Abstract
Field inhomogeneities caused by variations in magnetic susceptibility throughout the head lead to geometric distortions, mainly in the phase‐encode direction of echo‐planar images (EPI). The magnitude and spatial characteristics of the distortions depend on the orientation of the head in the magnetic field and will therefore vary with head movement. A new method is presented, based on a phase informed model for motion and susceptibility (PIMMS), which estimates the change in geometric distortion associated with head motion. This method fits a model of the head motion parameters and scanner hardware characteristics to EPI phase time series. The resulting maps of the model fit parameters are used to correct for susceptibility artifacts in the magnitude images. Results are shown for EPI‐based fMRI time‐series acquired at 3T, demonstrating that compared with conventional rigid body realignment, PIMMS removes residual variance associated with motion‐related distortion effects. Furthermore, PIMMS can lead to a reduction in false negatives compared with the widely accepted approach which uses standard rigid body realignment and includes the head motion parameters in the statistical model. The PIMMS method can be used with any standard EPI sequence for which accurate phase information is available. Hum Brain Mapp 34:3086–3100, 2013. © 2012 The Authors. Human Brain Mapping Published byWiley Periodicals, Inc.
Highlights
Echo-planar imaging (EPI), the technique most commonly used for functional imaging studies, is sensitive to inhomogeneities of the B0 field due to its low bandwidth in the phase-encoding (PE) direction [Jezzard and Balaban, 1995]
Field inhomogeneities caused by variations in magnetic susceptibility throughout the head lead to geometric distortions in the PE direction of EPI volumes which change with head position in the magnetic field [Jezzard and Clare, 1999]
In this study we extend this Phase Informed Model for Motion and Susceptibility (PIMMS) to include knowledge about the linear change in phase caused by heating of the passive shims [Foerster et al, 2005] as well as the head motion parameters estimated from EPI magnitude data
Summary
Echo-planar imaging (EPI), the technique most commonly used for functional imaging studies, is sensitive to inhomogeneities of the B0 field due to its low bandwidth in the phase-encoding (PE) direction [Jezzard and Balaban, 1995]. Temporal changes in distortion caused by, for example, head movement, can lead to temporal signal fluctuations that remain after standard rigid body realignment procedures (e.g., [Friston et al, 1995]) have been applied. This residual variance can have a detrimental effect on the temporal SNR (tSNR) and fMRI studies which require maximal sensitivity to small BOLD signal changes. A widely accepted way of reducing motion related residual signal changes that remain after standard
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