Xavier Sieber^1,2, Jessica A. M. Bastiaansen^1,2,3,4, Christopher W. Roy^1,2, Jonas Richiardi^1,2, Matthias Stuber^1,2, and Ruud B. van Heeswijk^1,2
1Department of Medical Radiology, Lausanne University Hospital (CHUV), Lausanne, Switzerland, ²University of Lausanne (UNIL), Lausanne, Switzerland, ³Department of Diagnostic, Interventional and Pediatric Radiology (DIPR), Inselspital, Bern University Hospital, University of Bern, Bern, Switzerland, ⁴Translational Imaging Center, Sitem-insel, Bern, Switzerland

We present a flexible optimization framework to design spectral-selective radiofrequency pulses using numerical optimization on Bloch equation simulation. The RF pulse performance is assessed using a composite loss function for both fat suppression and power requirement. The framework was used to design three binomial (1-1, 1-2-1 and 1-3-3-1) and one cubic B-splines interpolated RF pulses. These pulses were then tested in vitro as well as in vivo in the knee and in the heart, where they demonstrated robust fat suppression comparable to a state-of-the-art water-selective pulse.

David Leitão^1,2, Raphael Tomi-Tricot^1,3, Pip Bridgen¹, Tom Wilkinson¹, Patrick Liebig⁴, Rene Gumbrecht⁴, Dieter Ritter⁴, Sharon Giles¹, Ana Baburamani¹, Joseph V. Hajnal^1,5, and Shaihan J. Malik^1,5
1Biomedical Engineering and Imaging Sciences, King's College London, London, United Kingdom, ²London Collaborative Ultra high field System (LoCUS), London, United Kingdom, ³MR Research Collaborations, Siemens Healthcare Limited, Frimley, United Kingdom, ⁴Siemens Healthcare GmbH, Erlangen, Germany, ⁵Centre for the Developing Brain, King's College London, London, United Kingdom

In MT imaging, particularly at ultra-high field, results can be influenced by spatial variations in both (1) on-resonance flip angle and (2) RF saturation specified by B₁^rms including on- and off-resonance contributions. Conventional pulse design with parallel transmission focuses on homogenizing flip angle (α) distributions but does not explicitly account for B₁^rms distribution.
In this work we propose a generalized pulse design framework that considers both α and B₁^rms and apply it to achieve uniform excitation and saturation at the same time. Performance is confirmed in phantom experiments at 7T, resulting in more uniform MTR compared to conventionally-designed pulses.
 

Sebastian Diaz¹, Yamin Arefeen², Borjan Gagoski^3,4, Ellen Grant^3,4, and Elfar Adalsteinsson^2,5,6
1Department of Biomedical Engineering, University of Arizona, Tucson, AZ, United States, ²Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, ³Department of Radiology, Harvard Medical School, Boston, MA, United States, ⁴Fetal-Neonatal Neuroimaging and Developmental Science Center, Boston Children's Hospital, Boston, MA, United States, ⁵Harvard-MIT Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, United States, ⁶Institute for Medical Engineering and Science, Massachusetts Institute of Technology, Cambridge, MA, United States

Fetal MRI is inherently time-constrained due to unpredictable motion and comfort concerns. Single-shot spin-echo sequences address these concerns by rapidly acquiring images with desired tissue contrast. However, these sequences typically operate at specific absorption rate (SAR) limitations due to a train of refocusing pulses, which impose long TR times and reduce acquisition efficiency. This study leverages SLfRank, the use of ranked factorization to jointly solve for SLR bi-coupled spin parameters, to design refocusing profiles that reduce SAR while maintaining slice-profile performance. The proposed refocusing pulse reduced SAR by over 20% while producing similar image quality to the standard clinical sequence.

Vencel Somai^1,2, Alan J Wright¹, Ming Li Chia¹, and Kevin M Brindle^1,3
1CRUK CI, University of Cambridge, Cambridge, United Kingdom, ²Department of Radiology, University Of Cambridge, Cambridge, United Kingdom, ³Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom

The BIR4 pulse achieves adiabatic excitation over a moderate frequency range but has a relatively high adiabatic threshold, which is demanding on the hardware and deposits significant energy when the repetition rate is high. We show here using simulations and an experiment in vivo that optimal control can provide a better alternative with a significantly lowered adiabatic threshold.

Jiye Kim¹, Dongmyung Shin¹, Hongjun An¹, Hwihun Jeong¹, Minjun Kim¹, and Jongho Lee^1
1Department of Electrical and Computer Engineering, Seoul National University, Seoul, Korea, Republic of

 DeepRF¹ is a recently proposed RF pulse design method using deep reinforcement learning and optimization, generating RF defined by a reward (e.g., slice profile and energy constraint) from self-learning. Here, we proposed an accelerated algorithm for DeepRF that utilizes a modified optimal control, replacing the computationally complex gradient ascent-based optimizer. The new algorithm is tested for slice-selective inversion and slice-selective excitation and compared with original DeepRF and SLR RF pulses, reporting improved computation efficiency while preserving performances. Additionally, a short-duration B₁-insensitive inversion pulse, which was difficult to produce in conventional RF algorithms, is designed to demonstrate the usefulness of DeepRF.

Zungho Zun¹ and Taehoon Shin^2,3,4
1Weill Cornell Medicine, New York, NY, United States, ²Division of Mechanical and Biomedical Engineering, Ewha Womans University, Seoul, Korea, Republic of, ³Graduate Program in Smart Factory, Ewha Womans University, Seoul, Korea, Republic of, ⁴Department of Medicine, Case Western Reserve University, Cleveland, OH, United States

Conventional k-space analysis of excitation based on Fourier relationship between excitation profile and RF deposit in k-space is extremely useful for pulse sequence design but is strictly limited to the small-tip-angle regime. We propose a new theoretical viewpoint for including refocusing pulses in the k-space formalism and demonstrate that the effect of refocusing pulse can be viewed as origin symmetric rotation of RF deposit in k-space. This new viewpoint may enable analytic approaches to designing RF pulses with multiple refocusing pulses and offer new opportunities to develop complex RF pulses that require insensitivity to off-resonance. 
 

Bahadir Alp Barlas^1,2 and Emine Ulku Saritas^1,2,3
1Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey, ²National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey, ³Neuroscience Graduate Program, Bilkent University, Ankara, Turkey

The utilization of low-cost low-field MRI systems has been rising due to the recent image quality improvements. However, these systems cannot incorporate numerous popular MRI techniques due to their hardware limitations. This work proposes a sheared 2DRF pulse design to make reduced FOV imaging applicable for low-cost MRI systems. The proposed sheared 2DRF pulse design provides significant reduction in pulse duration together with improved signal under B₀ field inhomogeneities, while ensuring robust sidelobe suppression and unlimited slice coverage. We demonstrate the proof-of-concept applicability of the proposed approach for a 0.35T and a 1.5T MRI scanner.

Sudhir Ramanna¹, Harsh Agarwal¹, Rajagopalan Sundaresan¹, Sajith Rajamani¹, Ashokkumar P Reddy¹, Bhairav Mehta¹, and Ramesh Venkatesan^1
1GE Healthcare, Bangalore, India

Multiplexing of slices using the NEX/averaging dimension is an option for clinical MR protocols to improve SNR without imposing a scan time penalty. Hadamard encoding and decoding imposes Hadamard factors as limits where in only powers of 2 is used. We propose to use a Hadamard like encoding scheme but decode differently to yield a reduced effective NEX and improved motion sensitivity. This enables us to use any integer multiband factors.

Seger Nelson¹ and Rebecca Emily Feldman^2,3
1Computer Science, Mathematics, Physics, and Statistics, University of British Columbia, Kelowna, BC, Canada, ²Computer Science, Mathematics, Physics, and Statistics, University of British Columbia Okanagan, Kelowna, BC, Canada, ³Biomedical Engineering and Imaging Institute, Icahn School of Medicine at Mount Sinai, New York, NY, United States

We investigate a method for the design of radio frequency (RF) pulses for custom simultaneous multi slice spatial profiles. We trained a variation of residual neural networks on a dataset of 96,000 generated adiabatic “power independent of the number of slices” (PINS) RF pulses, varying parameters such as gradient duty cycle, number of pulse lobes, quadratic phase strength, and transition and filter bandwidth. We compared the direct-design RF pulses to those generated by our model.

Luca Nagel¹, Geoffrey J. Topping¹, and Franz Schilling^1
1Department of Nuclear Medicine, TUM School of Medicine, Klinikum Rechts der Isar, Technical University of Munich, Munich, Germany

Cryogenically cooled transmit/receive RF surface coils (cryocoils) can improve signal-to-noise ratio (SNR) up to 4X¹. With flat profile RF pulses, surface coil excitation profiles are highly inhomogeneous, complicating the use of most pulse sequences, and especially the fast and efficient bSSFP sequence. A transmit B₁-compensating RF pulse that produces uniform excitation with a Tx/Rx ¹³C cryo-coil  was developed and tested in 3D-FLASH and 3D-bSSFP. Thermal ¹³C phantom imaging showed a regional uniform excitation, and proof-of-concept time-resolved 3D bSSFP imaging of hyperpolarized [¹³C]urea was demonstrated in mice.

Tanya Deniz Ipek¹, Victor Han¹, Jingjia Chen¹, 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

Simultaneous multislice (SMS) imaging accelerates data acquisition by exciting several slices simultaneously but has a higher specific absorption rate (SAR) due to additional radiofrequency (RF) pulses required to excite multiple frequencies. Multiphoton excitation achieved by oscillating gradient fields provides frequency sidebands, eliminating the need for additional excitation pulses. We have demonstrated SMS imaging for multiphoton MRI and used SENSE reconstruction to separate simultaneously acquired slices with the help of coil sensitivity profiles. Multiphoton SMS images achieve a similar quality to standard one-photon SMS images but offer the benefit of reduced SAR as they do not require extra RF power.

Arun Joseph^1,2,3, Patrick Liebig⁴, Gabriele Bonanno^1,2,3, Jin Jin^5,6, Kurt Majewski⁷, Tobias Kober^8,9,10, and Tom Hilbert^8,9,10
1Advanced Clinical Imaging Technology, Siemens Healthcare AG, Bern, Switzerland, ²Translational Imaging Center, sitem-insel AG, Bern, Switzerland, ³Magnetic Resonance Methodology, Institute of Diagnostic and Interventional Neuroradiology, University of Bern, Bern, Switzerland, ⁴Siemens Healthcare GmbH, Erlangen, Germany, ⁵Siemens Healthcare Pty Ltd, Brisbane, Australia, ⁶ARC Training Centre for Innovation in Biomedical Imaging Technology, The University of Queensland, Brisbane, Australia, ⁷Siemens AG, Munich, Germany, ⁸Advanced Clinical Imaging Technology, Siemens Healthcare AG, Lausanne, Switzerland, ⁹Department of Radiology, Lausanne University Hospital and University of Lausanne, Lausanne, Switzerland, ¹⁰LTS5, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

Echo planar imaging is an important MR technique used for different applications in both structural and functional imaging. It is typically used with fat suppression to avoid phase inconsistencies induced by the chemical shift of the fat signal. pTx pulses are used to mitigate the B1⁺ bias field occurring at higher fields strengths; typical pTx pulses do however not suppress the fat signal. In this work, we designed a frequency-selective water excitation pTx pulse to be used in 3D EPI acquisitions, enabling homogenous, high-resolution whole-brain fat-free imaging at 7T.

Chia-Yin Wu^1,2,3, Jin Jin^2,4, Carl Dixon¹, Donald Maillet¹, Markus Barth^1,2,3, and Martijn Cloos^1,2
1Centre for Advanced Imaging, The University of Queensland, Brisbane, Australia, ²ARC Training Centre for Innovation in Biomedical Imaging Technology, The University of Queensland, Brisbane, Australia, ³School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, Australia, ⁴Siemens Healthcare Pty Ltd, Brisbane, Australia

In this work, we demonstrate a parallel transmit implementation of velocity selective arterial spin labelling (VSASL) at 7 Tesla on a custom flow phantom developed in house. The use of tailored pTx pulses can greatly mitigate the transmit field inhomogeneities present at higher field strengths. By manipulating the time symmetry of the pTx pulses in the VSASL module we were able to show significant improvement in velocity selective inversion fidelity.