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Devon Karl Overson^1,2, Dean Darnell^1,2, Fraser Robb³, Allen Song^1,2, and Trong-Kha Truong^1,2
1Duke-UNC Brain Imaging & Analysis Center, Duke University, Durham, NC, United States, ²Duke Medical Physics Graduate Program, Duke University, Durham, NC, United States, ³GE Healthcare, Aurora, OH, United States

Keywords: Shims, RF Arrays & Systems, flexible coil array, localized B0 shimming, integrated RF/shim coil array, iPRES

Susceptibility-induced B₀ inhomogeneities can cause artifacts that severely degrade the image quality in many applications. Integrated parallel reception, excitation, and shimming (iPRES) coil arrays can perform MRI and localized B₀ shimming with a single coil array, but are so far rigid, which limits their SNR and shimming performance. Here, we develop a flexible iPRES coil array that can conform to the subject’s anatomy and substantially improve the SNR and reduce the B₀ inhomogeneities and geometric distortions in diffusion-weighted imaging of the knee compared to a rigid iPRES coil array, which will be valuable for many anatomical regions and applications.

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Shengyue Su¹ and Anke Henning^1
1Advance Imaging Research Center, University of Texas Southwestern Medical Center, Dallas, TX, United States

Keywords: Shims, New Devices, Simulations

In this work, we propose a B₀ shim coil design method based on high-density wire pattern discretized from stream function optimization. We demonstrate that the magnetic field generated by the designed shim coil can fully match the theoretical magnetic field generated by continuous stream functions. We compared the performance of different current-carry surfaces over shim ability for stream function optimization. We also evaluated the impact of current limitation for the discretized coil on B₀ shim result and power dissipation. 

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Tzu Cheng Chao¹, Daniel D. Borup², and James G. Pipe^1
1Department of Radiology, Mayo Clinic, Rochester, MN, United States, ²MR R&D, Royal Philips, Rochester, MN, United States

Keywords: System Imperfections: Measurement & Correction, Diffusion Tensor Imaging

Diffusion MRI provides useful microstructural information of the soft tissue. Scans using a single-shot Spiral trajectory have demonstrated better geometric fidelity and SNR than those with a Cartesian EPI counterpart. This study shows that the field drifting during a diffusion scan can be a confounding factor to the image quality. A correction strategy exploiting the average off-resonance measurement before and after a scan was applied to mitigate the blurring artifact caused by field drifting.

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Ali Aghaeifar¹ and Klaus Scheffler ^1,2
1Max Planck Institute for Biological Cybernetics, Tübingen, Germany, ²Biomedical Magnetic Resonance, University Hospital Tübingen, Tübingen, Germany

Keywords: Shims, Shims, multi-coil

Multi-Coil shimming is advantageous because of its ease of use for effective local and slice-wise shimming. Shim coils are constructed of current-carrying conductor wires and undergo Lorenz force while placed in the magnet. Currents alteration in slice-wise shimming changes the magnitude and direction of force and imposes mechanical vibrations and acoustic noise. Here we propose strategies to control the level of inter-slice shims current change. The proposed strategies showed that inter-slice shims current change can be appropriately reduced by applying a minor change into shimming routine and without degrading shimming performance significantly .

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Gabriel Ramos Llorden¹, Daniel Park¹, Hong-Hsi Lee¹, Alina Scholz², Boris Keil², Lawrence L. Wald^1,3, Thomas L. Witzel⁴, and Susie Y. Huang^1,3
1Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Boston, MA, United States, ²Institute of Medical Physics and Radiation Protection, Mittelhessen University of Applied Sciences, Giessen, Germany, ³Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, United States, ⁴Q Bio Inc, San Carlos, CA, United States

Keywords: Gradients, Brain

Most of EPI ghosting algorithms  assume that odd/even of phase modulations, either of first or higher spatial order, are stationary through the readout. Hence, a phase maps (or a linear function) for the subset of odd and even k-space lines is judged enough to model Nyquist ghosting and obtaining ghosting free images. Nevertheless, we demonstrate with field monitoring that for high-gradient strength diffusion MRI, persistent eddy currents from the diffusion module interfere non-linearly with the image encoding, leading to phase modulations that are time-varying during the readout. Implications for existing ghosting correction algorithms and potential alternatives are discussed.

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Faezeh Rahimi^1,2, Thomas Wilhelm Eigentler^1,3, Bilguun Nurzed¹, Jason M. Millward^1,4, and Thoralf Niendorf^1,4,5
1Berlin Ultrahigh Field Facility, Max-Delbrück Center for Molecular Medicine in the Helmholtz Association, Berlin, Germany, ²Theoretische Elektrotechnik Institut Hochfrequenz- und Halbleiter-Systemtechnologien, Technische Universität Berlin, Berlin, Germany, ³Chair of Medical Engineering, Technische Universität Berlin, Berlin, Germany, ⁴a joint cooperation between the Charité Medical Faculty and the Max-Delbrück Center for Molecular Medicine, Berlin, Germany, ⁵MRI.TOOLS GmbH, Berlin, Germany

Keywords: RF Arrays & Systems, Modelling, Brain MRI, UHF MRI, PTx

UHF-MR uses RF arrays to shape the magnetic field generated inside the brain. For brain MRI at 7T, a helmet shaped transmit RF array could be beneficial versus conventional annular configurations. Recognizing this opportunity this work examines the feasibility of a multi-channel TX helmet RF applicator using broadband Self-Grounded Bow-Tie (bbSGBT) antenna building blocks. The focus is on enhancing B₁⁺ uniformity and coverage for brain MRI at 7.0T. Our findings obtained for the human head voxel model Duke demonstrate improved mean B₁⁺ coverage and increased B₁⁺ uniformity for the helmet configuration. 

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Hanne Vanduffel¹, Nick Arango², Willy Gsell¹, Uwe Himmelreich³, Wim Vanduffel¹, Cesar Parra¹, Berkin Bilgic⁴, Clarissa Cooley⁴, Dimitios Sakellariou¹, Rodrigo de Oliviera SIlva¹, Lawrence Wald⁴, Rob Ameloot¹, and Jason Stockmann^4
1KULeuven, Leuven, Belgium, ²MIT, Cambridge, MA, United States, ³KULeuven, 3000, Belgium, ⁴Martinos Center, Charlestown, MA, United States

Keywords: Shims, High-Field MRI

We present a new method for passive shimming based on 3D printed ferromagnetic inks that are fully compatible with the scanner RF coils. We show simulated B0 shim performance for designs that are optimized to shim the human brain for both a subject-specific passive shim insert and a robust general  insert.

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Afis Ajala¹, Seung-Kyun Lee¹, Nastaren Abad¹, and Thomas Foo^1
1GE Global Research, Niskayuna, NY, United States

Keywords: Gradients, Arterial spin labelling, Concomitant Field Correction

Magnetic resonance imaging using 3D stack-of-spirals at 3T on a high-performance gradient system such as the MAGNUS can be subject to strong second-order concomitant magnetic fields (SOCG), which can lead to signal dropout and blurring artifacts that become more significant at locations that are farther from the gradient isocenter. SOCG cannot be corrected by pre-emphasis of gradient waveforms and/or radio frequency modulation alone, and hence requires higher-order hardware or software solution. We demonstrate a software-based correction of SOCG in 3D pseudo-continuous arterial spin labelling with a stack-of-spirals k-space trajectory by nulling the phase contributed by SOCG during reconstruction.

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Jonathan B Martin¹ and William A Grissom^2
1Biomedical Engineering, Vanderbilt University, Nashville, TN, United States, ²Vanderbilt University, Nashville, TN, United States

Keywords: RF Pulse Design & Fields, High-Field MRI

We present a magnitude-least-squares RF shimming algorithm which uses interleaved noisy and locally converging updates to escape local minima and find low-cost shim solutions. We introduce noise in the algorithm by performing updates using a small minibatch of the available B₁⁺ measurements.  The method is validated using in-vivo head B₁⁺ maps from a 7T scanner. An optimal minibatch size is found which consistently produces low-power and low-RMSE solutions across subjects and through head slices. This shim design method may be employed to more robustly correct for B₁⁺ inhomogeneities at ultra-high field strengths than is possible with conventional methods.

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Daehun Kang¹, Seung-Kyun Lee², Erin Gray¹, Eric Fiveland², Yunhong Shu¹, Thomas Foo², and Matt A Bernstein^1
1Radiology, Mayo Clinic, Rochester, MN, United States, ²GE Global Research, Niskayuna, NY, United States

Keywords: Gradients, Gradients, Eddy current heating

With a high-performance gradient system, gradient-induced eddy current in the RF shield can cause a substantial temperature rise in the patient bore. Especially, a multi-echo EPI sequence involving intense gradient switching and long echo trains, usually lasts for >10 minutes in many resting-state functional MRI studies. The temperature change caused by RF shield heating can jeopardize patient safety and hardware reliability. Based on theoretical and experimental investigation, we demonstrated two main factors that could contribute to the RF shield heating in a high-performance compact 3T scanner, namely the shield surface conductivity and mean-square slew rate of the gradient waveform.

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Saskia Wildenberg¹, Nico Egger², Sophia Nagelstraßer², Ralph Kimmlingen³, Titus Lanz⁴, Armin M. Nagel^2,5, and Andreas K. Bitz^1
1Electrical Engineering and Information Technology, University of Applied Sciences - FH Aachen, Aachen, Germany, Aachen, Germany, ²Institute of Radiology, Friedrich-Alexander Universität Erlangen-Nürnberg (FAU), Erlangen, Germany, Erlangen, Germany, ³Siemens Healthcare GmbH, Erlangen, Germany, Erlangen, Germany, ⁴Rapid Biomedical GmbH, Rimpar, Germany, Rimpar, Germany, ⁵Division of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany, Heidelberg, Germany

Keywords: RF Pulse Design & Fields, Simulations

Universal RF shimming is a proven method to avoid the time-consuming process of calculating patient-specific RF shims for every individual subject. Nevertheless, this method requires numerous B₁⁺ maps from in vivo measurements, which is a time-consuming process, especially in the context of product development. This work therefore investigates whether a universal RF shim based on simulated B₁⁺ maps, which are needed for the safety assessment of the coil anyways, is a viable alternative compared to shim optimization based on in vivo data. The study was performed using a torso coil at 7T for the prostate and cardiac region.

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Bilal Tasdelen¹, Ecrin Yagiz¹, Rajiv Ramasawmy², Ahsan Javed², Dogangun Uzun², Dursun Korel Yildirim², Adrienne Campbell-Washburn², and Krishna S Nayak^1
1Ming Hsieh Department of Electrical and Computer Engineering, University of Southern California, Los Angeles, CA, United States, ²Cardiovascular Branch, Division of Intramural Research, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD, United States

Keywords: System Imperfections: Measurement & Correction, System Imperfections: Measurement & Correction, EMI, interference, pilot tone

Five devices that are relevant in interventional setting and a Pilot-Tone generator are assessed in terms of their electromagnetic interference with 0.55T MRI. Two of the devices (ergometer and flat-panel television) generated significant interference, however acceptable image quality was achieved when using the EDITER interference cancellation technique. EDITER also enabled the use of higher Pilot-Tone amplitudes and use of cheap, general-purpose signal generator for Pilot-Tone generation.

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Sebastian Theilenberg¹, Carlotta Ianniello¹, Kay C Igwe¹, Adam M Brickman^2,3,4, and Christoph Juchem^1,5
1Biomedical Engineering, Columbia University in the City of New York, New York, NY, United States, ²Taub Institute for Research on Alzheimer’s Disease and the Aging Brain, Vagelos College of Physicians and Surgeons, Columbia University, New York, NY, United States, ³Gertrude H. Sergievsky Center, Vagelos College of Physicians and Surgeons, Columbia University, New York, NY, United States, ⁴Department of Neurology, Columbia University Medical Center, New York, NY, United States, ⁵Radiology, Columbia University Medical Center, New York, NY, United States

Keywords: Shims, Brain

The multi-coil (MC) technique utilizing a set of individually driven non-specific coils outperforms traditional spherical harmonics B₀ shimming in rodent and human brains. MC designs consisting of dedicated MC elements have shown the best shim performance, while designs utilizing a DC current on the RD coil of RF coil arrays have slightly lower performance but need significantly less physical space. Here, we optimized the number, size and placement of dedicated MC elements to be used in combination with RF-DC coils in a hybrid MC shim system for B₀ shimming of the human brain in a clinical 3 T MR scanner.

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Carlotta Ianniello¹, Sebastian Theilenberg¹, Jonah Majumder¹, John Thomas Vaughan^1,2,3, and Christoph Juchem^1,2
1Department of Biomedical Engineering, Columbia University in the City of New York, New York, NY, United States, ²Department of Radiology, Columbia University Medical Center, New York, NY, United States, ³Columbia Magnetic Resonance Research Center, Columbia University in the City of New York, New York, NY, United States

Keywords: Hybrid & Novel Systems Technology, RF Arrays & Systems, RF Rx array, AC/DC coil, Hybrid RF and shimming

We explore the potential of a hybrid approach in which RF receive elements are driven with direct current for B0 shimming (RF+MC) aided with multicoil shim-only elements (MC-only) to overcome the challenging B0 inhomogeneities in the frontal brain. A proof-of-concept implementation consisting of 16 RF+MC elements and 18 MC-only elements is presented. Simulations have shown minimal SNR loss despite the proximity of MC-only elements. Furthermore, the B0 shimming performance of this preliminary setup outperformed 3^rd and 5^th order SH shimming despite its rudimentary design. Future work will focus on optimizing the B0 performance while minimizing the MC-to-RF coupling.

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Ates Fettahoglu^1,2, Zhehong Zhang¹, Nahla Elsaid¹, Andrew Dewdney³, and Gigi Galiana^1
1Radiology and Biomedical Imaging, Yale School of Medicine, New Haven, CT, United States, ²Radiology, Stanford University, Stanford, CA, United States, ³Siemens Healthcare, Erlangen, Germany

Keywords: Hybrid & Novel Systems Technology, Prostate, Eddy-current, GIRF

Diffusion-weighted imaging requires strong sensitizing gradients, resulting in eddy currents that create image distortions and hinder the reliability of measured diffusion parameters. In this study, we characterize and and compensate for eddy currents on a compact inside-out nonlinear gradient using a 2D phase encoded FLASH sequence. Time series of the effective waveform was calculated from PCA of the field maps, which was then used to calculate a gradient impulse response function (GIRF). The GIRF was validated by comparing the output waveform of a simple vs pre-emphasized trapezoid. Results showed clear improvement of the pre-emphasized waveform.
 

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Volkert Roeloffs¹, Peter Hömmen¹, Andreas Voß¹, Yi Li¹, Robert Odenbach¹, Leander Bartsch^1,2, Steffen Lother¹, and Stefan Röll^1,3
1Neoscan Solutions GmbH, Magdeburg, Germany, ²Otto-von-Guericke University, Magdeburg, Germany, ³Research Campus STIMULATE, Otto-von-Guericke University, Magdeburg, Germany

Keywords: System Imperfections: Measurement & Correction, Gradients, eddy currents

We characterized our new 1.5 T HTS magnet via GIRF determination. Analysis of time scales showed that relevant first-order eddy currents exhibit time constant of less then 200 µs. This finding is a promising characterization of our system but requires further investigations.

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Andrew J Wheaton¹ and Wayne R Dannels^1
1Canon Medical Research USA, Mayfield, OH, United States

Keywords: System Imperfections: Measurement & Correction, System Imperfections: Measurement & Correction, B1 mapping

A simple data processing method for calculating B1 map data using the Bloch-Siegert shift method is described. The proposed method uses a phase conjugate approach instead of conventional phase subtraction. The proposed method has the dual advantages of 1) not requiring phase unwrapping and 2) enabling simple and SNR-efficient complex data combination for multi-channel phased array coils. One consequence of using a phase conjugate approach is a reduced range of resolvable B1 due to phase aliasing. A practical solution is offered to resolve this problem by making an assumption of the realistic range of expected B1.

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David Schote¹, Lukas Winter¹, Christoph Kolbitsch¹, and Andreas Kofler^1
1Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany

Keywords: System Imperfections: Measurement & Correction, Low-Field MRI, B0-field map

Heavy B₀-field inhomogeneities in low-field MRI can lead to geometric distortions in the reconstruction, if not compensated. We simulated low-field data to evaluate different approaches for estimating the field map from phase difference maps. By using spherical harmonic basis functions as physical constraints in our neural network approach we could improve the estimation of B₀-field maps compared to other network architectures and methods not involving neural networks.