Dianne's 2 favorite papers:
Jellison_2004: A very approachable introduction to DTI and the white matter tracts.
Wedeen_2012: A surprising paper that claims tracts intersect at right angles in a grid (I'm not kidding, at least look at the pictures).
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Jones_1999: A classic paper exploring the number of B0 images needed for a given number of b-weighted images.