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Keywords: Gradients, New Trajectories & Spatial Encoding Methods, Ultrasonic

In this work, we present peripheral nerve stimulation and imaging results for a dual-axis head gradient that operates at ultrasonic frequencies. PNS measurements on 4 volunteers did not yield any noticeable stimulation. Imaging was performed using an ultrasonic Wave-CAIPI MP-RAGE, which featured ~20-fold acceleration and fully-sampled K-space coverage. We show the first images with this approach and a g-factor close to unity.

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Keywords: Gradients, High-Field MRI, Ambiguity

Gradients with a conventional, bipolar design generally face a trade-off between performance, encoding ambiguity, and circumventing the latter by means of RF selectivity. This problem is particularly limiting in cutting-edge brain imaging performed at field strengths ≥ 7T and using high-performance head gradients. To address this issue, the present work proposes to fundamentally eliminate the encoding ambiguity in head gradients by using a unipolar z-gradient design that takes advantage of the signal-free range on one side of the imaging volume. This concept is demonstrated by design of a unipolar high-performance head gradient and simulated 7T imaging with such a system.

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Keywords: High-Field MRI, High-Field MRI

With increasing prevalence of ultra-high field magnets and high-performance gradients, it is important to understand gradient-induced magnet heating, which can yield significant helium boil-off. Here, we propose a method to model this effect requiring no knowledge of gradient coil construction. Using measured gradient stray fields, we construct equivalent current surface models of gradient coils that can be input into finite element model of magnet systems to predict the deposited energy. We validate this equivalent current surface method using a gradient with a known winding pattern and further validate the energy deposition models by measuring power deposition in a 7T magnet.

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Mathias Davids^1,2, Livia Vendramini¹, Valerie Klein^1,2,3, Natalie Ferris^4,5, Bastien Guerin^1,2, and Lawrence L. Wald^1,2,5
1Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, ²Harvard Medical School, Boston, MA, United States, ³University Clinics Mannheim, Computer Assisted Clinical Medicine, Mannheim, Germany, ⁴Harvard Graduate Program in Biophysics, Harvard University, Cambridge, MA, United States, ⁵Harvard-MIT Division of Health Sciences and Technology, Cambridge, MA, United States

Keywords: Gradients, Gradients, gradient coil design, high-performance imaging

We report experimental PNS threshold measurements of an asymmetric PNS optimized whole-body gradient coil and compare it to a standard symmetric coil designed without PNS optimization. Stimulation thresholds were measured in 10 healthy adult subjects for five clinically relevant scan positions. The optimized design raised thresholds by up to 47% in four out of the five studied scan positions (head, cardiac, pelvic, and knee imaging positions). These results support the potential value of PNS-optimized asymmetric whole-body gradients for maximizing image encoding performance

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Hannah Scholten¹, David Lohr², Tobias Wech¹, and Herbert Köstler^1
1Department of Diagnostic and Interventional Radiology, University Hospital Würzburg, Würzburg, Germany, ²Chair of Molecular and Cellular Imaging, Comprehensive Heart Failure Center (CHCF), University Hospital Würzburg, Würzburg, Germany

Keywords: Gradients, System Imperfections: Measurement & Correction

Gradient inaccuracies often deteriorate image quality in non-Cartesian MRI, raising a demand for accurate gradient waveform measurements. The recently proposed approach of “variable-prephasing” provides an efficient gradient measurement technique with high SNR. However, the original variable-prephasing sequence neglects lingering field effects from the prephasing gradients, which we show to produce erroneous results if the gradient system exhibits sharp mechanical resonances. We therefore propose “fully compensated variable-prephasing” and demonstrate its ability to remove all field effects not stemming from the test gradient of interest.

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Paulina Siuryte¹ and Sebastian Weingärtner^1
1TU Delft, Delft, Netherlands

Keywords: Gradients, Gradients, acoustic noise

Gradient acoustic noise in MRI remains a large source of patient discomfort, with sound pressure levels reaching 130dB. In this work, we explore Predictive Noise Canceling (PNC) to reduce the acoustic noise. The method, similarly to active noise canceling, uses anti-noise to create a quiet zone. However, PNC is based on a direct gradient noise prediction, and is hence robust to aperiodicity and latency. We use iterative equalization and external trigger signals to achieve improved signal fidelity. The method is, for the first time, applied regular MRI sequences, and achieves up to 13dB noise reduction in the 0.3-4kHz range.

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Afis Ajala¹, Seung-Kyun Lee¹, Nastaren Abad¹, Yihe Hua¹, and Thomas Foo^1
1GE Global Research, Niskayuna, NY, United States

Keywords: Gradients, Gradients, Concomitant Field Correction

The use of higher-performance gradient coils results in stronger second-order concomitant magnetic fields, which can lead to image artifacts such as signal dropout, blurring, and phase errors that cannot be corrected by pre-emphasis of gradient waveforms and/or radio frequency modulation alone. We have developed an axially symmetric second-order field coil that is insertable, and demonstrate its ability to prospectively correct the additional phase generated by second-order concomitant fields in 2D phase contrast and spiral-out gradient echo imaging in a 3.0 T high-performance head-gradient (MAGNUS) system.

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Xinqiang Yan^1,2, Shuyang Chai^1,2, Ming Lu^1,2, and John C Gore^1,2
1Vanderbilt University Institute of Imaging Science, Vanderbilt University Medical Center, Nashville, TN, United States, ²Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Nashville, TN, United States

Keywords: Shims, Shims

In this work, we propose a reverse RF/B0 shimming coil design for ultrahigh field MRI without using any large bridge choke inductors. Our simulation and experimental results reveal that this design does not impair the RF performance, but would able to reduce the DC resistance/inductance/heating and also save precious space in RF/B0 shimming coils.

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Congyu Liao^1,2, Jason Stockmann³, Xiaozhi Cao^1,2, Zhitao Li¹, Lincoln Craven-Brightman³, Monika Sliwiak³, Charles Biggs³, Zheng Zhong¹, Nan Wang¹, Hua Wu⁴, Thomas Grafendorfer⁵, Fraser Robb⁵, Bernhard Gruber^3,6, Azma Mareyam⁷, Adam B Kerr^2,4, and Kawin Setsompop^1,2
1Department of Radiology, Stanford University, Stanford, CA, United States, ²Department of Electrical Engineering, Stanford University, Stanford, CA, United States, ³Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital and Harvard Medical School, Charlestown, MA, United States, ⁴Stanford Center for Cognitive and Neurobiological Imaging, Stanford University, Stanford, CA, United States, ⁵GE Healthcare, Milwaukee, WI, United States, ⁶BARNLabs, Muenzkirchen, Austria, ⁷Harvard Medical School, Charlestown, MA, United States

Keywords: Shims, Shims

This work provides a demonstration that the AC/DC shim-array can be flexibly used to correct undesirable eddy-current and concomitant fields effect in MRI acquisitions, with simulation results showing its effectiveness at mitigating eddy-current induced phase in diffusion-prepared acquisition and in mitigating concomitant fields in non-Cartesian trajectories, such as spiral. Phantom and invivo experiments were also performed on a 46-channel AC/DC shim-array to demonstrate high-fidelity multi-shot 3D diffusion-prepared acquisition without need for SNR-zapping amplitude stabilizer.

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Keywords: Shims, Shims

An MRI scanner equipped with global shim systems fails to reach state-of-the-art when shimming two isolated ROIs simultaneously for two reasons: non-optimal spherical harmonic based shimming routine, and significant high-order B0 inhomogeneities, even though the two-area shimming can be essential in scan scenarios, such as bilateral breasts or dyadic brains. To address these challenges, a hybrid active and passive local shimming (HAPLS) technique is proposed to shim two isolated areas in one FOV simultaneously. Both the simulation and experimental results validated that HAPLS can complementarily address the bifocal and high-order inhomogeneities, and locally improve in vivo B0 homogeneity as well.

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Chathura Kumaragamage¹, Scott McIntyre¹, Terence W Nixon¹, Henk M De Feyter¹, and Robin A de Graaf^1
1Radiology and Biomedical Imaging, Yale University, New Haven, CT, United States

Keywords: New Devices, Spectroscopy

ECLIPSE is a pulsed second order gradient insert that allows unparalleled extracranial lipid suppression over an elliptical ROI for applications in human brain proton MRSI.  While ECLIPSE provides excellent axial slice coverage for brain shapes that closely resemble an ellipse, coverage is compromised for head shapes that are asymmetrical. We recently constructed an ECLIPSE gradient coil in combination with a 54-channel multi-coil array (MC-ECLIPSE) for human brain ROI shaping and B₀ shimming. Here we demonstrate >95% axial slice coverage with ROI shaping over challenging head shapes, in addition to improved B₀ shimming capabilities relative to second order spherical harmonic shims.

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Daniel Papp¹, Gaspard Cereza¹, Alexandre D'Astous¹, Eva Alonso-Ortiz¹, and Julien Cohen-Adad^1,2,3,4
1NeuroPoly Lab, Institute of Biomedical Engineering, Polytechnique Montreal, Montreal, QC, Canada, ²Mila - Quebec AI Institute, Montreal, QC, Canada, ³Functional Neuroimaging Unit, Centre de recherche de l'Institut universitaire de gériatrie de Montréal, Montreal, QC, Canada, ⁴Centre de recherche du CHU Sainte-Justine, Université de Montréal, Montreal, QC, Canada

Keywords: Parallel Transmit & Multiband, Spinal Cord

Spinal cord MRI at 7T suffers from Tx inhomogeneity. RF shimming is therefore potentially useful in this region. Here, we evaluate for the first time the effect, both on signal intensity and signal homogeneity, of multiple RF shimming algorithms implemented in the open-source ‘Shimming Toolbox’, and compare the results with vendor solutions.

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Keywords: High-Field MRI, Body

In this work we introduce the concept of universal modes, which consists of the application of the universal pulse concept to time interleaved acquisition of modes (TIAMO). Based on simulated B₁⁺ maps of a 16-channel body array at 10.5T, the ability to mitigate B₁⁺ inhomogeneities is successfully demonstrated in six different human body models, targeting the pelvis.

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Niklas Wallstein¹, Roland Müller¹, André Pampel¹, and Harald E Möller^1,2
1NMR group, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, ²Department of Physics, Leipzig University, Leipzig, Germany

Keywords: System Imperfections: Measurement & Correction, Ex-Vivo Applications, Radiation Damping

Radiation damping (RD) is, in principial, well understood but commonly unconsidered in MRI. It results from inductive coupling of the spin system and the detection circuit, leading to nonlinearities in the Bloch equations. Previous research established that RD increases with the filling factor and quality factor of the coil and the magnetic field strength. However, little attention has been paid to a quantitative characterization. Moreover, implications for the RF pulse performance must be considered given similar timescales (milliseconds) of RD and pulse length. Our findings indicate that RD can impact typical MRI sequences and, consequently, parameters determined from their application.

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Daniel West¹, Felix Glang², Jonathan Endres³, Moritz Zaiss³, Jo Hajnal^1,4, and Shaihan Malik^1,4
1Department of Biomedical Engineering, King's College London, London, United Kingdom, ²Magnetic Resonance Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany, ³Department of Neuroradiology, Universitätsklinik Erlangen, Erlangen, Germany, ⁴Centre for the Developing Brain, King's College London, London, United Kingdom

Keywords: System Imperfections: Measurement & Correction, Pulse Sequence Design

MRI systems are usually engineered to give 'ideal' performance, and acquisition methods are generally developed using this assumption. This work proposes an alternative sequence learning framework that includes a model of realistic scanner performance, allowing acquisition sequences to be designed to directly account for system imperfections. In this proof-of-concept demonstration we designed pulse sequences to account for eddy current perturbations with different realistic time constants, while also respecting hardware limits. The flexibility of this approach could be used to design new types of pulse sequence to operate on lower performance, lower cost hardware in future.