OPENING SESSION - ismrm

Monday AM
17:18 143. Analysis of Quadratic Field Distortions Using the Fractional Fourier Transform
Carlos Sing-Long 1,2 , Vicente Parot 1,2 , Carlos Lizama 3 , Sergio Uribe, 2,4 , Cristian Tejos 1,2 ,
Pablo Irarrazaval 1,2
1 Department of Electrical Engineering, Pontificia Universidad Catolica de Chile, Santiago, Chile; 2 Biomedical
Imaging Center, Pontificia Universidad Catolica de Chile, Santiago, Chile; 3 Department of Mathematics and
Computer Science, Universidad de Santiago de Chile, Santiago, Chile; 4 Department 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 Splitthoff 1 , Maxim Zaitsev 1
1 Dept. 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 Vannesjö 1 , Lars Kasper 1 , Matteo Pavan 1 , Christoph Barmet 1 , Klaas Paul
Pruessmann 1
1 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
Jochen Leupold 1 , Jürgen Hennig 1
1 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 Wade 1,2 , Charles McKenzie 1,3 , Brian Rutt 4
1 Imaging Research Laboratories, Robarts Research Institute, London, ON, Canada; 2 Biomedical Engineering,
The University of Western Ontario, London, ON, Canada; 3 Medical Biophysics, The University of Western
Ontario, London, ON, Canada; 4 Diagnostic 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 Huang 1 , Wei Lin 1 , Yu Li 1 , Arne Reykowski 1
1 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 nonrigid
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.