Artifacts & Correction: Non-Motion
Monday 3 May 2010
Room A6 16:30-18:30 Moderators: Erik B. Beall and Bruno Madore

16:30 139.

Z-Selective Multi-Spectral 3D Imaging: A MAVRIC-SEMAC Hybrid
Kevin M. Koch1, Kevin F. King1, Brian A. Hargreaves2, Graeme C. McKinnon
Applied Science Laboratory, GE Healthcare, Waukesha, WI, United States; 2Department of Radiology, Stanford University, Palo Alto, CA, United States

Both the MAVRIC and SEMAC methods have been shown capable of significantly reducing susceptibility artifacts near metallic implants.  Here, we demonstrate that advantageous features of both methods can be utilized in combination.  In particular, the z-selectivity of the SEMAC can be interfaced with the encoding mechanisms and spectral overlap utilized by MAVRIC.  In doing so, a technique that offers increased volume selectivity while maintaining smooth spectral image combinations is demonstrated.  Demonstrations of this hybrid approach on phantom and in-vivo implant scenarios are presented.

16:42 140.

Dipole-Based Filtering for Improved Removal of Background Field Effects from 3D Phase Data
Samuel James Wharton1, Richard Bowtell1
Sir Peter Mansfield Magnetic Resonance Centre, University of Nottingham, Nottingham, United Kingdom

A robust method for filtering 3D phase data dominated by rapidly spatially varying externally generated fields is presented. One or more dipole point sources situated outside of the region of interest are used to model and remove the unwanted background fields caused by remote tissue/air interfaces such as those that are present in the sinuses. The method was tested on simulated and experimentally acquired phase data and compared to other commonly used filtering methods, including Fourier filtering and polynomial fitting. The results show that the dipole-based filter outperformed the other methods in removing unwanted fields and preserving image contrast.

16:54 141. 

Improved Background Field Correction Using Effective Dipole Fitting
Tian Liu1, Ildar Khalidov2, Ludovic de Rochefort3, Pascal Spincemaille2, Jing Liu2, Yi Wang1
Biomedical Engineering, Cornell University, New York, NY, United States; 2Radiology, Weill Cornell Medical College, New York, NY, United States; 3MIRCen, I2BM, DSV, CEA, Fontenay-aux-Roses, France

Effective dipole fitting removes susceptibility induced global background field. It assumes that each independent voxel outside a given region of interest (ROI) are responsible for the background field inside that ROI. It removes the field generated by these sources, while preserving the field arising from local susceptibility variations inside the ROI.

17:06 142

A Novel Approach for Separation of Background Phase in SWI Phase Data Utilizing the Harmonic Function Mean Value Property
Ferdinand Schweser1, Berengar Wendel Lehr2, Andreas Deistung2, Jürgen Rainer Reichenbach2
Medical Physics Group, Department of Diagnostic and Interventional Radiology, Jena University Hospital, Jena, Germany; 2Medical Physics Group, Department of Diagnostic and Interventional Radiology, Jena University Hospital, Jena, Germany

In this contribution, we present, for the first time, a non-heuristic, parameter-free approach for high-precision separation of local phase and background phase contributions for in vivo SWI-data.

17:18 143.  

Analysis of Quadratic Field Distortions Using the Fractional Fourier Transform
Carlos Sing-Long1,2, Vicente Parot1,2, Carlos Lizama3, Sergio Uribe, 2,4, Cristian Tejos1,2, Pablo Irarrazaval1,2
Department of Electrical Engineering, Pontificia Universidad Catolica de Chile, Santiago, Chile; 2Biomedical Imaging Center, Pontificia Universidad Catolica de Chile, Santiago, Chile; 3Department of Mathematics and Computer Science, Universidad de Santiago de Chile, Santiago, Chile; 4Department of Radiology, Pontificia Universidad Catolica de Chile, Santiago, Chile

In Magnetic Resonance Imaging (MRI) the distortions produced by field inhomogeneities can be corrected with post processing techniques, e.g. linear correction and conjugate phase reconstruction methods. However, these methods do not provide a theoretical framework to analyze the distortions. In this work, we propose the Fractional Fourier Transform (FrFT) as a way to study the distortions produced by quadratic field inhomogeneities. We analyze some commonly used sequences to exemplify the usefulness of this method. We also show how this analysis can be used to reconstruct artifact-free images obtained from non homogeneous fields.

17:30 144

Generalized Non-Linear SENSE Shimming
Daniel Nicolas Splitthoff1, Maxim Zaitsev1

1Dept. of Diagnostic Radiology, Medical Physics, University Hospital Freiburg, Freiburg, Germany

With the SENSE Shimming (SSH) approach a method was introduced recently that allows for estimating B0 field inhomogeneities based on a reference image and a series of points on a single free induction decay (FID). In the original approach the temporal evolution of the FID data is explained by field inhomogeneities, using linear approximations. Effects caused by relaxation and those caused by inhomogeneities can therefore not be distinguished and values can only be given relative to a baseline measurement. We here present an extension to the method, which takes into account a larger range of the FID in order to explain not only B0 inhomogeneities but relaxation as well and which therefore allows for accurate field map estimation based on a reference image and a single FID. Since the signal equation is non-linear, the linear fitting of the original approach has to be replaced by a non-linear optimization. The feasibility of the method is shown on in vivo data.

17:42 145. 

On the Feasibility of Single-Shot EPI During Higher-Order Shim Settling
Signe Johanna Vannesjö1, Lars Kasper1, Matteo Pavan1, Christoph Barmet1, Klaas Paul Pruessmann1
Institute for Biomedical Engineering, ETH and University Zürich, Zürich, Switzerland

Susceptibility artefacts is a major problem in MRI, becoming more severe with higher field strengths and longer read-out trajectories. Updating the shim settings between acquisition of different slices allows for optimizing the shims to smaller subvolumes, but puts high requirements on the timing characteristics of the shim switching. Here the settling dynamics of the higher order shims were measured using a 3rd order dynamic field camera. Long-living (seconds) eddy currents were found, which had a significant effect on image quality. Based on measured k-space trajectories, it was possible to reconstruct phantom images acquired during eddy current settling.

17:54 146

Increasing Spoiling Efficiency in RF-Spoiled Gradient Echo Sequences by Averaging of Phase-Cycle Adapted K-Spaces - not available
Jochen Leupold1, Jürgen Hennig1
Dept. of Diagnostic Radiology, Medical Physics, University Hospital Freiburg, Freiburg, Germany

RF-spoiled gradient echo sequences (FLASH, SPGR, T1-FFE) require a spoiler gradient in order to suppress ghost artefacts. Here we show that two k-spaces can be adapted to the RF phase cycle such that averaging of them leads to elimination of these artefacts even if the spoiler gradient has only half of the moment that is requried for common RF-spoiled gradient echo acquisition.

18:06 147.  

Transient RF Spoiling for 3D Look-Locker Acquisitions
Trevor Wade1,2, Charles McKenzie1,3, Brian Rutt4
Imaging Research Laboratories, Robarts Research Institute, London, ON, Canada; 2Biomedical Engineering, The University of Western Ontario, London, ON, Canada; 3Medical Biophysics, The University of Western Ontario, London, ON, Canada; 4Diagnostic Radiology and Richard M Lucas Center for Imaging, Stanford University, Stanford, CA, United States

Theoretical and experimental investigation of RF spoiling in the special case of accelerated 3D Look-Locker imaging has led to an improved value for the phase increment used in the standard RF spoiling scheme.  Poor choice of phase increment leads to an inversion recovery curve that deviates significantly from the theoretical ideal, leading to an inaccurate estimate of the recovery time constant.  Simulations were used to determine improved values for the phase increment based on minimizing summed squared differences, or time constant measurement accuracy. These were tested experimentally and found to be superior to previously reported values for most imaging parameters.

18:18 148

Parallel Imaging for Efficient Spike Noise Detection and Correction
Feng Huang1, Wei Lin1, Yu Li1, Arne Reykowski1
Invivo Corporation, Gainesville, FL, United States

Spike noise is a term used to describe broadband electrical interference in an MRI system.  The result of spike noise can be seen in k-space as a bright dot, which will translate into some type of striping in the final image. Usually, the scan has to be repeated if random spike occurs.  A parallel imaging based method, COnvolution and Combination OperAtion (COCOA), has been proposed for non-rigid motion compensation. In this work, it is shown that COCOA can be used to robustly detect and correct random spikes in an efficient way. Hence repeated scan can be avoided.



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