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 the design and application of respiration-specific and respiration-robust three-dimensional 4kT-point pTx pulses using respiration-resolved 3D B₁⁺ maps. The subject-specific pulses were tested on 20 B₁⁺ maps (shallow/deep breathing) of 10 volunteers with different age and BMI and were experimentally validated in the last three volunteers at 7T. Compared to respiration-specific pulses, respiration-robust pulses resulted in a negligible overall decrease of the FA homogeneity with clear benefits of achieving homogeneous 3D FA across all respiration states.

Caroline Le Ster¹, Franck Mauconduit¹, Aurélien Massire², Vincent Gras¹, and Nicolas Boulant^1
1Paris-Saclay University, CEA, CNRS, BAOBAB, NeuroSpin, Gif-sur-Yvette, France, ²Siemens Healthcare SAS, Saint-Denis, France

Parallel transmission allows flip angle homogenization at ultra-high field. The pulse design process can be made transparent to the user with calibration-free universal pulses (UP) that are designed over a database of field maps. Here a new method is proposed where UPs are designed over a standardized database, i.e. normalized to a reference. During a scan, these standardized UPs (SUPs) are adjusted to the subject through a fast calibration that relies on a linear transformation of the actual B1⁺ map to the database reference. Adjusted SUPs improve excitation performance and reduce intersubject variability compared to UPs.

Bastien Guerin¹, Eugene Milshteyn¹, Yulin Chang², Mads S Vinding³, Mathias Davids¹, Wald L Lawrence¹, and Jason Stockmann^1
1Massachusetts General Hospital, Charlestown, MA, United States, ²Siemens Medical Solutions, Malvern, PA, United States, ³Center for functionally integrative neuroscience, Aarhus, Denmark

We design “universal” kT-point and fully optimized pulses for flip-angle uniformization in the brain at 7 Tesla using a birdcage coil and a B₀ shim array coil. The fully optimized pulses are RF + gradient and RF + gradient + shim current waveforms joint optimization with system constraints (amplitude, slew-rate and acceleration). We design the universal pulses using 3 subjects’ field maps and evaluate them on 4 additional subjects.

Jürgen Herrler¹, Sydney Nicole Williams², Patrick Liebig³, Shajan Gunamony^4,5, Christian Meixner⁶, Andreas Maier⁷, Arnd Dörfler¹, David Porter², and Armin Michael Nagel^6
1Institue of Neuroradiology, University Hospital Erlangen, Erlangen, Germany, ²Imaging Centre of Excellence, University of Glasgow, Glasgow, Scotland, ³SIEMENS Healthineers, Erlangen, Germany, ⁴Institute of Neuroscience & Psychology, University of Glasgow, Glasgow, Scotland, ⁵MR CoilTech Limited, Glasgow, Scotland, ⁶Institue of Radiology, University Hospital Erlangen, Erlangen, Germany, ⁷Friedrich Alexander University Erlangen Nürnberg, Erlangen, Germany

To investigate the universality of using pre-optimized parallel-transmit (pTx) pulses with different RF coils for routine imaging, Universal Pulses and Fast Online Customized Pulses were generated using eight respective datasets from both a commercial and a self-built 8Tx/32Rx coil. The pTx pulse types were designed for both excitation and inversion pulses in a 3D MPRAGE sequence. For each coil, one subject was examined with all combinations of pTx pulses. All pTx pulses outperform CP pulses on the coil they were trained on. When using a different coil, Universal Pulses may fail, especially for inversion, whereas FOCUS pulses achieve robust homogeneity.

Luke Watkins¹, Alix Plumley², Kevin Murphy¹, and Emre Kopanoglu^2
1Department of Physics and Astronomy, CUBRIC, Cardiff University, Cardiff, United Kingdom, ²Department of Psychology, CUBRIC, Cardiff University, Cardiff, United Kingdom

Within-scan patient motion reduces PTx pulse performance. A motion-robust PTx 5-spoke pulse (MRP) was designed using simulated B1 maps at multiple head positions, and was compared to a conventional 3-spoke reference pulse. For a 5° rotated head orientation, magnitude nRMSE was reduced from 14% to 5.4%, phase RMSE from 17° to 3.6°, maximum magnitude and phase errors from 64% to 20% and 68° to 15° respectively. Although a longer pulse duration, the MRP maintained similar magnitude nRMSE to the reference pulse at the centre, and superior performance in 97% of all four error metrics for 46 off-centre positions.

Christina Graf¹, Martin Soellradl¹, Armin Rund², Christoph Stefan Aigner³, and Rudolf Stollberger^1
1Graz University of Technology, Institute of Medical Engineering, Graz, Austria, ²Institute of Mathematics and Scientific Computing, University of Graz, Graz, Austria, ³Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Berlin, Germany

The aim of this work is to design short and B₀- and B₁-robust inversion pulses by optimal control. A time-optimal control framework is used that incorporates variations within B₀- and B₁-fields. The optimized RF pulse is compared numerically to two hyperbolic-secant pulses and shows a very good efficiency over a broad set of B₀-offsets and B₁-scalings. Two phantom measurements are performed on a 3T MRI system for various scalings of B₁ that verify the results, one with a cylindrical MRI phantom, the other one with oil and water with a contrast agent, demonstrating also the B₀-robustness of the proposed pulse.

Jun Ma^1,2, Bernhard Gruber^3,4, Xinqiang Yan⁵, and William Grissom^2
1Department of Radiology, Stanford University, Stanford, CA, United States, ²Department of Biomedical Engineering, Vanderbilt University, Nashville, TN, United States, ³A. A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA, United States, ⁴Division MR Physics, Center for Medical Physics and Biomedical Engineering, Medical University Vienna, Vienna, Austria, ⁵Department of Radiology and Radiological Sciences, Vanderbilt University, Nashville, TN, United States

Current parallel transmit pulse design is based on a spatial domain formulation that has prohibitive memory and computational requirements when the number of coils or the number of dimensions is large. We previously introduced a k-space domain method that produces a sparse matrix relating any target excitation pattern in k-space to the pulses that produce it, which can be finely parallelized, has much smaller memory footprint, and can compensate off-resonance. Here we validate the algorithm for 3D inner-volume excitation using a simulated 24-channel transmit array and a SPINS trajectory, with comparisons to conventional iterative spatial domain designs.

Ziwei Zhao¹, Nam G. Lee², and Krishna S. Nayak^1,2
1Ming Hsieh Department of Electrical and Computer Engineering, Viterbi School of Engineering, University of Southern California, Los Angeles, CA, United States, ²Department of Biomedical Engineering, Viterbi School of Engineering, University of Southern California, Los Angeles, CA, United States

We describe a multidimensional small-tip RF pulse design procedure that incorporates concomitant field effects and gradient imperfections. Nonrotating concomitant fields in the reference frame are modeled as a Bloch-Siegert shift in the rotating frame and treated as higher-order phase terms in the excitation k-space formalism. We evaluate the effects of concomitant fields using simulations that mimic current 0.55T, 1.5T, 3T, and 7T systems. The proposed procedure produces more accurate excitation patterns, especially when concomitant field effects are strongest, i.e., low field strengths, off-isocenter, and longer pulse durations.

Belinda Ding¹, Iulius Dragonu², Patrick Liebig³, and Christopher T Rodgers^1
1Wolfson Brain Imaging Centre, University of Cambridge, Cambridge, United Kingdom, ²Siemens Healthcare Limited, Firmley, United Kingdom, ³Siemens Healthineers, Erlangen, Germany

In this study, we assessed the performance of jointly optimised pTx excitation and refocusing pulses in 2D spin-echo based sequences on a 7T Terra scanner. In conventional pTx acquisitions, the excitation and refocusing pulses are designed independently based a set of fieldmaps. Here, we compared two approaches for jointly optimising the excitation and refocussing against an approach of separately optimised pTx pulses and the traditional circularly polarised pulses. We observed that pTx pulses significantly improve image quality in both phantom and in vivo acquisitions at 7T. The image quality is further improved with joint optimisation of excitation and refocusing pulses.

Tsuyoshi Matsuda¹, Ikuko Uwano¹, Yuji Iwadate², and Makoto Sasaki^1
1Division of Ultrahigh Field MRI, Institute for Biomedical Sciences, Iwate Medical University, Iwate, Japan, ²Global MR Applications and Workflow, GE Healthcare Japan, Hino, Japan

Flip angles (FAs) might be underestimated during the actual FA imaging (AFI) method due to the aliasing phenomenon that occurs when the actual FAs exceeds 90°. To optimize nominal FA values for the AFI scan performed at 7 T, at which in-plane FA values markedly vary, we attempted to detect pixels showing erroneous FAs by comparing the phase difference between two types of AFI source images. AFI-FA maps with nominal FAs of ≥ 60° include substantial areas with FA underestimation, which is unsuitable for accurate FA measurements.

Jiye Kim¹, Dongmyung Shin¹, Juhyung Park¹, Hwihun Jeong¹, and Jongho Lee^1
1Department of Electrical and Computer Engineering, Seoul National University, Seoul, Korea, Republic of

A deep reinforcement learning method referred to as DeepRF-Grad, is newly developed to design an RF pulse and a slice selective gradient waveform. The method is demonstrated for slice-selective inversion and compared with SLR and VERSE-designed pulses. The DeepRF-Grad designed pulse showed lower SAR (SLR: 13.2mG²s, VERSE: 6.37mG²s, DeepRF-Grad: 5.00mG²s). When designed for off-resonance robustness, the DeepRF-Grad generated enhanced off-resonance characteristics compared to that of VERSE-designed pulse, while showing similar SAR. 

Tianrui Luo¹, Douglas C. Noll¹, Jeffrey A. Fessler¹, and Jon-Fredrik Nielsen^1
1University of Michigan, Ann Arbor, MI, United States

A recently proposed auto-differentiable Bloch simulation approach allows joint design of RF and gradient waveforms for large-tip objectives. However, that approach requires a relatively long design time, preventing  online pulse design. As a refinement, we propose to accelerate such simulation-based pulse design approaches by dividing it into two stages: 1) fast coarse dwell time design; 2) fine dwell time tuning. This combination substantially reduced the design time (from 8.5 min to 3 min), while still attaining high excitation accuracy, enabling online pulse design applications.

Dongmyung Shin¹, Jiye Kim¹, Juhyung Park¹, and Jongho Lee^1
1Electrical and Computer Engineering, Seoul National University, Seoul, Korea, Republic of

A recently developed RF pulse design method, DeepRF, is investigated using consistency, ablation, and scalability studies. The consistency of DeepRF designs is confirmed by repeating the same slice-selective inversion pulse design. The importance of the combination of two modules in DeepRF (i.e., RF generation and RF refinement) is verified through the ablation of each module. The scalability of DeepRF for a range of a design parameter is validated by designing several slice-selective inversion pulses with different time-bandwidth products.

Xiaodong Ma¹, Kamil Uğurbil¹, and Xiaoping Wu^1
1Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, MN, United States

In this study, we expand the application of a deep reinforcement learning (DRL) pulse design framework to designing four basic types of RF pulses and more complicated multi-band RF pulses. Our results showed that the DRL framework can be used to effectively design all types of RF pulses, improving slice profiles with reduced ripple levels in comparison to the conventional SLR algorithm.

Ziwei Zhao¹, Nam G. Lee², and Krishna S. Nayak^1,2
1Ming Hsieh Department of Electrical and Computer Engineering, Viterbi School of Engineering, University of Southern California, Los Angeles, CA, United States, ²Department of Biomedical Engineering, Viterbi School of Engineering, University of Southern California, Los Angeles, CA, United States

Velocity‐selective (VS) RF pulses can differentiate flowing blood from background tissue, and have been utilized for non-contrast intracranial, abdominal, and peripheral angiography at 3T. Here, we explore RF pulse designs for intracranial VS-MRA at low field strengths including 0.55T. We evaluate pulse performance using simulations that incorporate realistic levels of B₀ variation, B₁⁺ variation, and gradient distortions. Compared to 3T, simulations indicate 22% - 38% sharper velocity transitions and/or 50% - 60% less signal loss at 0.55T. We also show that gradient distortions can lead to “stripe” artifacts and can be mitigated with pre-emphasis.

Jürgen Herrler¹, Patrick Liebig², Rene Gumbrecht², Sydney Nicole Williams³, Christian Meixner⁴, Andreas Maier⁵, Arnd Dörfler¹, and Armin Michael Nagel^4
1Institue of Neuroradiology, University Hospital Erlangen, Erlangen, Germany, ²SIEMENS Healthineers, Erlangen, Germany, ³Imaging Centre of Excellence, University of Glasgow, Glasgow, Scotland, ⁴Institue of Radiology, University Hospital Erlangen, Erlangen, Germany, ⁵Friedrich Alexander University Erlangen Nürnberg, Erlangen, Germany

B₀ mapping was performed with a circularly polarized pulse and with a non-selective universal parallel transmit (pTx) pulse. The pTx pulse showed improved homogeneity, which resulted in different B₀ values, mainly present at tissue interfaces. Based on the two resulting B₀ maps, patient specific B₀ shimming was performed. Further, based on the two shimmed B₀ maps, individually optimized pTx excitation and inversion pulses were designed  for use in a 3D MPRAGE sequence. The pTx inversion pulse based on the B₀ map, which itself used universal pTx pulse, achieved a reduction of B₀ related artifacts in the frontal sinus region.

Ole Geldschläger¹, Dario Bosch^1,2, and Anke Henning^1,3
1High-field Magnetic Resonance, Max-Planck-Institute for biological Cybernetics, Tübingen, Germany, ²Biomedical Magnetic Resonance, University Hospital Tübingen, Tübingen, Germany, ³Advanced Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, TX, United States

In this study, the concept of ‘Universal pTx pulses’ for local excitation is tested in vivo at 9.4T. Based on B₀/B₁₊ maps from eight different subject heads, universal pulses for a 2-dimensional local excitation target pattern were designed. The pulses aiming to excite the visual cortex of the human brain (with a flip angle of 90 and 7 degree, respectively), while the remaining areas should experience no effective excitation.

In simulations and in vivo at 9.4T, the resulting universal pules perform just slightly worse compared to the subject specific tailored pulses (on non-database heads).

Yan Tong¹, Peter Jezzard¹, Caitlin O'Brien^1,2, and William T Clarke^1
1Wellcome Centre for Integrative Neuroimaging, FMRIB Division, NDCN, University of Oxford, Oxford, United Kingdom, ²Guy's and St Thomas' NHS Foundation Trust, London, United Kingdom

T2‐relaxation under‐spin‐tagging (TRUST) is a robust spin tagging-based method to quantify oxygen extraction fraction (OEF), but it lacks spatial specificity. Recently, O’Brien et al. proposed a method at 3 T involving multiple saturation pulses to achieve spatial specificity. Parallel transmission (pTx) provides additional degrees of freedom for spatial localisation. A pTx RF pulse design strategy based on a shells trajectory was applied to perform regional OEF measurement at 7 T. Phantom experiments showed that repeating the RF pulses significantly improved the saturation efficiency.