Correcting Warps in Diffusion-Weighted Echo-Planar Images

Most diffusion-weighted images are acquired using the echo-planar imaging (EPI) pulse sequence, and because of this they are prone to distortions caused by the application of the diffusion-weighting gradient pulses. These distortions can be severe, particularly on older MRI scanners, and lead to errors in calculation of the tensor. The diffusion analysis toolkit provides a pre-processing step for the correction of these distortions, prior to tensor calculation.

Below, you can see two images. The image on the left is from an older-generation MRI scanner; you can see a slice with different diffusion weightings with severe distortion of the image when diffusion weighting is applied in different directions. On the right is an image from a more modern scanner using a double-refocused EPI sequence with minimal distortion that probably does not need eddy-current warping correction.
Example images showing eddy-current distortion in EPI images.
eddy_warping eddy_no_warping
Warp correction needed Warp correction
probably not needed

To perform warp correction as a pre-processing step prior to tensor calculation, ensure that the warp_corr_check check-box is selected. The warp correction panel will then be visible:


You can change various setting that will alter the speed of warp correction, and the degree of success and quality of the correction.

With the warp_corr_check check-box selected, when you go on to calculate the diffusion tensor, the warp correction will be automatically performed. Warp correction produced a new set images, which are like the set of input images that you use, but their name will have a prefix "wc", and which have been warp-corrected. If the warp correction finds that a set of images exists on disk which have already been warp corrected (having the file name prefix "wc"), you will see this pop-up:



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