Sorting Out Some Artifacts
Wednesday 22 April 2009
Room 314 16:00-18:00

Moderators:

 Brian A. Hargreaves and X. J. Zhou
 
16:00 567. Embedded PLACE Correction for Geometric Distortion and N/2 Ghosting in Single-Shot EPI
    Qing-San Xiang1, Frank Q. Ye2
1Radiology, University of British Columbia, Vancouver, BC, Canada; 2Neurophysiology Imaging Facility, National Institute of Mental Health, NIH, Bethesda, MD, USA
    Geometric distortion and N/2 ghosting are two major artifacts in EPI. They can be simultaneously and instantaneously suppressed by a new method based upon Phase Labeling for Additional Coordinate Encoding (PLACE). Since all data are acquired within a single RF excitation, the correction is instant and thus the new method is termed iPLACE. A few central k-space lines are collected 3 times. This allows an effective N/2 ghost suppression, as well as further distortion correction. iPLACE has been demonstrated to be effective by phantom experiments performed on a 4.7 T scanner.
     
16:12 568. Understanding the Origin of Image Intensity Displacement in Spiral-In Versus Spiral-Out Acquisitions
    Kimberly D. Brewer1,2, Chris V. Bowen2,3, Steven D. Beyea2,3
1Department of Physics, Dalhousie University, Halifax, Nova Scotia, Canada; 2Institute for Biodiagnostics (Atlantic), National Research Council of Canada, Halifax, Nova Scotia, Canada; 3Departments of Physics, Radiology and Biomedical Engineering, Dalhousie University, Halifax, Nova Scotia, Canada
    Pulse sequences using reverse spiral trajectories (i.e. Spiral-In) are commonly used to avoid signal loss and distortion in regions with susceptibility field gradients (SFGs). Although there have been theories postulated as to why Spiral-In is superior to Spiral-Out, none of them explain why Spiral-In continues to recover more signal in SFG regions, even when acquired with an acquisition window that begins after that of Spiral-Out. We explored this phenomenon further through use of a phantom that produces well-known field patterns as well as computer simulations.
     
16:24 569. Simple Robust Estimation of Gradient Delays for Spiral MRI
    Ryan Keith Robison1, James Grant Pipe1
1Keller Center for Imaging Innovation, Barrow Neurological Institute, Phoenix, AZ, USA
    Timing delays between gradient transmission and data sampling are a major source of artifact in spiral MRI. These delays result most often from eddy currents, among other things. Many of the common methods used to measure these delays require specialized hardware, advanced pulse sequences, or very small phantoms. A simple technique is proposed to measure gradient delays associated with a spiral sequence on all three gradient axes in six excitations.
     
16:36 570. Eliminating Metal Artifact Distortion Using 3D-PLACE
    Michael Nicholas Hoff1, Qing-San Xiang1,2
1Department of Physics, University of British Columbia, Vancouver, BC, Canada; 2Department of Radiology, University of British Columbia, Vancouver, BC, Canada
    A method is presented which removes in-plane and through-plane distortion stemming from MRI of a metallic hip prosthesis. Two images are acquired using 3D turbo spin-echo sequences differing only by added gradient lobes along the frequency encoding (FE) direction, which yield a relative phase ramp across the field-of-view. The undistorted FE coordinate for each voxel is encoded into the additional phase information contained in the phase difference image. Distortion correction of a Lego structure surrounding the prosthesis is demonstrated. The sequence modification is straightforward, the required scan time is minimal, and phase unwrapping is unnecessary for this technique.
     
16:48 571. A Method to Remove Nyquist Ghosts from Echo Planar Images (EPI) Using UNFOLD
    W. Scott Hoge1, Huan Tan2, Robert A. Kraft2
1Radiology, Brigham and Women's Hospital, Boston, MA, USA; 2Wake Forest University School of Medicine, Winston-Salem, NC, USA
    Nyquist ghosts are a persistent artifact in echo planar imaging (EPI), and occur when data sampled along positive and negative read-out gradients is inconsistent. Previous methods to correct these effects include double-sampled EPI, which doubles the echo train length leading to greater magnetic susceptibility artifacts, and echo interleave strategies that carry a cost of reduced temporal resolution. Here, we present an approach using the slightly modified trajectory common to interleave strategies, but utilizing UNFOLD in place of interleaving to combine the data. This has the advantage of greatly reducing visible Nyquist ghosts while maintaining 90% of the original temporal sampling bandwidth.
     
17:00 572. B1 Correction Using Dual Tau Look-Locker (DτLL)
    Trevor Wade1,2, Brian Rutt1
1Robarts Research Institute, London, Ontario, Canada; 2Biomedical Engineering, University of Western Ontario, London, Ontario, Canada
    A new method of mapping the transmit B1 field is introduced that is capable of rapidly producing 3D B1 maps at low flip angles without relying on the double angle assumption. It is based on the accelerated 3D Look-Locker sequence and has been compared to both the standard double angle method as well as a double angle implementation of the 3D Look-Locker sequence.
     
17:12 573. Sense Shimming (SSH), First In-Vivo Results
    Daniel Nicolas Splitthoff1, Maxim Zaitsev1
1Dept. of Diagnostic Radiology, Medical Physics, University Hospital Freiburg, Freiburg, BW, Germany
    Recently, a new shimming method has been introduced, named Sense Shimming, or SSH. Due to expected complication regarding chemical shift artefacts and physiological noise, up to now only phantom measurements have been shown. We here present the first in-vivo measurements.
     
17:24 574. Simultaneous Fat Suppression and Band Reduction with Large-Angle Multiple-Acquisition BSSFP
    Neal Kepler Bangerter1,2, Garry E. Gold 3, Glen R. Morrell2, Brian Andrew Hargreaves3
1Electrical & Computer Engineering, Brigham Young University, Provo, UT, USA; 2Radiology, University of Utah, Salt Lake City, UT, USA; 3Radiology, Stanford University, Stanford, CA, USA
    Balanced SSFP (bSSFP) is a fast and highly SNR-efficient imaging technique, but can suffer from characteristic bands of signal loss in the presence of field inhomogeneity. Effective fat suppression with bSSFP is also challenging in regions with large field inhomogeneities. While techniques exist for both banding artifact reduction and fat suppression with bSSFP, they are sometimes incompatible and typically require a significant increase in scan time. In this work, we present a novel approach to simultaneous fat suppression and banding artifact reduction in bSSFP.
     
17:36 575. Oscillating Radial Trajectories for Reduced Undersampling Artifacts
    Rizwan Ahmad1, Lee C. Potter2, Periannan Kuppusamy1
1Davis Heart and Lung Research Institute, Department of Internal Medicine,, The Ohio State University, Columbus, OH, USA; 2Department of Electrical and Computer Engineering, The Ohio State University, Columbus, OH, USA
    We have proposed an oscillating radial trajectory of k-space which significantly improves the reconstruction quality as compared to the traditional radial sampling. Adding oscillations reduces the coherency of the radial trajectory and hence minimizes the aliasing artifacts. In contrast to the other methods for generating randomized trajectories, the proposed k-space trajectories are smooth and hence easy to implement on a conventional MRI gradient coil system. We present a systematic way of generating oscillating radial trajectories and show an improvement over the traditional radial sampling using simulations.
     
17:48 576. Effects of Concomitant Fields on Short-Time-Scale Noble Gas Diffusion Measurements
    Michael Carl1,2, John P. Mugler III3, Gordon D. Cates2, Wilson Miller3
1GE Healthcare, Applied Science Lab, Milwaukee, WI, USA; 2Physics, University of Virginia, Charlottesville, VA, USA; 3Radiology, University of Virginia, Charlottesville, VA, USA
    In previous work we described a specialized pulse sequence designed to access very short diffusion times using noble-gas NMR. In the present work we study the quantitative effects that concomitant fields have on the resulting diffusion measurements, and develop strategies for minimizing or correcting for these effects. We derived an approximate threshold criterion to determine under what circumstances concomitant field distortions remain insignificant compared to the desired diffusion attenuation. Phantom diffusion experiments were performed to confirm our results.