Simon Gross¹, Yolanda Duerst¹, Laetitia Maëlle Vionnet¹, Christoph Barmet^1,2, and Klaas Paul Pruessmann^1
1Institute for Biomedical Engineering, ETH and University of Zurich, Zurich, Switzerland, ²Skope Magnetic Resonance Inc., Zurich, Switzerland

A novel model for the prediction of B₀-maps from external field measurements is presented. It is based on the joint analysis of training data from simultaneously acquired B0-maps and magnetic field evolution measured with NMR field probes. A first application to real-time shim feedback is demonstrated. 

Yuhang Shi¹, Johanna Vannesjo¹, Karla L. Miller¹, and Stuart Clare^1
1Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom

This work presents a rapid field map prediction method based on the individual subject's quick localizer scan and a large brain field map database to accelerate the field map acquisition stage for dynamic shimming applications. Our model-based method is able to better identify the steep change in the field associated with some slices in the lower part of the brain, however a low-resolution field map performs better for the rest of the brain.

Ali Aghaeifar^1,2, Alexander Loktyushin¹, Christian Mirkes^1,3, Axel Thielscher¹, and Klaus Scheffler^1,3
1Max Planck Institute for Biological Cybernetics, Tübingen, Germany, ²IMPRS for Cognitive and Systems Neuroscience, Tübingen, Germany, ³Department of Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany

B0 field inhomogeneity is a major source of distortion in MR images. Current approaches to dynamic shimming require extra acquisition time or external hardware. We propose a method that estimates first order shim errors by using projections of radial acquisition. The errors can be estimated from three projections multiple times in each measurement, which makes the method highly robust. The proposed method is evaluated in simulation and in vivo. Obtained results show a strong agreement between applied and measured first order shim errors.

Corey Allan Baron¹ and Dwight G. Nishimura^1
1Electrical Engineering, Stanford University, Stanford, CA, United States

B₀ inhomogeneity leads to image artifacts and/or blurring. These issues can be addressed by using a B₀ map, which typically requires an extra scan. In addition to the longer total scan time required, motion occurring between the acquisition of the imaging data and B₀ map can lead to misregistration. The proposed method utilizes images reconstructed from rewind trajectories to construct a B₀ map. In pulse sequences that already use gradient rewinds (e.g., bSSFP), a B₀ map that is inherently registered to the imaging data can be created with no additional scan time.

Oliver Bieri^1,2, Grzegorz Bauman^1,2, and Carl Ganter^3
1Radiology, University Hospital Basel, Basel, Switzerland, ²Biomedical Engineering, University of Basel, Basel, Switzerland, ³Diagnostic Radiology, Technical University Munich, Munich, Germany

A new method for accurate and fast broadband frequency mapping with balanced steady state free precession is introduced. The method mitigates the need for advanced phase unwrapping algorithms from a matrix pencil analysis of sequentially shifted echo times. Typically, the new method offers a spectral resolution in the range of Hertz with a sensitivity range in the order of several thousands of Hertz.

Iulius Dragonu¹, Craig Buckley¹, Peter Weale¹, Matthew D Robson², and Aaron T Hess^2
1Siemens Healthcare Ltd, Frimley, Camberley, United Kingdom, ²University of Oxford Centre for Clinical Magnetic Resonance Research (OCMR), John Radcliffe Hospital, Headington, United Kingdom

Radiofrequency shimming with multiple channel excitation is a well established method to increase the transverse magnetic field homogeneity and reduce SAR at high magnetic field strength(≥7T). To harness the benefits of a parallel transmit system, the magnitude and relative phase of each transmit channel must be determined during a calibration scan. We propose a new strategy to accelerate the acquisition of such calibration images by simultaneously exciting several transmit channels and reconstructing the calibration images using the technique similar to simultaneous multi-slice acquisitions.

Sascha Brunheim^1,2, Stephan Orzada¹, Soeren Johst¹, Marcel Gratz^1,2, Maximilian N. Voelker¹, Oliver Kraff¹, Martina Floeser³, Andreas K. Bitz³, Mark E. Ladd^1,3, and Harald H. Quick^1,2
1Erwin L. Hahn Institute for Magentic Resonance Imaging, University Duisburg-Essen, Essen, Germany, ²High Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany, ³Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany

With current methods the mitigation of transmit field inhomogeneity at ultrahigh field by multi-channel RF shimming with conventional methods is relatively time consuming. This applies in particular for parallel transmit/receive in-vivo body imaging within breath-hold and during organ motion. Therefore, we propose a new technique merging fast acquired relative single channel maps and the spatial-dependent flip-angle distribution of two complementary shims to define absolute transmit coil maps for fast and accurate RF shim calculation. The performance of this technique is validated against established methods in phantom measurements and its reliability is shown in comparison to simulation data serving as reference.

Kay Nehrke¹ and Peter Börnert^1,2
1Philips Research, Hamburg, Germany, ²Radiology, LUMC, Leiden, Netherlands

To improve the workflow for B₁⁺ calibration on a dual transmit MRI system, the DREAM B₁⁺ mapping sequence has been streamlined for acoustic noise reduction and free-breathing acquisition using a standard external respiratory motion sensor. About 10 dB reduction in sound pressure level were achieved by optimizing the echo order with respect to gradient strength reduction. Feasibility was shown in volunteer experiments on abdominal  B₁⁺ mapping.

Wyger Brink¹ and Andrew Webb^1
1Radiology, Leiden University Medical Center, Leiden, Netherlands

Imaging methods at high fields can suffer from receive non-uniformities from the body coil, particularly when the body coil is used as a reference for intensity correction. In this work we show that the DREAM B1 mapping sequence can be used for receive uniformity correction in RF-shimmed whole-body imaging at 3T.

Wenwen Jiang¹, Peder E.Z Larson², and Michael Lustig^3
1Bioengineering, UC Berkeley/ UCSF, Berkeley, CA, United States, ²Radiology and Biomedical Imaging, UCSF, San francisco, CA, United States, ³Electrical Engineering and Computer Science, UC Berkeley, Berkeley, CA, United States

Gradient timing delay errors in non-Cartesian trajectories often induce spurious image artifacts. More importantly, misaligned k-space center data results in auto-calibration errors for parallel imaging methods. We propose a general approach that simultaneously estimates consistent calibration data and corrects for gradient delays. We pose the joint estimation problem as a low-rank minimization problem, and solve it using a Gauss-Newton method. We demonstrate the feasibility of the proposed method by simulation and phantom experiments.