Correction of high‐order eddy current induced geometric distortion in diffusion‐weighted echo‐planar images

Abstract

Diffusion-weighted images acquired with the echo-planar imaging technique are highly sensitive to eddy current induced geometric distortions that vary with the magnitude and direction of the diffusion sensitizing gradients. Such distortions cause misalignment of images acquired with different diffusion strengths and orientations. This in turn can result in errors when calculating maps of the apparent diffusion coefficient and diffusion tensor. Previous correction methods either require separate calibration data or only deal with low-order errors. In this study, we demonstrate a method that can correct for higher-order errors. The method relies on collecting pairs of images with diffusion sensitizing gradients reversed. This paired data are first corrected for shifts and linear distortion and then combined to cancel higher-order errors. All acquired data contribute to the final results. The method has been tested by simulation, on phantoms, on adult volunteers, and on neonatal brain Key words: echo-planar imaging; diffusion-weighted image; diffusion tensor imaging; eddy current induced distortion Diffusion-weighted (DW) imaging provides an important means of early detection of acute stroke (1-3) and is a key tool for probing white matter fiber tracts and tissue microstructures (4 -6). In vivo measurement of the diffusion tensor by diffusion tensor imaging (DTI) requires multiple image acquisitions with diffusion sensitivities in at least six noncollinear directions. Recently it was recognized that a simple diffusion ellipse representation may not be sufficient where there are mixed fiber directions within a voxel and so it is becoming more common to acquire data sensitized in a large number of directions. This approach requires rapid imaging and diffusion-weighted echo-planar imaging (DW-EPI) using pulsed gradient (StejskalTanner) spin echo sequences is the current method of choice. To improve signal-to-noise ratio (SNR), it is common to acquire several images for each sensitization direction.DW-EPI is highly sensitive to eddy currents, which cause image shifts and spatial distortions, particularly in the phase encoding direction. Eddy currents produced by the diffusion sensitizing gradient pulses vary with direction of diffusion sensitization, so that image distortions vary from image to image within data sets acquired for DTI. To avoid substantial errors in the calculated diffusion properties, these distortions must be fully corrected.Previous methods for dealing with this problem involved sequence modifications, acquisition of extra calibration data, and/or postprocessing. Sequence modifications can reduce the effects of eddy currents during the EPI readout period. Examples include the use of bipolar gradient pulses (7,8) or two opposite polarity gradient pulses, separated by a 180°radiofrequency refocusing pulse, to replace each Stejskal-Tanner diffusion pulse (9). These methods usually reduce diffusion sensitivity compared to the original Stejskal-Tanner diffusion pulses. A more efficient, twice-refocused me...

Higher order reconstruction for MRI in the presence of spatiotemporal field perturbations

“…However, besides requiring substantial additional scan time, these techniques usually do not fully address dynamic higher order fields. Finally, image distortions can often be mitigated, albeit rarely completely removed, by co-registration (23,24) or related imagedomain correction methods (25,26). Yet, these approaches are suitable for image data only and do not apply when data fusion is necessary to reconstruct a faithful image in the first place.…”

Integrated SENSE DTI with correction of susceptibility- and eddy current-induced geometric distortions

“…Furthermore, full characterization of the eddy current-induced magnetic fields is not required for every DTI acquisition, but can be performed on a much less frequent basis as long as the eddy currents remain temporally stable. As such, this method has an overall much improved efficiency as compared to several previously proposed correction methods (Jezzard et al, 1998;Haselgrove and Moore, 1996;Andersson et al, 2003;Bodammer et al, 2004;Shen et al, 2004). This is particularly advantageous when a large number of diffusion-weighting directions is used, such as in high angular resolution diffusion imaging.…”

“…Other correction methods require the acquisition of eddy current-induced magnetic field profiles for each diffusion-weighting direction (Jezzard et al, 1998) or the acquisition of pairs of diffusion-weighted images with low and high diffusionweighting (Haselgrove and Moore, 1996), with opposite phase-encoding directions (Andersson et al, 2003), or with opposite diffusion-weighting directions (Bodammer et al, 2004;Shen et al, 2004), all of which result in a substantially lengthened acquisition time. Post-processing methods based on cross-correlation (Haselgrove and Moore, 1996;Bastin, 1999) or mutual information (Horsfield, 1999;Rohde et al, 2004;Ardekani and Sinha, 2005) typically involve the coregistration of non-diffusion-weighted and diffusion-weighted images with highly variable contrasts and are generally computationally intensive.…”

Correcting eddy current and motion effects by affine whole‐brain registrations: Evaluation of three‐dimensional distortions and comparison with slicewise correction

“…Most previous studies applied their EC correction scheme on phantoms or on individual subjects (14,(19)(20)(21)(22)(23)(24)(25)(26)(37)(38)(39). However, interindividual comparisons of EC distortions in a group of healthy subjects have not yet been published.…”