ISMRM 24th Annual Meeting & Exhibition • 07-13 May 2016 • Singapore

Scientific Session: RF Pulse Design

Thursday, May 12, 2016
Room 331-332
16:00 - 18:00
Moderators: Douglas Noll, Holden Wu

Multi-Dimensional Reduced Field-of-View Excitation by Integrated RF Pulse and DYNAMITE B0 Field Design
Suryanarayana Umesh Rudrapatna1, Robin de Graaf1, Terrance Nixon1, and Christoph Juchem1
1Yale University, New Haven, CT, United States
Spatially selective multi-dimensional excitation with large flip angles is challenging, as current RF pulse design methods are computationally involved and typically yield low time-bandwidth product (TB < 10) pulses. The difficulty stems from Bloch equation non-linearity and the inflexibility in B0 pattern generation using linear gradients. This study uses the dynamic multi-coil technique (DYNAMITE), that provides unprecedented B0 shaping flexibility, facilitating the use of 1-dimensional Shinnar-Le Roux (SLR) pulses for 2-dimensional excitation. Selective mouse brain excitation was accomplished by adaptively designing the SLR pulse and the underlying B0 fields generated by DYNAMITE. The resultant zoomed MRI achieved more than two-fold acquisition acceleration at < 4% undesired excitation with TB > 12 pulses.

Designing 2D and 3D selective adiabatic pulses
Albert Jang1,2 and Michael Garwood1
1Center for Magnetic Resonance Research and Department of Radiology, University of Minnesota, Minneapolis, MN, United States, 2Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN, United States
Two-dimensional (2D) adiabatic pulses based on sampling k-space have previously been developed using amplitude modulation in one orthogonal direction and frequency modulation in the other2. Here, a new method for designing two and three-dimensional adiabatic pulses using a sub-pulse approach is introduced. Namely, a parent adiabatic pulse is divided into sub-pulse elements, each of which is a 2D selective pulse. Using this approach, selective excitation is achieved through the 2D pulse while being adiabatically driven by the parent adiabatic pulse. This can be extended to three-dimensions by applying blips along the remaining direction between sub-pulses. Simulation and experimental results are presented, confirming the validity of this approach.

Tailored Spiral In-Out Spectral-Spatial Saturation Pulses for Short and Effective Water Suppression in High Resolution MRSI
Jun Ma1, Carrie Wismans2, Zhipeng Cao1, Dennis W. J. Klomp2, Jannie P. Wijnen2, and William A. Grissom1,3
1Vanderbilt University Institute of Imaging Science, Nashville, TN, United States, 2Department of Radiology, University Medical Centre Utrecht, Utrecht, Netherlands, 3Biomedical Engineering, Vanderbilt University, Nashville, TN, United States
At ultra-high field (7T and above), the increased SNR can be used to significantly improve MRSI spatial resolution, but scan time is a challenge with large acquisition matrixes, so time-efficient water signal suppression is critical. However, at ultra-high field, B1+ and B0 inhomogeneities degrade the performance of time-efficient CHESS water suppression strategies. To address this, we propose to replace conventional spectrally-selective pulses with subject-tailored spiral in-out spectral-spatial (SPSP) saturation pulses that are designed using subject-specific B1+ and B0 maps. The pulses were validated in in vivo experiments.

Improved gradient waveforms for small-tip 3D spatially tailored excitation using Iterated Local Search
Jon-Fredrik Nielsen1, Hao Sun2, Jeffrey A Fessler1,2, and Douglas C Noll1
1Biomedical Engineering, University of Michigan, Ann Arbor, MI, United States, 2Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI, United States
We propose a strategy for the joint design of gradient and radiofrequency waveforms for small-tip 3D spatially tailored excitation, that may lead to more globally optimal excitation k-space trajectories. Currently, gradients are either pre-defined or restricted to certain classes such as echo-planar or concentric shells. Our method makes use of a recently proposed optimization method that expresses k-space with a 2nd-order B-spline basis permitting arbitrary k-space trajectories. We employ this method in an Iterated Local Search strategy, and show that this approach reduces the sensitivity of the excited pattern to the choice of initial k-space trajectory that "seeds" the optimization.

Short-T2 specific excitation by a 'back-and-forth' composite RF pulse
Ethan M Johnson1, Adam B Kerr1, Kim Butts Pauly2, and John M Pauly1
1Electrical Engineering, Stanford University, Stanford, CA, United States, 2Radiology, Stanford University, Stanford, CA, United States
Images of cortical bone have previously been created by selection of RF pulse parameters giving short-$$$T_2$$$-specificity in excitation for a 3D UTE sequence.  The previous demonstration required multiple excitations.  Here a composite pulse is described that creates similar contrast for depicting cortical bone with bright signal.

B0-robust slice-selective excitations for ultra-high field with flip-angle mitigation using parallel transmission
Mathias Davids1,2, Bastien Guérin2,3, Lawrence L Wald2,3,4, and Lothar R Schad1
1Computer Assisted Clinical Medicine, Medical Faculty Mannheim, Heidelberg University, Mannheim, Germany, 2A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, United States, 3Harvard Medical School, Boston, MA, United States, 4Harvard-MIT, Division of Health Sciences and Technology, Cambridge, MA, United States
High field MRI suffers from non-uniform transmit fields and B0 variations due to increased susceptibility effects, making uniform slice-excitation very difficult. We developed a new pulse trajectory – the “twisted spokes” RF pulse – to achieve accurate slice-selection with high in-plane uniformity and greatly improved B0 robustness. The twisted spokes trajectory consists of helical k-space segments oriented along the slice-selection direction (e.g., kz). We found that, when the helical segments are designed appropriately, the resulting RF pulses are short, achieve sharp slice profiles and uniform flip-angle distributions, and – at the same time – are very robust to off-resonance effects.

Optimal Control Design of Turbo Spin-Echo Sequences with Applications to Parallel-Transmit Systems
Alessandro Sbrizzi1, Hans Hoogduin1, Joseph V Hajnal2, Cornelis AT van den Berg1, Peter R Luijten1, and Shaihan Malik2
1UMC Utrecht, Utrecht, Netherlands, 2King's College London, London, United Kingdom
We cast the design of variable refocusing angles in TSE sequences as an optimal control problem. By application of the Adjoint States method (ASM), we are able to design dynamic shimming setting for pTx systems in a patient-specific, online fashion.

IMPULSE-SMS: Local SAR and peak power optimized pTx pulse design for simultaneous multislice imaging at high fields
Mihir Pendse1 and Brian Rutt1
1Stanford University, Stanford, CA, United States
We describe an extension of the IMPULSE pTx design algorithm to enable simultaneous multislice (SMS) excitation. We introduce a strategy for integrating the optimal control method for reducing peak power in SMS with the optimization of pTx channel weightings. Desirable features of IMPULSE, including the ability to optimize spoke locations and to design pulses without SAR compression, are retained in this extension. We demonstrate that, even for large multiband acceleration factors, our approach enables design of pTx pulses that minimize local SAR while achieving acceptable in-slice homogeneity under strict peak power constraints.  

Universal pulses: a new concept for calibration-free parallel transmission
Vincent Gras1, Alexandre Vignaud1, Alexis Amadon1, Denis Le Bihan1, and Nicolas Boulant1
1Neurospin, CEA/DSV/I2BM, Gif-sur-Yvette, France
At ultra-high field, a drawback of parallel transmission to mitigate the RF inhomogeneity problem is the necessity to measure subject-specific field maps in order to return optimized RF pulses, thereby decreasing the time available for clinically-relevant scans.  In this work, we investigate numerically and experimentally at 7T the design of "universal" kT-points pulses, which does not require the aforementioned calibration step but yet considerably improves excitation homogeneity compared to the standard circularly-polarized and RF shim modes. Such approach can simplify considerably the workflow of parallel transmission and render the potential of ultra-high field scanners more accessible to anyone in routine. 

RF Shimming for High Field MRI using Multi-channel Receive-Signals
Abhinav V. Sambasivan1, Lance DelaBarre2, Emad S. Ebbini1, Thomas J. Vaughan1,2, and Anand Gopinath1
1Electrical and Computer Engineering, University of Minnesota-Twin Cities, Minneapolis, MN, United States, 2Center for Magnetic Resonance Research, UMN-Twin Cities, Minneapolis, MN, United States
Counteracting the effects of B1 heterogeneities has been a major challenge for High field MRI systems. We propose here, a receiver-based approach called the Receive-RF Shimming (Rx-RFS) algorithm for multichannel MR systems which offers potential advantages in terms of reducing image acquisition time and mitigating SAR concerns. RX-RFS involves computing an optimal spatially-varying weight vector for combining the images from different receive elements. The reconstructed images (using Rx-RFS) exhibit enhanced contrast and more uniform signal levels when compared to standard reconstruction schemes throughout the entire Field-of-View. Rx-RFS also offers clinicians the flexibility to obtain local reconstructions at arbitrary Regions-of-Interest. 

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