Victor Han¹, Jianshu Chi¹, and Chunlei Liu^1,2
1Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, CA, United States, ²Helen Wills Neuroscience Institute, University of California, Berkeley, Berkeley, CA, United States

When multiple RF fields are applied at different frequencies, multiphoton excitation can occur when the sums or differences of integer multiples of these frequencies equal the Larmor frequency. No RF at the Larmor frequency is required. In this work, we describe the general principles of multiphoton pulsed selective excitation, providing a formalized treatment with design examples and implementations on a 3T scanner with an additional homemade z-direction (B_z) coil. With the additional B_z coil, we demonstrate additional flexibility, where the same excitation can be accomplished in several different ways with the same pulse duration.

Simon Lévy¹, Jürgen Herrler², Andrzej Liebert¹, Katharina Tkotz¹, Moritz S. Fabian², Christian Eisen¹, Michael Uder¹, David Grodzki³, Moritz Zaiss², and Armin M. Nagel^1,4
1Institute of Radiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany, ²Department of Neuroradiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany, ³MR Application Predevelopment, Siemens Healthcare, Erlangen, Germany, ⁴Division of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany

Fat signal can cause strong artifacts in MR images and/or bias measurements such as in Chemical Exchange Saturation Transfer (CEST) where metabolites peaks are reduced. At 7T, the signal-to-noise ratio and the peak separation are increased but the Specific Absorption Rate (SAR) constraints too, limiting the use of fat suppression pulses. Parallel transmit dynamic pulses for fat saturation (using k_T-points) and water-selection (using a Spiral Non-Selective trajectory) were implemented for a whole-brain CEST protocol and optimized to reduce SAR compared to the standard circularly-polarized approach. Results showed high improvements in flip angle homogeneity and fat suppression efficiency, with reduced SAR.

Sydney Nicole Williams¹, Iulius Dragonu², Belinda Ding^1,2, Patrick Liebig³, and David A. Porter^1
1Imaging Centre of Excellence, University of Glasgow, Glasgow, United Kingdom, ²Siemens Healthcare Ltd. United Kingdom, Frimley, United Kingdom, ³Siemens Healthcare GmbH, Erlangen, Germany

This abstract addresses the challenges of 7T diffusion MRI associated with shorter T2/T2* and inhomogeneous B₁⁺ field by implementing simultaneous multislice (SMS) parallel-transmit (pTx) slice-by-slice shimming in a readout-segmented diffusion-weighted sequence, RESOLVE. The benefits of pTx were compared to conventional single-transmit circularly polarized (CP) mode both with and without SMS using a matched high-resolution diffusion protocol in the same healthy volunteer. Slice-by-slice shimming significantly improves signal homogeneity while SMS opens the door for improved scan efficiency. The simplicity of slice-by-slice shimming and sequence integration makes this a practical solution for routine 7T imaging.

Franck Mauconduit¹, Vincent Gras¹, Alexandre Vignaud¹, and Nicolas Boulant^1
1Paris-Saclay University, CEA, CNRS, BAOBAB, NeuroSpin, Gif-sur-yvette, France

When using a slice-by-slice RF shimming or multi-spokes pulses in 2D parallel transmission MRI, the solution is valid for a specific slice position and orientation. Here, we introduce a new pulse concept were the spoke coefficients and gradient blips are smooth functions of the slice position and orientation. We show that this design performs better than CP mode and equivalent to other designs. However, it enables to tilt or reposition slices without requiring additional optimization. This solution promotes signal smoothness through slices. Additionally, it can be pre-computed using the Universal Pulse concept or computed online using a tailored approach.

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 work demonstrates the design and application of frequency robust, subject-tailored or universal, non-selective kT-point pulses to achieve a homogeneous flip-angle within the 3D human heart at 7T across a 1400Hz wide frequency range that includes water and six fat frequencies. Frequency robust universal pulses were computed offline based on 31 3D B₁⁺-maps and could be used in time-critical situations for calibration-free 3D heart flip-angle homogenization. Experimental data at 7T validates the flip-angle predictions and demonstrates successful excitation of fat and water in the human heart at 7T.

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. We designed and evaluated three new adaptations to a pre-saturation TurboFLASH B₁⁺ mapping method. The proposed methods enabled significantly shorter TRs (on the order of 1 s), were insensitive to T₁, and had comparable accuracy to the original method whilst maintaining the other desirable characteristics of the sequence. The method was shown to work in 3D allowing for volumetric B_1­⁺ maps to be acquired in 18 seconds.

Alix Plumley¹, Nathan Goodwin¹, and Emre Kopanoglu^1
1Cardiff University, Cardiff, United Kingdom

Specific absorption rate (SAR) is sensitive to head motion, especially in parallel-transmit (pTx) due to channel interference. It is known that SAR distribution varies according to subject anatomy. Here, we investigate whether SAR sensitivity to motion also depends on the subject. We designed quadrature-mode and pTx pulses at a centred position and evaluated them at 29 displaced positions. We compared the motion-induced SAR change across 4 body-models. We observed some variation across models, but local-SAR at least doubled in all models’ worst-cases. Our findings suggest that the safety concerns surrounding motion effects on SAR are relevant for various populations.