ISMRM 21st Annual Meeting & Exhibition 20-26 April 2013 Salt Lake City, Utah, USA

RF Pulse Design
Monday 22 April 2013
Room 151 AG  14:15 - 16:15 Moderators: William A. Grissom, John M. Pauly

14:15 0071.   
Improved Excitation Fidelity in Cardiac Imaging with 2-Spoke Parallel Excitation at 7 Tesla
Sebastian Schmitter1, Lance DelaBarre1, Xiaoping Wu1, Andreas Greiser2, Dingxin Wang3, Kamil Ugurbil1, and Pierre-Francois Van de Moortele1
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States, 2Siemens AG, Erlangen, Bavaria, Germany, 3Siemens Medical Solutions USA, Inc., Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States

Cardiac MRI may greatly benefit from ultra-high field (UHF) providing higher SNR and intrinsic contrast. However, shorter RF wavelength at UHF results in transmit B1 (B1+) and contrast variations through the heart. This problem has been addressed using multi-channel transmit coils and B1-shimming, but further improvements are expected by using parallel transmission (pTX) with multi-spoke RF-pulses as shown in brain and liver at 7T. However, this involves additional challenges, including rapid and robust multi-channel ECG-triggered B1+ calibration, and sensitivity to motion. Here, we investigate the impact of 2-spoke RF-pulse design on 7T cardiac imaging using a 16-channel pTX system.

14:27 0072.   Improvement in B1+ Homogeneity of 3T Cardiac MRI in Swine with Dual-Source Parallel RF Excitation
Daniel A. Herzka1, Haiyan Ding2,3, and Michael Schar4,5
1Biomedical Engineering, Johns Hopkins University, Baltimore, MD, United States, 2Biomedical Engineering, Tsinghua University, Beijing, China, 3Biomedical Engineering, Johns Hopkins School of Medicine, Baltimore, MD, United States, 4Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins School of Medicine, Baltimore, MD, United States, 5Clinical Science, Philips Healthcare, Cleveland, OH, United States

MRI scanners use an integrated birdcage coil to generate radio frequency (RF) excitation fields (B1+). It has been reported that at 3T variations in flip angle can range from 31 to 66% (B1+ inhomogeneities) over the left ventricle and average flip angles can be reduced by ~20% (RF power). Multi-channel transmit systems allow RF-shimming to locally improve the B1+ field. We quantify improvements in B1+ field homogeneity and average RF power resulting from dual-source parallel RF excitation in cardiac swine imaging. Correlation with animal size demonstrates that larger subjects suffer more B1+ field inhomogeneity and benefit more from RF shimming.

14:39 0073.   
kT-PINS RF Pulses for Low-Power Field Inhomogeneity-Compensated Multislice Excitation
Anuj Sharma1, Samantha J. Holdsworth2, Rafael O'Halloran2, Eric Aboussouan2, Anh Tu Van2, Julian R. Maclaren2, Murat Aksoy2, Victor Andrew Stenger3, Roland Bammer2, and William A. Grissom1
1Biomedical Engineering, Vanderbilt University, Nashville, Tennessee, United States, 2Radiology, Stanford University, Stanford, California, United States, 3Medicine, University of Hawaii, Honolulu, Hawaii, United States

A new class of patient-tailored multiband RF pulses "kT-PINS" is presented that combines PINS multiband excitation pulses with kT-points 3D patient-tailored excitation pulses, enabling simultaneous excitation of multiple slices and compensation of transmit field inhomogeneities. An interleaved greedy and local optimization algorithm was developed to design the pulses. Phantom experiments demonstrate the effectiveness of the pulses in reducing flip angle inhomogeneity caused by transmit field inhomogeneity at 7T.

14:51 0074.   
Simultaneous Multi-Slice Parallel RF Excitation with In-Plane B1+ Homogenization
Xiaoping Wu1, Sebastian Schmitter1, Edward J. Auerbach1, Steen Moeller1, Kamil Ugurbil1, and Pierre-Francois Van de Moortele1
1CMRR, Radiology, University of Minnesota, Minneapolis, MN, United States

Simultaneous multi-band (MB) RF excitation, along with subsequent unaliasing via parallel imaging principles, provides an effective means to accelerate volume coverage along the slice direction. Recently, the approach has been exploited with significant success in functional and diffusion-weighted imaging studies of the brain. So far this technique has only been demonstrated in the context of single channel transmit. In this study, we extend this technique to multi-channel transmit and introduce parallel transmit (pTX) MB pulse design in order to tackle the issues of B1+ inhomogeneity at high and ultrahigh field strengths, and RF power deposition. The new extension is validated in the human brain at 7T and is demonstrated capable of providing good B1+ homogenization in addition to simultaneous MB excitation without necessitating the use of higher RF energy relative to a single channel application.

15:03 0075.   
Simultaneous Multi-Slice Excitation by Parallel Transmission Using a Dual-Row PTX Head Array
Benedikt A. Poser1, Robert James Anderson1, Peter Serano2, Azma Mareyam2, Bastien Guérin2, Weiran Deng1, Lawrence L. Wald2, and Victor Andrew Stenger1
1John A Burns School of Medicine, University of Hawaii, Honolulu, Hawaii, United States, 2Radiology, Massachusetts General Hospital, Charlestown, MA, United States

Simultaneous multi-slice (SMS) acquisitions have become popular for single-shot sequences such as BOLD and DW-EPI. SMS excitations are commonly achieved with multiband pulses. We explore parallel transmission (pTX) for SMS excitation, using differently frequency-shifted pulses on subsets of transmitters that define the excited slices. Using a dual-ring pTX coil and blippedCAIPIRINHA EPI, we show factor-2 SMS with conventional single-band pulses and factor-4 SMS using dual-band pulses on each ring. For pTX coils with inherent transmit-sensitivity encoding along the slice direction, the approach can reduce required RF power (global SAR) compared to MB pulses with as many frequency bands as slices applied on all coil elements.

15:15 0076.   
Subject- And Resource-Specific Monitoring and Proactive Management of Parallel RF Transmission
Cem Murat Deniz1,2, Leeor Alon2,3, Ryan Brown3, Daniel K. Sodickson2,3, and Yudong Zhu2,3
1Department of Radiology, Bernard and Irene Schwartz Center for Biomedical Imaging, New York University, New York, NY, United States, 2Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY, United States, 3Department of Radiology, Bernard and Irene Schwartz Center for Biomedical Imaging, New York University School of Medicine, New York, NY, United States

Given the constraints imposed by the hardware resource in terms of RF power delivery and reflection handling capabilities, any practical RF pulse used on an MR scanner must be designed with those capabilities on mind. In parallel transmission, coupling and interaction taking place in the multi-port structure as well as in the subject can significantly affect individual channel RF power transmission and the demands placed upon the power amplifiers. By using pre-scan based multi-channel calibration, we designed parallel RF excitation pulses obeying the forward / reflected peak and average power limits of the RF power amplifier. Additionally, global SAR limits were incorporated in the RF pulse design. Results showed that the prediction capability of this new calibration method enables the design of parallel RF excitation pulses respecting strict and multifaceted power limits.

15:27 0077.   
Improvement in T2-Weighted Imaging at 7T by Using KT-Points
Florent Eggenschwiler1, Kieran O'Brien2, Rolf Gruetter1,3, and José P. Marques4
1EPFL, Laboratory for Functional and Metabolic Imaging, Lausanne, Vaud, Switzerland, 2University of Geneva, Department of Radiology, Geneva, Geneva, Switzerland, 3Universities of Geneva and Lausanne, École Polytechnique Fédérale de Lausanne, Lausanne, Vaud, Switzerland, 4University of Lausanne, Department of Radiology, Lausanne, Vaud, Switzerland

At high magnetic field strengths (B0 > 3T), the inhomogeneous distribution of the B1+ field causes undesirable signal and contrast variations of GM/WM across the brain. These artifacts impair the quality of T2-weigthed images at high field and may mislead any medical diagnosis that depends on these images. In this study, short 3D tailored RF pulses (kT-points) were combined with a variable flip angle TSE sequence to obtain T2-weighted anatomical images with uniform contrast throughout the whole brain at 7T. A symmetric k-space trajectory ensured that the excitation profile associated with the kT-points remained close to the predicted STA approximation over the large range of flip angles used in the TSE sequence and gave optimal contrast homogeneity. The proposed methodology was tested at 7T, on both single-channel and PTx systems and demonstrates very promising achievements in T2-weighted brain imaging.

15:39 0078.   
Fast Reconstruction for RF Monitored Sweep Imaging with Sideband Excitation
David Otto Brunner1, Benjamin E. Dietrich1, Matteo Pavan1, and Klaas P. Pruessmann1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Zurich, Switzerland

Pulsed NMR spectroscopy and imaging using stochastic1 or swept2 excitation during signal acquisition requires sets high requirements on the transmit chain in order to accurately deconvolve the received signal. Monitoring the RF pulse concurrently with the acquisition alleviates many of those requirements, however the computational effort is increased in particular if other confounding, such as propagation delay differences or temporally variable off-resonances need to be corrected. We present a fast FFT based reconstruction approach which is applied to SWIFT imaging using sideband excitation showing the benefits of the applied corrections.

15:51 0079.   B1-Insensitive Slice-Selective Pseudo-Adiabatic Pulse
Benoît Bourassa-Moreau1, Guillaume Gilbert1,2, and Gilles Beaudoin1
1Department of Radiology, Centre Hospitalier de l'Université de Montréal, Montreal, Quebec, Canada, 2Philips Healthcare, Montreal, Quebec, Canada

In this work, we present a slice-selective pseudo-adiabatic excitation (pBIR4-S1s2) which offers a B1-insensitive excitation at an arbitrary flip angle in a comparatively short duration (<10 ms). Slice-selection is achieved by replacing the hard RF sub-pulses of the pseudo-adiabatic pulse by slice-selective sub-pulses (SLR) of the same time-integrated areas and driven with an oscillating gradient. Simulated magnetization response and experimental results are shown.

16:03 0080.   
A Simple Fat Suppression Method for Accelerated and Low-SAR 3D-EPI
Rüdiger Stirnberg1, Daniel Brenner1, Tony Stöcker1, and Nadim Jon Shah1,2
1Institute of Neuroscience and Medicine - 4, Research Centre Juelich GmbH, Jülich, Germany, 2Department of Neurology, Faculty of Medicine, JARA, RWTH Aachen University, Aachen, Germany

A simple modification of a 3D-EPI sequence is proposed which renders additional time- and SAR-demanding fat suppression obsolete. Instead, using a rectangular excitation pulse yields robust fat suppression with minimal SAR if the pulse duration is selected carefully. This method, particularly useful for high field application, facilitates high-resolution functional imaging (fMRI) at temporal resolutions of two seconds or less. The basic concept and an expression for the optimal pulse duration (also valid for large flip angles) are introduced, validated in vivo at 3T and applied to finger tapping fMRI at 1.5mm isotropic resolution.