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Ruth, at least in the context of that discussion, I was talking about VERY small shifts--- I was asking about averaging data that was taken within 5-10 minutes of each other and within the same scanning session. Since there is likely some small head movement between individual gradients, the question pertained more to how you align the images to one another to get the best tractography (i.e. align them before or after averaging the two individual scans). dag On 8/15/07, Ruth <[log in to unmask]> wrote: > > Hi, > I was just rereading the thread on averaging multiple DTI > acquisitions. As > I understand it, the suggestion was to rotate the 2nd acquisition to line > up > with the first acquisition, then continue with tensor calculation. Am I > correct that this would only be valid with small rotations? For example, > if > one of the acquisitions had to be rotated 30 degrees in order to line up > with the other (because the subject had moved their head within the > scanner), the directions recorded in it's original bvecs file would no > longer be accurate. That being the case, the 2 acquisitions, with > different > bvecs, couldn't be directly averaged. Is this correct? > > --Ruth > -- David A Gutman, M.D. Ph.D. Department of Psychiatry & Behavioral Sciences Emory University School of Medicine