Christoph Stefan Aigner¹, Sebastian Dietrich¹, Tobias Schaeffter^1,2, and Sebastian Schmitter^1,3
1Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany, ²Division of Imaging Sciences and Biomedical Engineering, King's College London, London, United Kingdom, ³Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany

We demonstrate calibration free universal 4kT-points pulse design to achieve a subject independent, homogeneous flip-angle within the human heart at 7T. The proposed universal pulse was computed offline based on 22 three-dimensional B₁⁺-maps of 20 volunteers with varying BMI and age (two of them were scanned twice with different coil placement). The optimized universal pulse was successfully applied experimentally to one volunteer from the library and four new unseen volunteers. In total we have analyzed 27 B₁⁺ maps from 24 volunteers. Experimental data at 7T validate the B₁⁺ predictions and demonstrate successful plug-and-play 3D pTx of the human heart.

Zhipeng Cao¹, Adrian Paez², Chunming Gu², and Jun Hua^2
1Vanderbilt University, Nashville, TN, United States, ²Johns Hopkins University, Baltimore, MD, United States

This study presents a finite difference regularized magnitude-least-squares algorithm that ensures robust RF shimming and small-tip-angle multi-spoke pulse design against excitation nulls and sub-optimal pulse solutions. It also calculates a monotonic trade-off between flip angle error and RF power. It was validated in simulations and experiments, and was effective for brain and knee imaging. During an EPI-based fMRI at 7T with dynamic RF shimming, the algorithm ensured high SNR throughout the human brain, compared to a near-complete local signal loss by the conventional magnitude-least-squares algorithm. Overall, the algorithm streamlines the workflow for patient-tailored 2D multislice imaging at UHF.
 

Bahadir Alp Barlas^1,2, Cagla Deniz Bahadir^1,2,3, Ugur Yilmaz², and Emine Ulku Saritas^1,2,4
1Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey, ²National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey, ³Department of Biomedical Engineering, Cornell University, New York, NY, United States, ⁴Neuroscience Graduate Program, Bilkent University, Ankara, Turkey

The use of 2D echo-planar radiofrequency (2DRF) excitation has been widely applied in reduced field-of-view imaging of targeted regions, especially for diffusion weighted imaging (DWI). This work proposes effective and efficient coverage of the excitation k-space by shearing the 2DRF trajectory. This approach enables significant improvement in off-resonance robustness of 2DRF pulses by minimizing the pulse duration. It also promises improved clinical utility by increasing SNR in problem regions, eliminating slice coverage limitations and providing inherent fat suppression capability.

Chathura Kumaragamage¹, Peter B Brown¹, Scott McIntyre¹, Terence W Nixon¹, Henk M De Feyter¹, and Robin A de Graaf^1
1Department of Radiology and Biomedical Imaging, Magnetic Resonance Research Center, Yale University, New Haven, CT, United States

An asymmetric GOIA pulse was developed (Tp = 6.66 ms, BW = 20 kHz) by combining two adiabatic half passage pulses (hyperbolic secant and hyperbolic tangent GOIA modulations). The pulse achieves an asymmetrical excitation/inversion profile, thus was used in a multi-pulse OVS sequence to achieve an efficient, highly-selective, B₁ and T₁-independent signal suppression with a transition width of only 1.7% of the bandwidth.

Daniel Löwen¹, Eberhard Daniel Pracht¹, Rüdiger Stirnberg¹, and Tony Stöcker^1,2
1German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany, ²Department of Physics and Astronomy, University of Bonn, Bonn, Germany

Achieving homogeneous fat suppression is important for a wide range of MRI applications. In this work, we present time-efficient water-selective, parallel transmit RF excitation pulses for ultra-high field applications. This method combines the properties of kT-points with water-selective binomial pulses to achieve short, B1 insensitive water excitation pulses.

Jinmin Xu^1,2, Nicolas Arango², Congyu Liao^2,3, Berkin Bilgic^2,3, Zijing Zhang^1,2, Lawrence L Wald^2,3, Setsompop Kawin^2,3, Huafeng Liu¹, and Jason P Stockmann^2,3
1State key Laboratory of Modern Optical Science and Engineering, Zhejiang University, Hangzhou, China, ²A. A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, ³Harvard Medical School, Boston, MA, United States

We show that a switched B0 offset field can be used to improve lipid suppression pulse performance in 2D imaging by pushing water and lipids apart in the frequency domain.  The method is realized using multi-coil B0 shim arrays with rapidly switchable output currents that can be turned on during the lipid suppression pulse. Convex optimization is used to jointly solve for the shim currents and the lipid suppression pulse center frequency to optimize lipid suppression while minimize water signal loss.  Applications to brain and body imaging are considered.

Molin Zhang¹, Nicolas Arango¹, Jason Stockmann², Jacob White¹, and Elfar Adalsteinsson^1,3,4
1Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, ²Athinoula A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States, ³Harvard-MIT Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, United States, ⁴Institute for Medical Engineering and Science, Massachusetts Institute of Technology, Cambridge, MA, United States

Inner volume excitation is a promising technique to save scanning time or improve spatial resolution. Shim arrays provide nonlinear fields that extend the possibility of RF excitation of complicated spatial patterns. Yet previous work only employed static non-linear B field and predefined RF pulse which limits the performance. Validated on a fairly difficult tailored 3d volume pattern, jointly designed time-varying nonlinear B field and RF pulse within the auto-differentiable Bloch simulator framework shows substantial improvements. The accuracy improves 62% in terms of L2 norm with incorporated constraints on available RF pulse power.

Frank Ong¹, Michael Lustig², Shreyas Vasanawala¹, and John Pauly^1
1Stanford University, Stanford, CA, United States, ²University of California, Berkeley, Berkeley, CA, United States

The Shinnar-Le-Roux (SLR) algorithm is widely used for designing frequency selective pulses with large flip angles. We improve its design process to generate pulses with lower energy (by as much as 26%) and more accurate phase profiles.

Concretely, the SLR algorithm consists of designing two polynomials that represent Cayley-Klein (CK) parameters. Because the CK polynomial pair is bi-linearly coupled, the original algorithm sequentially solves for each polynomial. This results in sub-optimal pulses.

Instead, we leverage a convex relaxation technique to jointly recover the CK polynomials. Our experiments show that the resulting pulses almost always attain the global solution in practice.

Mads Sloth Vinding¹, Christoph Stefan Aigner², Jason Stockmann^3,4, Bastien Guérin^3,4, Sebastian Schmitter^2,5, and Torben Ellegaard Lund^1
1Center of Functionally Integrative Neuroscience, Aarhus University, Aarhus, Denmark, ²Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany, ³Harvard Medical School, Boston, MA, United States, ⁴Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, ⁵Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States

We test the DeepControl convolutional neural network for brain slice 2DRF 30^o excitations (single-channel) at 7 T with a uniform FA profile. While the DeepControl framework was originally designed for localized 2D spatial-selective excitations, we demonstrate robustness towards FA homogenization at 7 T. Our numerical study, a comparison to gradient-ascent-pulse-engineering optimal control pulses, shows good pulse performance and (near-)conformity to the optimal control training library, including the RF pulse constraints. The DeepControl pulse prediction time takes only ~9 ms, which is more than 1000 times faster than the optimal control.

David Leitão¹, Raphael Tomi-Tricot², Pip Bridgen¹, Tom Wilkinson¹, Patrick Liebig³, Rene Gumbrecht³, Dieter Ritter³, Sharon Giles¹, Ana Baburamani¹, Jan Sedlacik¹, Joseph V. Hajnal^1,4, and Shaihan J. Malik^1,4
1Biomedical Engineering and Imaging Sciences, King's College London, London, United Kingdom, ²MR Research Collaborations, Siemens Healthcare Limited, Frimley, United Kingdom, ³Siemens Healthcare GmbH, Erlangen, Germany, ⁴Centre for the Developing Brain, King's College London, London, United Kingdom

Conventional RF pulse design methods optimize for rotation of the magnetization. However, in systems with Magnetization Transfer (MT) with a semisolid pool, the dynamics do not follow the Bloch equations; saturation of the semisolid depends on |B₁⁺|², making it particularly sensitive to B₁⁺ inhomogeneities. This work employs a novel pulse design framework termed ‘Direct Saturation Control’ (DSatC) to directly consider semisolid saturation. We use this to design composite pulses with spatially uniform semisolid saturation using parallel transmition at 7T, where B₁⁺ is intrinsically highly inhomogeneous. In-vivo results show much more homogeneous MT contrast when using DSatC.