Static & Dynamic B0
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Tuesday May 10th
Room 511D-F  16:00 - 18:00 Moderators: Christoph Juchem and Kevin Koch

16:00 284.   Correction of breathing-induced artefacts in high-field brain MRI using concurrent field monitoring 
Johanna Vannesjö1, David Brunner1, Christoph Barmet1, and Klaas Paul Pruessmann1
1Institute for Biomedical Technology, University and ETH Zurich, Zurich, Switzerland

Artefacts in anatomical brain images stemming from field fluctuations caused by breathing have previously been reported, and a navigator-based correction scheme has been shown. In this work, concurrent field monitoring was used to observe field shifts correlated with breathing, and to correct for breathing-related artefacts by including the monitored field evolutions in the image reconstruction. One advantage of the field monitoring approach, is that no alteration or extension of the imaging sequence is required.

16:12 285.   Theoretical basis of projection based shim estimation 
Daniel Nicolas Splitthoff1, and Maxim Zaitsev1
1Dept. of Radiology, Medical Physics, Unversity Medical Center Freiburg, Freiburg, Germany

Projections have been suggested for estimating B0 inhomogeneities. A detailed analysis of the phase difference of projections is given and put into the framework of linear algebra. The insights gained from this analysis lead to the conclusion that for proper detection of inhomogeneities the cross talk of the different orders need to be taken into account. We here present a solution of how to measure the cross talk when no further a priori information is available and show the benefit in a phantom measurement.

16:24 286.   Instant Measurement of Point Spread Functions Using an NMR field probe 
Lars Kasper1,2, Bertram Jakob Wilm1, Christoph Barmet1, and Klaas Paul Prüssmann1
1University and ETH Zurich, Institute for Biomedical Engineering, Zurich, Zurich, Switzerland, 2University of Zurich, Laboratory for Social and Neural Systems Research, Zurich, Zurich, Switzerland

The point spread function (PSF) is a comprehensive concept to describe the imaging and reconstruction process in MRI. Its analysis gives insight into the signal model of MR sequences as well as their imperfections, thus revealing true resolution and typical artifacts. By definition, the PSF describes the mapping of a point source of MR signal onto pixels of an image reconstruction matrix. We take this definition literally and use a miniaturized water-filled NMR field probe to determine the PSF experimentally. Hereby, we treat the MR scanner as a black box and infer information about the reconstruction characteristics solely from probing.

16:36 287.   Accelerated Point Spread Function Mapping Using Signal Modelling for Accurate EPI Geometric Distortion Correction 
Iulius Dragonu1, Thomas Lange1, Nicoleta Baxan1, Jürgen Hennig1, and Maxim Zaitsev1
1Dept. of Radiology, Medical Physics, University Medical Center Freiburg, Freibug, Baden-Wuerttemberg, Germany

Single-shot EPI is a fast technique allowing the acquisition of an image following a single RF excitation. However, EPI is prone to geometric distortions in presence of magnetic field inhomogeneities. We propose here a new method based on the modelling of the PSF data signal to allow accelerated acquisition for accurate geometric distortion corrections. Fully sampled PSF data of healthy volunteers were acquired. Undersampling factors up to 12.8 in the PSF-encoding direction were subsequently simulated. The pixel shift map obtained with the undersampled data and fully sampled data were compared. For each experiment the maximum error was below 0.15 pixels.

16:48 288.   Comprehensive Correction of Artifacts due to Eddy Current-Induced Echo Shifts in Partial Fourier DTI 
Trong-Kha Truong1, Nan-kuei Chen1, and Allen W Song1
1Brain Imaging and Analysis Center, Duke University, Durham, NC, United States

Partial Fourier imaging is typically used in DTI to increase the SNR. However, eddy currents induced by the diffusion gradients lead to: 1) signal loss if the echo is shifted outside the acquired k-space, 2) partial Fourier reconstruction errors if the echo is shifted outside the central k-space, and 3) variation of the effective TE, resulting in additional T2*-weighting. All three types of artifact vary with location and diffusion direction, causing errors in the diffusion tensor. Here, we propose a novel acquisition and post-processing method that can effectively correct for all three types of artifact while maintaining a high SNR.

17:00 289.   Magnetic Resonance Imaging of Arthroplasty: Comparison of MAVRIC and conventional Fast Spin Echo Techniques 
Matthew F Koff1, Catherine Lee Hayter1, Parina Shah1, Kevin M Koch2, Theodore T Miller1,3, and Hollis G Potter1,3
1Department of Radiology and Imaging, Hospital for Special Surgery, New York, NY, United States, 2Applied Science Laboratory, General Electric Healthcare, Waukesha, Wisconsin, United States, 3Weill Cornell Medical College of Cornell University, New York, NY, United States

Significant susceptibility artifacts occur when performing MRI around orthopedic hardware. This study evaluated standard of care 2D FSE imaging with the multi-acquisition variable-resonance image combination (MAVRIC) technique. Patients with joint replacements (hip or shoulder) were scanned using optimized 2D FSE and MAVRIC sequences. MAVRIC scans improved the visualization of the synovium, bone and supraspinatus tendon in the shoulder. MAVRIC scans resulted in increased detection of synovitis, peri-prosthetic osteolysis and supraspinatus tendon tears, resulting in a change in diagnosis almost 50% of the cases. This study further supports the use of MAVRIC for clinical implementation.

17:12 290.   MRI Assessment of Wear-induced Synovitis 
Catherine Lee Hayter1, Hollis G Potter1,2, Douglas E Padgett3, Giorgio Perino4, and Bryan J Nestor3
1Department of Radiology and Imaging, Hospital for Special Surgery, New York, NY, United States, 2Weill Cornell Medical College of Cornell University, New York, NY, United States, 3Department of Orthopedic Surgery, Hospital for Special Surgery, New York, NY, United States, 4Department of Pathology, Hospital for Special Surgery, New York, NY, United States

This study assessed the ability of MRI to detect different qualitative patterns of synovitis in symptomatic patients with hip arthroplasty compared to asymptomatic controls. We hypothesized a distinct, qualitative synovial MRI pattern would exist for metal on metal wear, metal on polymeric debris and aseptic lymphocytic vasculitis-associated lesions (ALVAL), that would be concordant with histologic findings. MRI could distinguish between tissue containing particulate debris and tissue without debris. MRI was sensitive in detecting polymeric debris, but did not detect the presence of metal in all samples. ALVAL elicited a specific synovial pattern on MRI that was highly concordant with histology.

17:24 291.   New MR Imaging Methods for Metallic Implants in the Knee: Artifact Correction and Clinical Impact 
Christina A. Chen1, Weitian Chen2, Stuart B. Goodman3, Brian A. Hargreaves3, Kevin M. Koch2, Wenmiao Lu4, Anja C. Brau2, Hillary J. Braun3, and Garry E. Gold3
1Radiology, Stanford University, Stanford, CA, United States, 2GE Healthcare Applied Science Lab, 3Stanford University, 4Nanyang Technological University

Slice Encoding for Metal Artifact Correction (SEMAC) is a recently developed MRI method that corrects for the metal-induced artifact that has previously limited the diagnostic value of postoperative images. This study found SEMAC to accurately measure metallic implant rotation in the knee, as implant misalignment is an important cause of implant pain and revision surgery. In addition, SEMAC significantly reduces artifact compared to fast spin echo in subjects, allowing for visualization of knee pathology that was able to guide patient management.

17:36 292.   Metal Artifact Reduction using Slice Encoding with Shear Correction 
Pauline Wong Worters1, Kim Butts Pauly1, and Brian A Hargreaves1
1Stanford University, Stanford, CA, United States

Slice Encoding for Metal Artifact Correction (SEMAC) is a robust method for resolving spatial distortion of tissue around metal in MRI. This method, as well as an alternative hybrid MAVRIC sequence, uses View-Angle Tilting (VAT) to correct for distortion in the readout direction. However, VAT imposes timing limitations in order to reduce blurring due to the RF profile. In this work, a shear post-processing method is proposed to replace VAT to correct for readout distortions. Shear correction allows for longer readout acquisitions (e.g., by using a lower readout bandwidth) and avoids the timing limitations imposed by VAT, while maintaining effective artifact correction.

17:48 293.   Frequency Encoding in the Presence of Extreme Static Field Gradients 
Kevin M Koch1, Kevin F King1, Weitian Chen2, Garry Gold3, and Brian A Hargreaves3
1Global Applied Science Laboratory, GE Healthcare, Waukesha, WI, United States, 2Global Applied Science Laboratory, GE Healthcare, Menlo Park, CA, United States,3Department of Radiology, Stanford University, Stanford, CA, United States

MR capabilities when imaging in the direct vicinity of metallic devices have been substantially improved with the development of 3D-MSI methods. The MAVRIC, SEMAC, and VS-MSI techniques have shown promising clinical capabilities in diagnosing soft-tissue pathology in previously inaccessible regions. Here, we discuss some limitations on how close 3D-MSI, or any technique relying on frequency-encoding, can image near metal in the presence of substantial local induction gradients. The presented analysis and results can aid in explaining residual artifacts in 3D-MSI and in predicting the effective spectral coverage required by the techniques.