Elsevier

NeuroImage

Volume 18, Issue 1, January 2003, Pages 127-142
NeuroImage

Regular Article
An Evaluation of the Use of Magnetic Field Maps to Undistort Echo-Planar Images

https://doi.org/10.1006/nimg.2002.1281Get rights and content

Abstract

When a head is placed in an MRI scanner, differences between the magnetic susceptibility of tissue, bone, and air distort the magnetic field. While some of the resulting inhomogeneity can be corrected by the shimming process, much of it cannot, and this causes distortion (sometimes referred to as geometric distortion) of echo-planar images (EPIs). One strategy for the correction of distortion is to acquire a map of the magnetic field achieved in each subject and then to use this to undistort their EPIs after reconstruction (17). Here, we present five experiments to evaluate the application of such a strategy on data from a 3-T scanner. We show that after undistortion, the shape of EPIs is more similar to the true shape of the brain, and we investigate the effect of head movement on the efficacy of undistortion. If undistortion was applied first, it was found that less nonlinear warping was required to transform EPIs into a standard space, particularly in the phase-encode direction. We show that if SPM 99 normalization is used to perform a nonlinear warp to standard space, the prior application of undistortion increases the statistical power of group studies with motor and auditory tasks. We show that this increase in power is due to an increase in the overlap of activation of different subjects. Finally, we evaluate where in the brain undistorting EPIs might be expected to have the greatesteffect, in terms both of mislocalization of activationand of a reduction in power. Overall, undistorting EPIs using field maps has proved extremely successful, improving the anatomical localization of activation and increasing statistical power.

References (24)

  • N.K. Chen et al.

    Correction for EPI distortions using multi-echo gradient-echo imaging

    Magn. Reson. Med.

    (1999)
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      For example, in the open PRIME-DE database (Milham et al., 2018), of the 19 imaging centers that provided resting-state fMRI (rfMRI) data, only seven provided B0 field-maps and only five provided EPI echo-spacing (time between neighboring k-space lines). Since movement during data acquisition can also change the pattern of B0 inhomogeneities and image distortion, combining corrections for B0 distortion and its interaction with motion may further increase spatial fidelity and statistical power (Andersson et al., 2001; Andersson et al., 2018; Andersson et al., 2003; Cusack et al., 2003; Graham et al., 2017; Hutton et al., 2013). Figure 3 demonstrates the geometric distortion of spin-echo (SE) EPI acquired with left–right (L-R) (Fig. 3a) and right-left (R-L) (Fig. 3b) phase encoding directions at 3T.

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