsin, Milwaukee, Wisconsin, United States

A method for wavelet-based RF pulse optimization has been proposed. This technique has been demonstrated to reduce peak B1 transmit power for multiband refocusing pulses. This method has been applied to a pulse sequence for multiband (Simultaneous Multi-Slice) diffusion imaging at 7.0 Tesla.

16:36 0915.   
RF pulse design for simultaneous multislice excitation with highly reduced B1 peak amplitude
Christoph Stefan Aigner1, Christian Clason2, Armin Rund3, and Rudolf Stollberger1
1Institute of Medical Engineering, Graz University of Technology, Graz, Austria, 2Faculty of Mathematics, University of Duisburg-Essen, Essen, Germany,3Institute for Mathematics and Scientific Computing, University of Graz, Graz, Austria

RF pulses for simultaneous multislice excitation (SMS) are used to speed up imaging but typically lead to a linear increase of the B1 peak amplitude with the number of slices. We present a flexible design approach based on the optimal control of the full time-dependent Bloch equations and its application to the design of linear phase and low peak-B1 pulses for SMS excitation. We validate the numerical simulations with phantom experiments and show by using a CAIPIRINHA based excitation pattern and a slice GRAPPA reconstruction that our approach is applicable for in vivo experiments.

16:48 0916.   HENSIR: Hadamard Encoded Simultaneous Image Refocusing
Nikolai J Mickevicius1 and Eric S Paulson2
1Biophysics, Medical College of Wisconsin, Milwaukee, WI, United States, 2Radiation Oncology, Medical College of Wisconsin, Milwaukee, WI, United States

Hadamard encoding and simultaneous image refocusing acquisition methods were combined to create a simultaneous multi-slice sequence which allows for increased slice coverage, increased averaging, and reconstruction that does not depend on parallel imaging methods like GRAPPA or SENSE.

17:00 0917.   Transmit Array Spatial Encoding (TRASE) with broadband WURST pulses for robust spatial encoding in inhomogeneous B0 fields
Jason P Stockmann1,2, Clarissa Cooley3,4, Mathieu Sarracanie1,2, Matthew S Rosen1,2, and Lawrence L Wald1,4
1A. A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, 2Department of Physics, Harvard University, Cambridge, MA, United States, 3Massachusetts General Hospital, Charlestown, MA, United States, 4Harvard Medical School, Boston, MA, United States

Transmit array spatial encoding (TRASE) uses linear phase variation in B1+ to perform spatial encoding with spin echo trains. While early TRASE results are promising, the method suffers from artifacts in the presence of B0 off-resonance when refocusing pulse angles deviate significantly from 180°. We show that this problem can be remedied by using frequency-swept WURST RF pulses to accurately refocus all spin isochromats across a wide bandwidth. We further show a simple method to compensate for the quadratic phase imparted to half of the echoes by the frequency-swept RF pulses. TRASE-WURST could benefit applications involving highly inhomogeneous magnetic fields, such as lightweight portable MR scanners.

17:12 0918.   A parallel transmit spectral-spatial pulse design method for ultra-high field MRS combining LSQR and optimal control based optimization
Tingting Shao1, Yun Zhang2, Nikolai Avdievich1, Steffen Glaser2, and Anke Henning1,3
1Max Planck Institute for Biological Cybernetics, Tübingen, Baden-Württemberg, Germany, 2Department of Chemistry, Technical University of Munich, Garching, Germany, 3Institute for Biomedical Engineering, UZH and ETH Zurich, Zurich, Switzerland

This work presents a new spectral-spatial (SPSP) parallel transmit pulse design method for 1H MRS applications in ultra-high field MRI. Based on the recently developed parallel transmission technology, the pulse is first designed by using a subspace preconditioned LSQR method in purpose of locating a global minimum, and is afterwards optimized by using a quasi-Newton based optimal control (OC) method to find a local minimum answer. By combining these two algorithms in this way, SPSP pulses that offer increased robustness against B1+ inhomogeneity and minimized chemical shift displacement artifacts can be achieved and therefore adopted in MRS applications.

17:24 0919.   
A spectral-spatial pulse for improved signal recovery in the small-tip fast recovery (STFR) sequence
Sydney N Williams1, Hao Sun2, Jon-Fredrik Nielsen1, Jeffrey A Fessler2, and Douglas C Noll1
1Biomedical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2Electrical Engineering, University of Michigan, Ann Arbor, Michigan, United States

A novel RF pulse for the steady-state sequence, small-tip fast recovery (STFR), that improves signal loss from inhomogeneity effects by employing a spectral-spatial design pattern computed from an acquired field map. Results include improved signal recovery and lower SAR compared to a purely spectral design.

17:36 0920.   Impact of RF-shimming on the uniformity and specific absorption rate of spin-echo imaging at 7 Tesla
Filiz Yetisir1, Bastien Guerin2, Benedikt A. Poser3, Lawrence L. Wald2,4, and Elfar Adalsteinsson1,4
1Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, 2Dept. of Radiology, Martinos Center for Biomedical Imaging, Charlestown, MA, United States, 3Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands,4Harvard-MIT Division of Health Sciences Technology, Institute of Medical Engineering and Science, Cambridge, MA, United States

In this work, pTx RF-shimming large tip angle pulses were designed that dramatically improved the flip angle uniformity of slice selective spin echo images at 7 T. It was also demonstrated that the pulse design algorithm is capable of reducing local SAR by 48% and 55% for excitation and refocusing pulses respectively at constant flip angle error.

17:48 0921.   Delay-Insensitive Variable-Rate Selective Excitation (DIVERSE)
Adam B. Kerr1, Kangrong Zhu1, Matthew J. Middione2, Hua Wu3, Robert F. Dougherty3, and John M. Pauly1
1Electrical Engineering, Stanford University, Stanford, CA, United States, 2Applied Sciences Laboratory West, GE Healthcare, Menlo Park, CA, United States,3Center for Cognitive and Neurobiological Imaging, Stanford University, Stanford, CA, United States

A novel approach for VERSE RF pulse design is presented that reduces sensitivity to system gradient-RF delay. The delay-insensitive VERSE (DIVERSE) method introduces a constraint on the instantaneous product of the RF and gradient magnitudes, thus reducing the impact of a gradient-RF delay on the resultant excitation. DIVERSE pulses show substantial improvement in robust slice-selective excitation compared to minimum-time VERSE pulses for off-isocenter slices. The performance is validated in simulation, experimental slice profile measurements and in application in a diffusion-weighted spin-echo EPI sequence.