I think this reference is right.
R. W. Cox and D. Glen, "Efficient, Robust, Nonlinear, and Guaranteed Positive Definite Diffusion Tensor Estimation", Proc. Intl. Soc. Mag. Reson. Med. 14 (2006), p.349.
Abstract can be viewed here:
afni.nimh.nih.gov/sscc/staff/rwcox/ISMRM_2006/ISMRM%202006%20-%203340/files/00349.pdf
Bob's PowerPoint Presentation for the corresponding talk here:
afni.nimh.nih.gov/pub/tmp/ISMRM2006/ISMRM_2006_DTI_talk.ppt
My presentation on this:
afni.nimh.nih.gov/sscc/presentations/Diffusion%20Tensor.ppt