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

Scientific Session • Parallel Transmission Strategies

Wednesday 3 June 2015

Room 714 A/B

10:00 - 12:00


Ulrich Katscher, Ph.D., Mark E. Ladd, Ph.D.

10:00 0542.   Slab-selective pTX Multiband TOF Angiography at 7 Tesla
Sebastian Schmitter1, Xiaoping Wu1, Steen Moeller1, Edward John Auerbach1, Gregor Adriany1, Pierre-Francois Van de Moortele1, and Kamil Ugurbil1
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States

Time-of-flight (TOF) angiography significantly benefits from ultra-high field (greater than or equal to7T) allowing for improved angiographic contrast and higher spatial resolution. However, high-resolution TOF at 7T is associated with challenges: higher resolution requires longer acquisition and despite stronger contrast, the latter is spatially heterogeneous due to shorter RF wavelength at 7T. Previously, we have addressed spatial heterogeneity by utilizing a 16-channel pTX system together with spoke RF pulses. In this work we aim to address both challenges, heterogeneity AND acquisition time by applying a pTX multi-band technique on a 16-channel pTX system to simultaneously excite multiple TOF slabs while achieving homogeneous contrast.

10:12 0543.   
IMPULSE: A Generalized and Scalable Algorithm for Joint Design of Minimum SAR Parallel Transmit RF Pulses
Mihir Pendse1 and Brian Rutt1
1Radiology, Stanford University, Stanford, CA, United States

Joint design of pTx RF waveforms and excitation k-space trajectories to mitigate flip angle inhomogeneity while abiding by strict patient-specific SAR limits is a difficult nonconvex problem. We describe the “minSAR” formulation of the problem and an efficient new optimization algorithm (IMPULSE). Major benefits of the algorithm include (a) direct optimization using a complete SAR estimate without compression, (b) tractable optimization of k-space trajectories, and (c) ability to perform joint optimization across many pulses to allow temporal hotspot averaging. Compared to prior methods, we demonstrate reduced local SAR values and shorter computation times for identical levels of excitation homogeneity.

10:24 0544.   
Fully optimized time-shifted radio-frequency spoke pulses for simultaneous reduction of intra-voxel dephasing, flip-angle non-uniformity and the specific absorption rate at ultra-high field using parallel transmission
Bastien Guerin1, Jason Stockmann1,2, Mehran Baboli3, Andrew V Stenger3, and Lawrence L Wald1,4
1Department of Radiology, Massachusetts General Hospital, Charlestown, MA, United States, 2Physics department, Harvard University, Cambridge, MA, United States, 3John A. Burns School of Medicine, University of Honolulu, Honolulu, United States, 4Division of Health Sciences Technology, Harvard-MIT, Cambridge, MA, United States

We propose a slice-selective parallel transmit (pTx) pulse design that simultaneously addresses the problems of B1+ inhomogeneity, through-plane dephasing and SAR at ultra-high field. The technique is based on a new small tip-angle equation including the effects of thick-slice averaging, time-shifting of the spoke sub-pulses and the presence of a background through-plane B0 gradient. We fully optimize the spoke amplitudes and time-shifts using a fast primal-dual optimization with analytical gradients and Hessian. We also show that time-shifting the spoke sub-pulse also helps reducing SAR. We demonstrate the method on a 3D-printed B1/B0 phantom and an 8-channel 7T pTx system.

10:36 0545.   RF Shimming via Efficient Modes for Massively Parallel Transmit Coils
Christian Findeklee1, Christoph Leussler1, Peter Vernickel1, and Ulrich Katscher1
1Research Laboratories Hamburg, Philips GmbH Innovative Technologies, Hamburg, Hamburg, Germany

Instead of using single coil elements as basis for RF shimming, we propose to use eigenmodes, which can be sorted by efficiency w.r.t. average B1 vs. power or global SAR. By using just most efficient modes, we inherently achieve a good compromise between homogeneity and power/SAR in a direct approach instead of a time-consuming traditional L-curve evaluation.

10:48 0546.   High resolution GRE at 9.4T using spokes pulses
Desmond Ho Yan Tse1,2, Daniel Brenner3, Bastien Guerin4, and Benedikt A Poser1
1Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands, 2Department of Radiology, Maastricht University Medical Centre, Maastricht, Netherlands, 3German Centre for Neurodegenerative Diseases (DZNE), Bonn, Germany, 4Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Boston, Massachusetts, United States

Slice-specific spokes pulses were designed and applied for high resolution GRE imaging at 9.4T. The problems of SNR drop out and contrast variations due to RF inhomogeneity which is typical at ultra high field were mitigated by the spokes pulses. This led to improvements in both SNR and contrast, which ultimately allows fine vein structures in cortex to be observed in these high resolution images.

11:00 0547.   
Array-compressed parallel transmit pulse design
Zhipeng Cao1,2 and William A. Grissom1,2
1Biomedical Engineering, Vanderbilt University, Nashville, TN, United States, 2Vanderbilt University Institute of Imaging Science, Nashville, TN, United States

Synopsis: An array-compressed parallel transmit pulse design concept is proposed to enable many-coil transmit arrays to be optimally driven by a small number of RF amplifiers/channels. It is demonstrated in three pulse design applications that by integrating coil compression into parallel transmit pulse design, more accurate pulses can be designed than with approaches that do not consider the spatial encoding demands of the pulse design problem when computing coil array combination weights.

11:12 0548.   Direct control of the temperature rise in parallel transmission via temperature virtual observation points: simulations at 10.5 T
Nicolas Boulant1, Xiaoping Wu2, Gregor Adriany2, Sebastian Schmitter2, Kamil Ugurbil2, and Pierre-Francois Van de Moortele2
1NeuroSpin, CEA, Saclay, Ile de France, France, 2Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States

We report a parallel transmission radiofrequency pulse design algorithm under strict temperature rise constraints. The latter are directly enforced by using an equivalent SAR virtual observation point model, this time based on temperature that we shall name T(Temperature)VOPs. Simulations are performed at 10.5 T with a 16 channels coil on the Duke head model, and with Time-Of-Flight sequences of various durations. The algorithm here benefits from the lack of direct correspondence between the SAR and temperature to return in all cases more powerful and safer RF pulses than the ones returned by a traditional SAR-constrained pulse design.

11:24 0549.   Non-iterative Parallel Transmission RF Pulse Design with Strict Temperature Constraints
Cem M. Deniz1,2, Giuseppe Carluccio1,2, Daniel K. Sodickson1,2, and Christopher M. Collins1,2
1Center for Advanced Imaging Innovation and Research (CAI2R), Department of Radiology, New York University School of Medicine, New York, NY, United States, 2The Sackler Institute of Graduate Biomedical Sciences, New York University School of Medicine, New York, NY, United States

RF safety in parallel transmission (pTx) is generally ensured by imposing SAR limits during pTx RF pulse design. Several methods have been proposed for incorporating these limits into pulse design using strict constraints or Tikhonov regularizations. Recently, a temperature based iterative RF pulse design method was proposed using temperature simulations at each iteration and updating the SAR constraints accordingly. In this work, a non-iterative parallel transmission RF pulse design is demonstrated using strict temperature constraints that are derived from temperature correlation matrices. Temperature correlation matrices and B1+ maps were obtained from electrodynamic and thermal simulations of an 8 channel head array with a numerical model of human head.

11:36 0550.   Comparison of Local and Remote Transmit Arrays for Body Imaging at 7T under Power and Local SAR Constraints
Martina Flöser1,2, Andreas K. Bitz1, Stephan Orzada2, Klaus Solbach3, and Mark E. Ladd1,2
1Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany, 2Erwin L. Hahn Institute for MRI, University Duisburg-Essen, Essen, Germany, 3High Frequency Engineering, University Duisburg-Essen, Duisburg, Germany

So far, local coil arrays that are placed directly on the subject are most commonly used for body imaging at 7T. Placing the coil array under the bore liner would simplify the workflow and increase subject comfort. Therefore, we compare the performance of several local and remote body coil arrays under power and local SAR constraints in simulations. While tight-fitting coil arrays perform better in axial slices, remote arrays achieve better B1+ homogeneity in coronal and sagittal slices. Arranging the coil elements in multiple rings can be beneficial for B1+ homogeneity, but is power and SAR demanding.

11:48 0551.   
Ultra-fast inner volume excitations with parallel transmission at 7 Tesla using fully optimized B0-robust k-space trajectories
Mathias Davids1,2, Bastien Guérin2, Lawrence L. Wald2,3, and Lothar R. Schad1
1Computer Assisted Clinical Medicine, Medical Faculty Mannheim, Heidelberg University, Mannheim, BW, Germany, 2Martinos Center for Biomedical Imaging, Dept. of Radiology, Massachusetts General Hospital, Charlestown, MA, United States, 3Harvard-MIT Division of Health Sciences Technology, Cambridge, MA, United States

Three-dimensional spatially selective excitations remain difficult due to limitations of the pulse duration and off-resonance induced distortions, especially at high-field strengths. It is shown that the shape optimization of k-space trajectory increases the achieved flip angle accuracy substantially while maintaining acceptable pulse durations (less than 8 ms). In particular, the incorporation of off-resonance robustness constraints within the optimization creates trajectories with an optimal tradeoff between pulse duration, excitation fidelity, and off-resonance robustness. The impact is evaluated based on simulations as well as preliminary experimental data acquired using an 8-channel parallel transmit system at 7 Tesla.