James L. Kent¹, Ladislav Valkovic^2,3, Iulius Dragonu⁴, 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, University of Oxford, Oxford, United Kingdom, ³Department of Imaging Methods, Institute of Measurement Science, Slovak Academy of Sciences, Bratislava, Slovakia, ⁴Siemens Healthineers, Frimley, United Kingdom

Ultra-high field MRI with parallel transmit offers significant advantages but B₁ transmit maps must be acquired to utilize its full potential. Acquiring absolute B₁⁺ maps for each channel is a time-consuming process and limits the application of UHF to clinical practice. We compare two methods of accelerating the transmit mapping procedure to obtain fast volumetric multi-channel maps for an 8-channel transmit array. Both methods make use of a low-rank reconstruction (TxLR) from undersampled data and a modified pre-saturation TurboFLASH (satTFL) B₁⁺ mapping sequence in combination with either Fourier encoding or a B1TIAMO style acquisition.

Dario Bosch^1,2, Fabian Müller¹, and Klaus Scheffler^1,2
1Magnetic Resonance Center, MPI for Biological Cybernetics, Tuebingen, Germany, ²Department for Biomedical Magnetic Resonance, University Tuebingen, Tuebingen, Germany

With an increasing number of transmit channels, faster methods for acquiring $$$B_1^+$$$ maps on pTx MRI systems grow in importance. We compared different interferometry modes for accurate absolute $$$B_1^+$$$ mapping using satTFL. We also introduced weighted hybrid $$$B_1^+$$$ mapping as an attempt to improve hybrid $$$B_1^+$$$ mapping. In this, a second absolute $$$B_1^+$$$ map was introduced as an attempt to further shorten measurement duration while keeping acceptable accuracy. From the tested interferometry modes, phase-cycling performed best in both absolute and weighted mapping. Weighted hybrid mapping exceeded conventional hybrid mapping substantially and could therefore be sufficiently accurate for many applications.

Christina Graf¹, Adam Berrington², Martin Soellradl¹, Armin Rund³, and Rudolf Stollberger^1
1Institute of Medical Engineering, Graz University of Technology, Graz, Austria, ²Sir Peter Mansfield Imaging Centre, School of Physics and Astronomy, University of Nottingham, Nottingham, United Kingdom, ³Institute for Mathematics and Scientific Computing, University of Graz, Graz, Austria

In this work, an optimal control framework and its application for the development and improvement of $$$B_1^+-$$$ and $$$\Delta B_0-$$$robust, slice-selective inversion pulses is introduced. The optimized RF pulse shows superior performance compared to adiabatic, state of the art pulses through simulations, phantom experiments, and in vivo measurements. Measurements on a 3T MR system revealed improved slice-profile quality of the optimized pulse, especially for low $$$B_1^+$$$ and high $$$\Delta B_0$$$. Since the performance is independent of the field strength, its application in ultrahigh-field spectroscopy seems straightforward.

Jan Sedlacik^1,2,3,4, Raphael Tomi-Tricot^1,2,3,5, Tom Wilkinson^1,2,3, Pip Bridgen^1,2,3, Franck Mauconduit⁶, Alexis Amadon⁶, Sharon Giles^1,2,3, Joseph V Hajnal^1,2,3, and Shaihan J Malik^1,2,3
1Centre for the Developing Brain, School of Biomedical Engineering & Imaging Sciences, King’s College London, London, United Kingdom, ²Biomedical Engineering Department, School of Biomedical Engineering & Imaging Sciences, King’s College London, London, United Kingdom, ³London Collaborative Ultra high field System (LoCUS), London, United Kingdom, ⁴Radiology Department, Great Ormond Street Hospital for Children, London, United Kingdom, ⁵MR Research Collaborations, Siemens Healthcare Limited, Frimley, United Kingdom, ⁶Paris-Saclay University, CEA, CNRS, BAOBAB, NeuroSpin, Gif-sur-yvette, France

A fast and accurate B1+mapping method is essential for making proper use of a parallel transmit array with high number of coil elements in a clinical setting. The saturation-prepared turbo FLASH method offers this capability and is implemented in the fully automated pre-scan adjustments of the currently only clinical ultra-high magnetic field strength MRI system. However, some parallel transmit methods require an even higher accuracy. This can be achieved by calibrating the fast B1+measurements with a more accurate but longer B1+mapping method.

Bobby Runderkamp¹, Wietske van der Zwaag², Thomas Roos², Gustav Strijkers³, Matthan Caan³, and Aart Nederveen^1
1Radiology & Nuclear Medicine, Amsterdam UMC, Amsterdam, Netherlands, ²Spinoza Center for Neuroimaging, Amsterdam, Netherlands, ³Biomedical Engineering and Physics, Amsterdam UMC, Amsterdam, Netherlands

Liver MRI could benefit from the increased SNR at 7T but suffers from severe flip angle inhomogeneity. In this work, eight-channel parallel transmission is used for flip angle shimming, comparing circularly polarized transmission and phase shimming with a kt-points pulse. Fourier phase-encoded DREAM is used for artifact-free B1+ mapping. With kt-points, the nominal flip angle of 8˚ could be achieved with homogeneous signal over the entire liver, while phase shimming only delivers a flip angle of approximately 4˚ with less homogeneity. Future work is directed towards use of universal slab-selective kt-points pulses to make 7T liver MRI clinically feasible.

Alix Plumley¹ and Emre Kopanoglu^1
1Cardiff University, Cardiff, United Kingdom

Motion-resolved parallel-transmit (pTx) B1-maps can be predicted using neural networks, facilitating online pulse re-design – a prospective solution to motion. However, networks require large training-datasets. Since different pTx coils inherently produce different B1-distributions, it is unclear whether coil-specific training-datasets are needed. Here, we train networks on simulated data from one coil-model and test on 6 differently-sized coil-models. While performance was optimal for the coil on which networks were trained, B1-prediction yielded lower error than that caused by motion in ≥91% of magnitude, and ≥55% of phase evaluations for 5 out of the 6 models, demonstrating some generalisability across coils.

Emre Kopanoglu^1
1CUBRIC / Psychology, Cardiff University, Cardiff, United Kingdom

Safety models on scanners are unaware of the actual patient position while excitation pulses are inherently position dependent. This study investigates the effect of this positional mismatch on SAR estimation for axial, coronal and sagittal slice orientations. The positional mismatch yields up to 5.2-fold underestimation of peak local SAR. RF shimming and 2-spoke parallel-transmit pulses for axial slice orientation have reduced SAR-sensitivity to positional mismatch with the worst-case underestimation being <2.0-fold whereas a reduced SAR-sensitivity was not observed for coronal and sagittal slices. For extreme head positions not represented in safety-models, axial RF shimming / 2-spokes parallel-transmit pulses maybe beneficial.

Amer Ajanovic^1,2, Yannick Brackenier^1,2, Raphael Tomi-Tricot^1,2, Joseph V Hajnal^1,2,3, and Shaihan Malik^1,2
1Biomedical Engineering Department, School of Biomedical Engineering and Imaging Sciences, King's College London, LONDON, United Kingdom, ²London Collaborative Ultra high field System (LoCUS), London, United Kingdom, ³Centre for the Developing Brain, School of Biomedical Engineering and Imaging Sciences, King's College London, London, United Kingdom

Establishing a comprehensive model that addresses patient safety at UHF MRI with respect to motion is a non-trivial task because of the underlying computationally expensive frameworks. Using MARIE we computed a large number of pose simulations of 3 feasible coil models (birdcage, 4-channel and 8-channel pTx) in proximity to Duke body model to allow the parameter space to be well explored. It is found that a low order linear model can capture most of the variation in cpB1+. We also observe low variations in cpSAR for 2 realistic coil models across the conditions examined, with 8-channel coil being most robust.

Philipp Ehses¹, Lydiane Hirschler^1,2, Eberhard Daniel Pracht¹, Katerina Deike-Hofmann¹, Vincent Gras³, Franck Mauconduit³, Aurélien Massire⁴, Nicolas Boulant³, Matthias J. P. van Osch², and Tony Stoecker^1,5
1German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany, ²Radiology, Leiden University Medical Center, Leiden, Netherlands, ³University of Paris-Saclay, CEA, CNRS, BAOBAB, NeuroSpin, Gif sur Yvette, France, ⁴Siemens Healthcare SAS, Saint-Denis, France, ⁵Department of Physics and Astronomy, University of Bonn, Bonn, Germany

Better understanding of how the brain cleans itself is of high importance as its failure has been linked to various neurodegenerative diseases. A first step towards elucidating CSF-mediated clearance mechanisms is the measurement of CSF motion in the human brain. In this work, we present an imaging sequence for CSF mobility measurements employing calibration-free parallel transmission. During the imaging session, no pulse calculations or complex B1 shimming procedures are necessary, making this approach feasible for large scale patient studies.

Christoph Stefan Aigner¹, Sebastian Dietrich¹, Felix Krüger¹, Max Lutz¹, and Sebastian Schmitter^1,2,3
1Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany, ²German Cancer Research Center (DKFZ), Medical Physics in Radiology, Heidelberg, Germany, ³University of Minnesota, Center for Magnetic Resonance Research, Minneapolis, MN, United States

This study demonstrates the design and application of a new class of universal pulses (UP) designed for a library of 22 non-respiration resolved (NRR) B₁⁺-maps acquired in-vivo during shallow breathing (SB) and 30 respiration resolved (RR) B₁⁺-maps acquired during deep breathing (DB). The proposed UP was tested on 9 NRR SB (test-cases-SB)  and 15 RR DB B₁⁺-maps (test-cases-DB) and was applied experimentally in five volunteers (test-cases-DB) along with tailored pulses in multiple gradient echo measurements. Compared to tailored pulses, UP resulted in a slightly overall decrease of the FA homogeneity but similar image quality across all test-cases.

Thomas Vaughan¹, Michael Garwood², Steve Suddarth³, Lance DelaBarre², and Dan Myer^4
1BME, Radiology, Columbia University, New York, NY, United States, ²University of Minnesota, Minneapolis, MN, United States, ³University of Minneosta, Minneapolis, MN, United States, ⁴Communications Power Corp., Hauppauge, NY, United States

A new, laboratory grade RF power amplifier that offers eight channels of low, CW power for simultaneous transmit and receive applications,  and eight channels of high, peak-pulsed power for conventional MRI is presented. These FPGA controlled, multiple transmit channels can be matrix switched into 1, 2, 4, and 8 channel combinations for a range of 2W, 4W, 8W and 16W CW, and 2kW, 4kW, 8kW and 16kW peak power pulses. Further each channel can be used for independent phase, gain, frequency, space and time for maximum flexibility in RF field modulation. This power amplifier meets our wide ranging experimental requirements.