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Christina Graf¹, Clemens Diwoky², Armin Rund³, and Rudolf Stollberger^1
1Institute of Biomedical Imaging, Graz University of Technology, Graz, Austria, ²Institute of Molecular Biosciences, Karl-Franzens University Graz, Graz, Austria, ³Institute for Mathematics and Scientific Computing, Karl-Franzens University Graz, Graz, Austria

Keywords: RF Pulse Design & Fields, RF Pulse Design & Fields

In this work, robust excitation pulses for ³¹P spectroscopy at 7T are designed by optimal control. The particular challenges of inhomogeneous RF field due to use of surface RF coils and broad bandwidth requirements due to the nature of ³¹P were addressed thoroughly. The optimized RF pulses show superior performance compared to adiabatic RF pulses during numerical simulations and phantom experiments. Application of the optimized RF pulses to T₁ prediction by dual angle method is the logical consequence.

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Madison M Albert¹, Christopher E Vaughn¹, Jonathan B Martin¹, Sai Abitha Srinivas¹, and William A Grissom^1
1Biomedical Engineering, Vanderbilt University, Nashville, TN, United States

Keywords: RF Pulse Design & Fields, Low-Field MRI, Amplifiers, Predisortion

In low-field MR, low-cost amplifiers with reduced fidelity are used. Current solutions to this problem require additional hardware and do not optimize the pulse for the final excitation profile. The proposed methodology models the amplifier with a neural network, connects the network to a Bloch simulator, and optimizes the pulses for the desired excitation profile. In simulation, a windowed sinc pulse (time-bandwidth product = 10, phase = 0 radians, duration = 1 ms) was optimized to minimize the loss between the target profile and the generated profile after the pulse passes through the amplifier to 0.05% of the starting loss.

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Shota Hodono¹, Chia-yin Wu^1,2,3, Carl Dixon¹, Donald Maillet¹, Jin Jin⁴, Jonathan R Polimeni^5,6, and Martijn A Cloos^1
1Centre for Advanced Imaging, The University of Queensland, St. Lucia, Australia, ²ARC Training Centre for Innovation in Biomedical Imaging Technology, The University of Queensland, St. Lucia, Australia, ³chool of Information Technology and Electrical Engineering, The University of Queensland, St. Lucia, Australia, ⁴Siemens Healthcare Pty Ltd, Brisbane, Australia, ⁵Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, ⁶Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, United States

Keywords: RF Pulse Design & Fields, Velocity & Flow

We propose a new slice-selective excitation pulse that simultaneously saturates magnetization adjacent to it. The Saturation and Excitation (SatEx) pulse is a summation of two asymmetric sinc pulses whose leading lobes have been cut combined with an asymmetric excitation pulse with its final lobes truncated. The saturation and excitation slice profile were validated in simulations and a flow-phantom scan. The flow-phantom scan showed strong saturation of inflow effects. Since the SatEx pulse is as short as a conventional sinc excitation pulse, it can be implemented in a wide range of fast imaging sequences to suppress inflow effects without lengthening TR.

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Jason A Reich¹, Erin MacMillan^2,3,4, and Rebecca Feldman^1,5
1Computer Science, Mathematics, Physics and Statistics, University of British Columbia, Kelowna, BC, Canada, ²UBC MRI Research Centre, Department of Radiology, Faculty of Medicine, University of British Columbia, Vancouver, BC, Canada, ³SFU ImageTech Lab, Simon Fraser University, Surrey, BC, Canada, ⁴Philips Canada, Mississauga, ON, Canada, ⁵Biomedical Engineering and Imaging Institute, Icahn School of Medicine at Mount Sinai, New York, NY, United States

Keywords: RF Pulse Design & Fields, RF Pulse Design & Fields

It has been challenging to achieve ultrashort echo times with two-dimensional acquisitions. However, the use of a prepulse and a whole volume hard excitation may address these limitations. In addition, such a pulse sequence would allow for the use of simultaneous multi-slice pulses to reduce scan times. We aim to design a low specific absorption rate, short duration MultiPINS prepulse that excites a slice profile with sharp, narrow slice gaps. Optimization suggests that a pulse with time bandwidth product (TBW) of 49.43 is optimal, but further investigation suggests that a pulse with TBW as low as 36 may be feasible.

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Seung-Kyun Lee¹, Matthew Tarasek¹, Yihe Hua¹, Keith Park¹, Daehun Kang², Matt A Bernstein², and Thomas Foo^1
1GE Global Research, Niskayuna, NY, United States, ²Mayo Clinic, Rochester, MN, United States

Keywords: Gradients, Gradients, eddy current

Gradient-induced eddy current in an RF shield can cause significant temperature increase in the patient bore. As gradient coils produce higher amplitudes with faster rise time, understanding and controlling eddy-current heating become more and more important for patient safety and device longevity. Through time-domain analysis, we have derived a mathematical formula that relates the eddy-current heating of a uniform conductive shell to the properties of the conductor and the mean-square slew rate of the gradient waveform. The theoretical prediction was tested by experiments performed on a high-performance head-only gradient (MAGNUS) system.

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Koray Ertan¹, Peter B Roemer², and Brian Rutt^1
1Radiology, Stanford University, Stanford, CA, United States, ²Roemer Consulting, Lutz, FL, United States

Keywords: Gradients, Gradients, PNS, Peripheral Nerve Stimulation, Safety, Bioeffects, Magnetic Fields

Peripheral Nerve Stimulation (PNS) remains as a limiting factor for gradient coils despite the recent advances. PNS thresholds varies significantly across population group. In this study, E-fields are simulated on simplified body model population with randomized realistic body dimensions and different patient landmark positions. It is shown that multivariate linear model with body dimensions and z-offset location as the independent variables can accurately predict the maximum E-field on the surface which can also be used to predict PNS thresholds. This simple multivariate model may pave the way to estimate subject specific PNS thresholds using simple anatomical measurements.

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Stefan Sommer^1,2,3, Herbert Köstler⁴, and Hannah Scholten^4
1Siemens Healthineers International AG, Zurich, Switzerland, ²Swiss Center for Musculoskeletal Imaging (SCMI), Balgrist Campus, Zurich, Switzerland, ³Advanced Clinical Imaging Technology (ACIT), Siemens Healthineers International AG, Lausanne, Switzerland, ⁴Department of Diagnostic and Interventional Radiology, University Hospital Würzburg, Würzburg, Germany

Keywords: System Imperfections: Measurement & Correction, Gradients, GIRF, GSTF

The gradient system transfer function (GSTF) is a broadly used model to characterize the gradient chain and correct for imperfections e.g. in non-Cartestian sampling trajectories. The model assumes linearity and time-invariance (LTI) of the system. In this work, we indirectly examine the LTI compliance across two different field strengths by comparing different GSTFs based on varying subsets of input gradient shapes for the model computation. Overall, only small changes in the GSTF were observed upon adding different input gradients for the system characterization. However, the differences were more pronounced at 7T compared to 3T.

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Seung-Kyun Lee¹ and Afis Ajala^1
1GE Global Research, Niskayuna, NY, United States

Keywords: Gradients, Gradients, concomitant field

We show that concomitant field-induced phase in phase-contrast imaging allows measurement of the transverse (x,y) components of the magnetic field generated by a gradient coil. Combined with conventional B₀ mapping which gives the z-component, the method permits experimental determination of the full three-dimensional vector magnetic field of a gradient coil. We demonstrate the method on a high-performance head gradient (MAGNUS) system and discuss its application to concomitant-field artifact correction based on one-time calibration.

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Daniel West¹, David Leitão¹, Raphael Tomi-Tricot^1,2, Tobias C Wood³, Jo Hajnal^1,4, and Shaihan Malik^1,4
1Biomedical Engineering, King's College London, London, United Kingdom, ²MR Research Collaborations, Siemens, Frimley, United Kingdom, ³Department of Neuroimaging, King's College London, London, United Kingdom, ⁴Centre for the Developing Brain, King's College London, London, United Kingdom

Keywords: System Imperfections: Measurement & Correction, Gradients

In this work we use a general sequence to characterize gradient imperfections at low field (0.55T) by measuring the gradient impulse response function (GIRF). Maxwell fields become non-negligible at lower field strengths and so we incorporate these into our GIRF calculation to form a single processing pipeline. Once the predictive ability of the GIRF was confirmed, we use our data to estimate the Maxwell phase and compare this to an analytic approach; good agreement is observed. These results will inform future low-field investigations and enable an improved image quality for sequences that are particularly prone to gradient imperfections.

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Ansgar Adler¹, Nick Scholand¹, Jost Kollmeier², Yong Wang^3,4, and Martin Uecker^1,4,5
1Insitute of Biomedical Imaging, Graz University of Technology, Graz, Austria, ²Biomedical NMR, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany, ³Department of Fluid Physics, Pattern Formation, and Biocomplexity, Max Planck Institute for Dynamics and Self-Organization, Göttingen, Germany, ⁴DZHK (German Centre for Cardiovascular Research), Partner Site Göttingen, Göttingen, Germany, ⁵Institute of Diagnostic and Interventional Radiologie, Göttingen, Germany

Keywords: Phantoms, Velocity & Flow, Simulation

The lattice Boltzmann method (LBM) is a versatile technique to simulate fluid dynamics in complex environments. We extended the LBM to flows with a spin 1/2 degree of freedom in external magnetic fields. We performed 3D simulations for FLASH MRI of a laminar flow in a pipe and the complex flow of the Karman vortex street.

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Ege Aydın^1,2, Reza Babaloo^1,2, and Ergin Atalar^1,2
1Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey, ²National Magnetic Resonance Research Center (UMRAM), Bilkent University, Ankara, Turkey

Keywords: Gradients, System Imperfections: Measurement & Correction

Resistive elements in a gradient system, namely the gradient coil and the amplifier, causes heating and the total system resistance to increase. With feedforward controllers, such changes cause inaccurate gradient fields to occur, which in turn causes image artifacts. To mitigate self-heating caused resistance change issues, modeling the gradient system's thermal behavior is necessary. Such a model introduces additional parameters to the overall feedforward model, and identifying these parameters correctly is essential for accurate current waveforms. For this purpose, we propose a method to extract these parameters along with a more complete thermal model.

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Manouchehr Takrimi¹ and Ergin Atalar^1,2
1UMRAM, Bilkent University, Ankara, Turkey, ²Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey

Keywords: Gradients, Hybrid & Novel Systems Technology, Gradient Array Coils

We propose a fast computational method based on a series of electromagnetic simulations to calculate a proper set of feeding currents for a z-gradient array coil that dynamically provides the best shield for its primary array coil. The net magnetic field generated by each array element is calculated, and the total average eddy power (energy) loss resulting from fast-switching stray fields inside the cryostat assembly is estimated and controlled. Two design scenarios with comparable performance criteria are offered for a 48-element z-gradient array coil. The accuracy of the estimated eddy power losses is compared with that of commercial software.

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Yinhao Ren¹, Yunyu Gao², Bensheng Qiu³, Xiang Nan⁴, and Jijun Han^1
1School of Biomedical Engineering, Anhui Medical University, Hefei, China, ²School of Biomedical Engineering, Southern Medical University, Guangzhou, China, ³Center for Biomedical Imaging, University of Science and Technology of China, Hefei, China, ⁴Department of Anatomy, Anhui Medical University, Hefei, China

Keywords: RF Pulse Design & Fields, Brain

In the BSS method, WA and MLE as two methods in channel combination were proposed in |B1+| mapping for the multi-channel coils. We investigated the effects of the channel numbers on the performance of WA and MLE. The results showed the accuracy of |B1+| maps improved with the channel numbers increasing, even at low SNR. However, this trend slowed down when the channel numbers reached 24 or above. The RSD of WA and MLE was reduced to 6.6% and 4.0% for 24-channel coils, but the improvements were only 1.0% and 0.7% for 32-channel coils at SNR=25.

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Nahla M H Elsaid¹, Terence Nixon¹, Andrew Dewdney², Andrea De Simone³, Ates Fettahoglu¹, and Gigi Galiana^1,4
1Radiology and Biomedical Imaging, Yale University, New Haven, CT, United States, ²Magnetic Resonance, Siemens Healthcare GmbH, Erlangen, Germany, ³Università Cattolica del Sacro Cuore, Rome, Italy, ⁴Biomedical Engineering, Yale University, New Haven, CT, United States

Keywords: Gradients, Prostate

This work presents the early performance characteristics of an inside-out prostate nonlinear gradient (NLG). Diffusion-weighted imaging (DWI) could detect prostate cancers non-invasively. However, it requires high b-values, which in turn require a gradient pulse with a long duration, increasing the echo time. A longer echo time means a lower SNR and lower contribution from short T2 components. NLG coil circumvents this issue by having a gradient with a high amplitude within a limited field of view, which applies to prostate imaging.

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Folk W. Narongrit¹, Thejas Vishnu Ramesh², T Arthur Terlep¹, Antonia Susnjar², and Joseph V. Rispoli^1,2
1Elmore Family School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN, United States, ²Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN, United States

Keywords: Magnets (B0), New Devices

We present a low-cost (<$17 USD per channel), open-source, and scalable prototype for visualizing the three-axis static magnetic field components up to 3 T based on the GaAs Hall effect technology. The prototype showed accuracy and stability on the bench and accurately measured field values in a 0.25 T electromagnet and a 3 T MRI scanner.

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Philipp Amrein¹, Serhat Ilbey², Sebastian Littin², Feng Jia², Michael Bock², Maxim Zaitsev², and Ali Caglar Özen^2
1Division of Medical Physics, Department of Radiology, University Medical Center Freiburg, Germany, Freiburg, Germany, ²Division of Medical Physics, Department of Radiology, University Medical Center Freiburg, Freiburg, Germany, Freiburg, Germany

Keywords: Gradients, New Devices, Dental MRI

Dental MRI requires high resolution imaging, e.g., for detection of root canals, as well as, high readout bandwidth for short T2* tissues, such as dentin and nerve fibers. These requirements can only be satisfied by high-performance gradients. In this study, we show with Bloch simulations the necessity for high performance gradients, and consequently introduce a gradient and RF insert, which is optimized for dental MRI. We also explore the concept a cylindrical encoding for curved slices for 4-fold acceleration compared to conventional encoding.

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Zhehong Zhang¹, Nahla M H Elsaid², Terry Nixon^1,2, Andrew Dewdney³, and Gigi Galiana^1,2
1Department of Biomedical Engineering, School of Engineering and Applied Science, Yale University, New Haven, CT, United States, ²Department of Radiology and Biomedical Imaging, School of Medicine, Yale University, New Haven, CT, United States, ³Siemens Healthcare, Erlangen, Germany

Keywords: Gradients, New Devices, Insert gradient coil

A compact, lightweight device that generates a strong nonlinear gradient was recently developed for prostate DWI. In principle this hardware can be installed/removed for individual scans, but an additional question is whether the gradient field would need to be mapped and characterized with each installation. This work shows high reproducibility of the field after different installations, implying a single high quality field map can be reused. This result paves the way for practical clinical use of this insert gradient.

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Alireza Sadeghi-Tarakameh¹, Andrea Grant¹, Matt Waks¹, Nader Tavaf¹, Russell L Lagore¹, Lance DelaBarre¹, Edward Auerbach¹, Gregor Adriany¹, Pierre‐Francois Van de Moortele¹, Kamil Ugurbil¹, and Yigitcan Eryaman^1
1Center for Magnetic Resonance Research (CMRR), University of Minnesota, Minneapolis, MN, United States

Keywords: RF Arrays & Systems, High-Field MRI

Higher signal-to-noise ratio (SNR) is the primary motivation for increasing the magnetic field strength of MRI scanners. However, advanced technology, including state-of-art radiofrequency receiver coils, is strictly required to realize the SNR gain from field strength. In this study, we experimentally investigate the capability of conventional loop technology to capture central SNR gain from field strength for head applications. For this purpose, we evaluated the SNR performance of several loop receiver arrays at 3T, 7T, and 10.5T. We showed that realizing the SNR gain from the UHF will likely require RF coil technologies other than the conventional receiver loop arrays.