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Andrea N Sajewski¹, Tales Santini¹, Tiago Martins¹, Jacob Berardinelli¹, and Tamer Ibrahim^1
1University of Pittsburgh, Pittsburgh, PA, United States

Keywords: In Silico, RF Arrays & Systems

The recently developed 60-channel Tic-Tac-Toe transmit coil was analyzed for use in either single or parallel transmit systems at 7T. With the 60-channel TTT coil, excellent homogeneity can be achieved and safe levels of SAR can be provided regardless of which system (sTx or pTx) is used. 

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Maxim Terekhov¹, David Lohr¹, Christoph Aigner², Sebastian Dietrich², Sebastian Schmitter², and Laura M. Schreiber^1
1Chair of Molecular and Cellular Imaging, Comprehensive Heart Failure Center, University Hospital Würzburg, Wuerzburg, Germany, ²Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Berlin, Germany

Keywords: Parallel Transmit & Multiband, Cardiovascular

Parallel transmit (pTX) technology is an emerging tool for improving of the B₁⁺-field homogeneity in cardiac MRI (cMRI) at the ultra-high magnetic field. In this work, we propose a methodology to enhance the characteristics of the shaped B₁⁺ field using a tailored cost function for the optimization procedure computing complex transmit vectors of magnitudes and phases for driving  TX array. Using the cost functions (CF) based on the local B₁ gradients with fine-tuning by weighting coefficients allows for considerable improvement of static pTX B₁-shimming quality in compared to traditional CF using the coefficient-of-variation (CoV) of B₁.

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Bilguun Nurzed¹, Dennis Hieronymi¹, Thomas Wilhelm Eigentler¹, Christoph Stefan Aigner², Sebastian Schmitter², and Thoralf Niendorf^1,3,4
1Max-Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany, ²Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany, ³Experimental and Clinical Research Center (ECRC), a joint cooperation between the Charité Medical Faculty and the Max-Delbrück Center for Molecular Medicine, Berlin, Germany, ⁴MRI.TOOLS, Berlin, Germany

Keywords: Shims, RF Arrays & Systems, Cardiovascular

Transmission field inhomogeneities at ultrahigh and extreme field MRI can be offset using static or dynamic pTx. Responding to the challenges and recognizing the opportunities of cardiac MRI, this abstract examines the feasibility of parallel transmission (pTx) using self-grounded bow-tie (SGBT) RF array configurations for static and dynamic B₁⁺ homogenization of the heart at 21.0T. Our results demonstrate that static pTx provides limited performance at 21.0 T, but dynamic pTx enables uniform heart excitation at 21.0T

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Nejat Karadeniz¹, Jo Hajnal^1,2, and Özlem Ipek^1
1Biomedical Engineering, King's College London, London, United Kingdom, ²Centre for the Developing Brain, King's College London, London, United Kingdom

Keywords: RF Arrays & Systems, Safety

MRI multi-row array coils can offer increased flexibility to control and tailor the RF field distribution through RF Shimming. The performance of various multi-row configurations of parallel-transmit (pTx) RF coil array for brain imaging at 3T is investigated using electromagnetic field simulations with digital human computational model for overlapping and non-overlapping loop coil elements in 16- to 24-channel single, double, triple-row arrays. We found that triple-row coil arrays improve the transmit field homogeneity in the entire volume of the head.  The double-row coil array offers the best SAR efficiency among other investigated coils.

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Ehsan Kazemivalipour^1,2, Markus W. May^3,4, D. Rangaprakash^1,2, Berkin Bilgic^1,2,5, Jason P. Stockmann^1,2, Robert L. Barry^1,2, Boris Keil^6,7, Lawrence L. Wald^1,2,5, and Bastien Guerin^1,2
1A. A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, United States, ²Harvard Medical School, Boston, MA, United States, ³Erwin L. Hahn Institute for MRI, University Duisburg-Essen, Essen, Germany, ⁴High-Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany, ⁵Harvard-MIT Division of Health Sciences Technology, Cambridge, MA, United States, ⁶Institute of Medical Physics and Radiation Protection, Department of Life Science Engineering, Mittelhessen University of Applied Sciences, Giessen, Germany, ⁷Center for Mind, Brain and Behavior (CMBB), Philipps-University Marburg, Marburg, Germany

Keywords: RF Pulse Design & Fields, RF Pulse Design & Fields

We calculate “universal” RF-shimming excitations that provide maximum B₁⁺ coverage and homogeneity across the brain & c-spine using a patient-friendly 16-channel pTx array. We simulated three body models and three z-positions to ensure robustness to both B₁⁺ and SAR variation across a range of head/neck sizes and shapes (universal design). We provide two solutions, both of which may be saved on the scanner: a low-SAR solution that improves flip-angle uniformity by 13% compared to the BC mode excitation (same SAR) and a high-SAR excitation that improves flip-angle non-uniformity by 25% at the cost of a 2x local SAR increase.

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Igor Tyshchenko¹, Simon Lévy², Jin Jin³, Bahman Tahayori⁴, and Leigh A. Johnston^1
1Department of Biomedical Engineering, The University of Melbourne, Melbourne, Australia, ²MR Research Collaborations, Siemens Healthcare Pty Ltd, Melbourne, Australia, ³MR Research Collaborations, Siemens Healthcare Pty Ltd, Brisbane, Australia, ⁴The Florey Institute of Neuroscience and Mental Health, Melbourne, Australia

Keywords: Parallel Transmit & Multiband, Parallel Transmit & Multiband, Universal pulses, safety margins, 7T MRI

Universal pulse (UP) design in parallel transmit (pTx) imaging is a calibration-free technique that can effectively mitigate the radiofrequency (RF) field inhomogeneity over a population in ultra-high field MRI. In this work, we explore a middle-ground approach of using sub-population UPs. Assuming the entire population is divided into sub-populations by grouping similar anatomical shapes and positions, UPs can thus be designed with reduced intersubject variability. In this preliminary study of head imaging, it is demonstrated that sub-populations of various sizes centred at the mean shape and position can reduce safety margins and improve UP’s performance.

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James L. Kent¹, Ladislav Valkovič^2,3, Iulius Dragonu⁴, Mark Chiew^1,5,6, and Aaron T. Hess^1
1Wellcome Centre for Integrative Neuroimaging, FMRIB, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom, ²Oxford Centre for Clinical Magnetic Resonance Research, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom, ³Department of Imaging Methods, Institute of Measurement Science, Slovak Academy of Sciences, Bratislava, Slovakia, ⁴Research & Collaborations GB&I, Siemens Healthcare Ltd, Camberley, United Kingdom, ⁵Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada, ⁶Physical Sciences, Sunnybrook Research Institute, Toronto, ON, Canada

Keywords: Parallel Transmit & Multiband, Parallel Transmit & Multiband

Ultra-high field MRI with parallel transmit offers significant advantages but B₁⁺ maps are required to utilize its potential. Acquiring B₁⁺ maps for each channel is time-consuming and limits the application of ultra-high field in clinical practice. In this abstract we present an approach to acquire full 3D multi-channel B₁⁺ maps in 11 seconds. We do this by employing a time-interleaved acquisition of modes strategy using undersampled relative maps and two fast undersampled Sandwich B₁⁺ maps all reconstructed using TxLR, a calibrationless reconstruction for transmit field maps. We demonstrate this approach with simulations and scans in phantom and in the brain.

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Alexander Smerglia¹, Labros Petropoulos¹, Xiaoyu Yang¹, Tsinghua Zheng¹, Paul Taylor¹, Blaise Whitesell¹, Jagjit Sidhu², Ken Sakaie², and Mark Lowe^2
1Quality Electrodynamics (QED), Mayfield, OH, United States, ²Imaging Institute, Cleveland Clinic, Cleveland, OH, United States

Keywords: Parallel Transmit & Multiband, Brain

An 8-parallel transmit 32-Rx head coil was built and evaluated for performance in high-field MRI. The coil is comprised of an 8-rung curved degenerate birdcage transmitter and a 32-loop element receive array. The coil housing was designed with an anterior/posterior split and eye windows for patient comfort, and a sizable interior that can fit 99 percent of the general population. A volunteer was imaged to confirm transmit coverage, which includes the entire brain and upper cervical spine. Good B1 uniformity, and improved transmit efficiency over previously built non-curved Tx head coils were also confirmed.

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Celik Boga¹, Ole Geldschlager², and Anke Henning^1
1UT Southwestern Medical Center, Dallas, TX, United States, ²Max Planck Institute for Biological Cybernetics, Tuebingen, Germany

Keywords: RF Pulse Design & Fields, High-Field MRI

The Universal Pulse (UP) concept to parallel transmission radiofrequency (RF) pulse design was introduced to avoid time consuming B₁⁺ and ΔB₀ data acquisition before each clinical scan. Introduced large flip angle design methods utilize kT-point trajectories and small tip angle approximation. In this work, new quasi damping scheme for Newton's methods method is introduced for large flip angle universal pulse design using numerical solution of the Bloch equation. Introduced quasi-damping scheme can be utilized in large flip angle Universal Pulse design for all flip angles including 180^o pulses, without the loss of performance.

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Minghao Zhang¹, Belinda Ding^1,2, Iulius Dragonu², Patrick Liebig³, Robin M. Heidemann³, and Christopher T. Rodgers^1
1Wolfson Brain Imaging Centre, University of Cambridge, Cambridge, United Kingdom, ²Siemens Healthcare Limited, Camberley, United Kingdom, ³Siemens Healthcare GmbH, Erlangen, Germany

Keywords: Parallel Transmit & Multiband, Diffusion Tensor Imaging

Diffusion MRI (dMRI) is inherently limited by SNR. 7T scans increase intrinsic SNR but suffer from regions of signal dropout, especially in temporal lobes and cerebellum. We applied dynamic parallel transmit (pTx) to allow whole-brain 7T dMRI and scanned 7 volunteers comparing pTx 2-spoke and circularly polarized pulses.

PTx 2-spoke scans increased whole-brain mean temporal SNR increased by 22%, produced cleaner fractional anisotropy maps, and reduced fiber estimation uncertainty by 4% (P=0.016) for first fibers and 2% (P<0.001) for second fibers. However, less restrictive SAR limits will be needed for clinical translation of our approach due to long scan time.

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Shin-ichi Urayama¹, Masaki Fukunaga², and Martijn Cloos^3
1Human Brain Research Center, Graduate School of Medicine, Kyoto University, Kyoto, Japan, ²National Institute for Physiological Sciences, Okazaki, Japan, ³University of Queensland, Brisbane, Australia

Keywords: RF Arrays & Systems, RF Arrays & Systems, RF-mode, non-pTx

To mitigate B1 inhomogeneity problems in ultra-high-field MRI, we proposed RF-mode switching technique which doesn’t need an expensive parallel transmit (pTx) system. As a preliminary study, we developed an RF-switching circuit (one of the main parts of the system) and validated its function through phantom experiments. The results showed that the switching function works well, although a certain degree of power-loss was observed both at bench (< -1dB) and on the images. We have re-designed a new switching circuit and will continue to the development to demonstrate this technique at a fraction of the cost of a pTx system.

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Belinda Ding^1,2, Sydney Nicole Williams¹, Minghao Zhang³, Jürgen Herrler⁴, Patrick Liebig⁴, Iulius Dragonu⁵, Radhouene Neji⁵, Christopher T. Rodgers³, and David A Porter^1
1Imaging Centre of Excellence, University of Glasgow, Glasgow, Scotland, ²Siemens Healthcare Ltd., Glasgow, United Kingdom, ³Wolfson Brain Imaging Centre, University of Cambridge, Cambridge, United Kingdom, ⁴Siemens Healthcare GmbH, Erlangen, Germany, ⁵Siemens Healthcare Ltd., London, United Kingdom

Keywords: Parallel Transmit & Multiband, Parallel Transmit & Multiband, phase schedule

Multiband(MB) imaging can greatly reduce scan time, but MB pulses often encounter peak amplitude constraints. An optimal phase schedule has been published for traditional single-transmit (sTx) RF pulses. However, after an initial evaluation, we found that it only has marginal benefits for parallel-transmit (pTx) pulses. We compared four common optimisation algorithms (fmincon, fminsearch, simmulated annealing, global search) to determine the most suitable algorithm for choosing the best offset phases for a pTx pulse. Overall, fmincon efficiently selected the optimal phase schedule of a pTx pulse and lowered peak amplitude by an average of 15% when MB-factor=3.

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Belinda Ding^1,2, Sydney Nicole Williams¹, Iulius Dragonu³, Jürgen Herrler⁴, Sarah Allwood-Spiers⁵, Patrick Liebig⁴, and David A Porter^1
1Imaging Centre of Excellence, University of Glasgow, Glasgow, Scotland, ²Siemens Healthcare Ltd., Glasgow, United Kingdom, ³Siemens Healthcare Ltd., London, United Kingdom, ⁴Siemens Healthcare GmbH, Erlangen, Germany, ⁵NHS Greater Clyde and Glasgow, Glasgow, Scotland

Keywords: Parallel Transmit & Multiband, Diffusion/other diffusion imaging techniques, high-field, 7T, DWI, repeatability

Parallel transmission (pTx) can significantly improve readout-segment EPI (rsEPI) diffusion-weighted imaging (DWI) at 7T when compared to the non-pTx sequence. However, no study has been done to assess the repeatability of pTx-DWI. Thus, we conducted a test-retest study to evaluate the impact pTx pulses have on the repeatability of ADC measures in a rsEPI[DP1]  DWI sequence at 7T.   Overall, pTx-DWI had higher SNR and can potentially improve the repeatability for intra- and inter-session ADC measures even when different B1-shim coefficients are used for different sessions. This suggests that pTx has an important role in quantittaive imaging studies at 7T.

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Aurelien Destruel^1,2,3, Franck Mauconduit⁴, Aurélien Massire⁵, Vincent Gras⁴, and Virginie Callot^1,2,3
1Aix-Marseille Univ, CNRS, CRMBM, Marseille, France, ²APHM, Hôpital Universitaire Timone, CEMEREM, Marseille, France, ³iLab-Spine, International Associated Laboratory, Montréal, Canada, Marseille, France, ⁴Paris-Saclay University, CEA, CNRS, BAOBAB, NeuroSpin, Gif-sur-yvette, France, ⁵Siemens Healthcare SAS, Saint-Denis, France

Keywords: RF Pulse Design & Fields, Spinal Cord

One of the best solutions to B₁ inhomogeneities observed at ultra-high field MRI is to use parallel transmit techniques (pTx). However, fast and accurate knowledge of the B₁⁺ is required. The presaturated TurboFLASH (satTFL)-based B₁-mapping sequence, with interferometric encoding, has shown great promise due to its speed and reliability, but it was observed that its performance may drop when used with unconventional radiofrequency coils and applications. In this study, a novel optimization of the interferometric encoding is presented and applied to cervical spinal cord MRI at 7T. The impact of having improved B₁-maps is evaluated on pTx MP2RAGE-based T₁-mapping.

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John M Drago^1,2,3, Bastien Guerin^2,3, Stephen F Cauley^2,3, and Lawrence L Wald^2,3,4
1Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, ²Harvard Medical School, Boston, MA, United States, ³A. A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States, ⁴Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, United States

Keywords: RF Pulse Design & Fields, Parallel Transmit & Multiband

We introduce multiphoton parallel transmission to mitigate nonuniform excitation in high-field MRI. Following an on-resonance birdcage subpulse, a multiphoton subpulse consisting of a single, off-resonance, high-frequency field from a birdcage coil, is supplemented with low-frequency parallel irradiation from a shim array. RF heating concerns are simplified compared to pTx, because only the birdcage coil produces significant SAR. Using a small-tip forward model, we optimize a nonconvex problem to find the amplitudes/phases of the low-frequency coils. Simulations demonstrate flip angle variation reduction across the brain ROI of 31.5% and NRMSE reduction of 42.0% compared to a conventional 7 T birdcage excitation.