Sebastian Schmitter¹, Xiaoping Wu¹, Steen Moeller¹, Edward John Auerbach¹, Gregor Adriany¹, Pierre-Francois Van de Moortele¹, and Kamil Ugurbil^1
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States

Time-of-flight (TOF) angiography significantly benefits from ultra-high field (7T) 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.

Mihir Pendse¹ and Brian Rutt^1
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.

Bastien Guerin¹, Jason Stockmann^1,2, Mehran Baboli³, Andrew V Stenger³, and Lawrence L Wald^1,4
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.

Christian Findeklee¹, Christoph Leussler¹, Peter Vernickel¹, and Ulrich Katscher^1
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.

Desmond Ho Yan Tse^1,2, Daniel Brenner³, Bastien Guerin⁴, and Benedikt A Poser^1
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.

Zhipeng Cao^1,2 and William A. Grissom^1,2
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.

Nicolas Boulant¹, Xiaoping Wu², Gregor Adriany², Sebastian Schmitter², Kamil Ugurbil², and Pierre-Francois Van de Moortele^2
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.

Cem M. Deniz^1,2, Giuseppe Carluccio^1,2, Daniel K. Sodickson^1,2, and Christopher M. Collins^1,2
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.

Martina Flöser^1,2, Andreas K. Bitz¹, Stephan Orzada², Klaus Solbach³, and Mark E. Ladd^1,2
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.

Mathias Davids^1,2, Bastien Guérin², Lawrence L. Wald^2,3, and Lothar R. Schad^1
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.