Hans Stærkind^1,2, Vincent Oltman Boer¹, Kasper Jensen³, Eugene Simon Polzik², and Esben Thade Petersen^1,4
1Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Hvidovre, Hvidovre, Denmark, ²Quantop, Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark, ³School of Physics and Astronomy, University of Nottingham, Nottingham, United Kingdom, ⁴Department of Health Technology, Technical University of Denmark, Kgs. Lyngby, Denmark

A novel optical field probe is described for the precision measurement of high magnetic fields. The advantages over traditional field probes are two-fold. First, there is no electromagnetic interference with the other parts of the MRI system such as the RF coil. Secondly, the setup allows for a continuous and long readout of the magnetic field during an MRI sequence, where traditional NMR probes typically operate in a pulsed mode.

Philipp Eirich^1,2, Tobias Wech¹, Julius F. Heidenreich¹, Manuel Stich^1,3, Nils Petri⁴, Peter Nordbeck⁴, Thorsten A. Bley¹, and Herbert Köstler^1
1Department of Diagnostic and Interventional Radiology, University Hospital Würzburg, Würzburg, Germany, ²Comprehensive Heart Failure Center Würzburg, Würzburg, Germany, ³Siemens Healthcare, Erlangen, Germany, ⁴Department of Internal Medicine I, University Hospital Würzburg, Würzburg, Germany

An automatic pre-emphasis based on the Gradient System Transfer Function (GSTF) was applied to real-time cardiac MR imaging at 3T to compensate for deviations of spiral k-space trajectories. This yielded cine series with coverage of the whole heart in free-breathing, less than 40 s total scan time and a temporal resolution of 50 ms. The developed framework was compared to a gated Cartesian acquisition in multiple breath-holds, in one healthy volunteer and one patient suffering from cardiac arrhythmia. 

Jürgen Rahmer¹, Ingo Schmale¹, Peter Mazurkewitz¹, and Peter Börnert^1
1Philips Research, Hamburg, Germany

Spiral sequences sample data during gradient variation and are therefore susceptible to dynamic field deviations caused by eddy currents, timing inaccuracies, or gradient amplifier non-linearities. Linear effects can be corrected using a gradient impulse response function for trajectory calculation. Non-linear effects require a measurement-based approach, e.g. measurement of the gradient fields during imaging using a field camera or an induction-based field measurement. To avoid the need for additional hardware, we propose a hybrid method that combines current measurements using the amplifier-internal current sensors with correction based on the current-to-gradient impulse response function. The approach improves trajectory accuracy in spiral imaging.

Ruoyun Emily Ma¹, Mehmet Akçakaya^1,2, Steen Moeller¹, Connor Benson¹, Edward Auerbach¹, Kâmil Uğurbil¹, and Pierre-François Van de Moortele^1
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States, ²Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN, United States

DW-EPI at high diffusion gradients suffers from eddy current induced signal blurring and geometric distortion. In this study, concurrent monitoring of field evolution with NMR probes was implemented for HCP diffusion MRI acquisition at 7T. After image reconstruction with field correction, eddy current induced geometric distortion was largely removed, yielding similar level of correction obtained by data driven approaches such as EDDY. Signal blurring was further reduced than with EDDY. Future efforts will be made to quantify the impact on cortical tractography of this improved DW-EPI reconstruction.

Koji Fujimoto¹, Martijn A. Cloos², and Tomohisa Okada^1
1Human Brain Research Center, Graduate School of Medicine, Kyoto University, Kyoto, Japan, ²Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, NY, United States

A self-contained method to estimate the gradient delay for radial based MRF sequences based on the residual error after dictionary matching was presented. Under IRB approval, MRF was performed in two healthy volunteers at 7T. Images were reconstructed with varying degrees of gradient offset in the readout direction. The correlation of the dictionary match (i.e. inner product of the compressed data with the matched dictionary entry) was recorded. The average signal intensity within the head ROI in the “match map” gave the best result, and hence could be an easy and reliable metric for gradient delay correction.

Tess E. Wallace^1,2, Jonathan R. Polimeni^2,3, Jason P. Stockmann^2,3, W. Scott Hoge^2,4, Tobias Kober^5,6,7, Simon K. Warfield^1,2, and Onur Afacan^1,2
1Computational Radiology Laboratory, Boston Children's Hospital, Boston, MA, United States, ²Department of Radiology, Harvard Medical School, Boston, MA, United States, ³Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Boston, MA, United States, ⁴Brigham and Women's Hospital, Boston, MA, United States, ⁵Advanced Clinical Imaging Technology, Siemens Healthcare, Lausanne, Switzerland, ⁶Radiology, Lausanne University Hospital and University of Lausanne, Lausanne, Switzerland, ⁷LTS5, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

Spatiotemporal B₀ field fluctuations give rise to dynamic susceptibility-induced distortions in EPI time-series, which reduces signal stability, particularly at higher field strengths. In this work, we propose a novel method for rapid measurement of B₀ field changes from FIDnavs embedded in an EPI sequence. We demonstrate the ability of the proposed method to accurately characterize field changes up to second order in controlled phantom and volunteer experiments. Dynamic slice-wise distortion correction using FIDnav field estimates reduced normalized root-mean-square error and improved temporal SNR in volunteers performing deliberate arm motion. 

Dominik Weidlich¹, Mark Zamskiy¹, Marcus Maeder², Stefan Ruschke¹, Steffen Marburg², and Dimitrios C. Karampinos^1
1Department of Diagnostic and Interventional Radiology, Technical University of Munich, Munich, Germany, ²Chair of Vibroacoustics of Vehicles and Machines, Technical University of Munich, Munich, Germany

Diffusion gradients are known to yield MR hardware vibrations, which may lead to signal loss in DW measurements especially at high b-values. The present work proposes to mitigate vibration-induced signal loss by introducing a vibration matching gradient (VMG). Laser interferometry was employed to measure the displacements induced by high b-value DW MR spectroscopy, focusing on the quantification of lipids ADC. The measured displacement patterns during the two diffusion gradients were more similar when using the VMG. The application of VMG showed an improvement in lipid ADC quantification in both a water-fat phantom and a volunteer’s tibial bone marrow.

Sebastian Dietrich¹, Christoph Stefan Aigner¹, Juliane Ludwig¹, Johannes Mayer¹, Simon Schmidt², Christoph Kolbitsch^1,3, Tobias Schaeffter^1,3, and Sebastian Schmitter^1,2
1Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany, ²Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany, ³Division of Imaging Sciences and Biomedical Engineering, King's College London, London, United Kingdom

A major challenge of ultra-high field MRI is the spatially inhomogeneous transmit radio frequency field that induces spatial contrast variations. In this work we present a novel technique to retrieve channel-wise, motion-resolved absolute 3D FA maps of the human body at 7T. Free breathing 3D scans of the thorax were performed and B1 maps without motion artifacts are obtained for the two motion states inhale and exhale. It allows acquiring B1 libraries that can be used for offline RF pulse calculation including motion-robust RF pulses.  

Siddharth Srinivasan Iyer^1,2, Congyu Liao², Qing Li³, Mary Katherine Manhard², Avery Berman², Berkin Bilgic², and Kawin Setsompop^2
1Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, ²Athinoula A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States, ³MR Collaborations, Siemens Healthcare Ltd, Shanghai, China

Calibration scans that acquire coil sensitivity, B₀ and B₁+ inhomogeneities information play an important role in enabling modern acquisition and reconstruction techniques. This work proposes a unified, rapid calibration sequence termed Physics Calibration (PhysiCal) to obtain accurate B₀, Eddy, B₁+ and coil sensitivity maps. PhysiCal utilizes a carefully designed mix of full and variable density sampling acquisitions across echoes with synergistic constrained and eigenvalue reconstruction for robust and accurate recovery of whole-brain B₀, B₁+, Eddy and 32-channel coil sensitivity maps in just 11 seconds at 1 mm x 2 mm x 2mm resolution at 3T.

Andreas Lesch¹, Christoph Aigner², and Rudolf Stollberger^1
1Institute of Medical Engineering, Graz University of Technology, Graz, Austria, ²Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Berlin-Charlottenburg, Germany

For many applications in MRI fast and accurate $$$B_1^+$$$ -mapping is an important prerequisite to correct for spatially varying RF-field variations. We recently proposed a reconstruction algorithm to reconstruct highly undersampled Bloch Siegert data, which was applied to 3T data. In this work we investigated the applicability of this algorithm to 7T data in terms of accuracy and possible acquisition time. We could successfully show that the proposed algorithm can be applied to 7T data with only slightly increased error compared to 3T. The minimum scan time at 7T is in the order of 45s-1min for a 3D volume.