Thanks for the bump, Ben. I am sorry for missing on the original post.
Distortions do not track with a person’s head. If a subject tilts their head back, the distortion will not simply follow that movement. It may indeed be altered a bit, but the distortion field is not going to follow the volreg parameters, for example.
The distortions that this is trying to correct are mostly along the phase encode (y) direction of the acquisition slices. Those distortions are indeed affected by the subject, and as the subject moves over time, a single estimate of the distortion field may become less accurate. But keep in mind that while the subject may move, the y-axis of the slices remains fixed.
Since the distortion correction is mostly along that one axis, the distortion correction is actually applied BEFORE volreg, so that the y-axis is not altered by the registration step.
Getting to the actual questions now…
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No. Rotating the blip images to match the volreg base would assume the distortions follow the anatomy. But while the estimates may not be perfect, distortions are mostly along the y-axis of the acquisition.
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The accuracy of the distortion correction will tend to decay (not so simple) as the subject gets farther from the B0 position. So the basic question is, how much does the distortion field change as the subject moves? If it changes a lot, this could be a destructive operation.
However, if the distortion fields are significantly changing, that would apply to the data if you do nothing to correct it, too. I would think the changes would have to be pretty big for distortion correction to be WORSE than not doing anything. Though that also depends on how strongly the corrections are applied.
We are “actively” working on these things now, so hopefully we will develop a better feel for the interplay between motion and distortion. But that will take some time…
- rick