This work helps elucidate how background noise introduces statistical artif
acts in the distribution of the sorted eigenvalues and eigenvectors in diff
usion tensor MRI (DT-MRI) data, Although it was known that sorting eigenval
ues (principal diffusivities) by magnitude introduces a bias in their sampl
e mean within a homogeneous region of interest (ROI), here it is shown that
magnitude sorting also introduces a significant bias in the variance of th
e sample mean eigenvalues. New methods are presented to calculate the mean
and variance of the eigenvectors of the diffusion tensor, based on a dyadic
tensor representation of eigenvalue-eigenvector pairs. Based on their use
it is shown that sorting eigenvalues by magnitude also introduces a bias in
the mean and the variance of the sample eigenvectors (principal directions
), This required the development of new methods to calculate the mean and v
ariance of the eigenvectors of the diffusion tensor, based on a dyadic tens
or representation of eigenvalue-eigenvector pairs. Moreover, a new approach
is proposed to order these pairs within an ROI, To do this, a corresponden
ce between each principal axis of the diffusion ellipsoid, an eigenvalue-ei
genvector pair, and a dyadic tensor constructed from it is exploited, A mea
sure of overlap between principal axes of diffusion ellipsoids in different
voxels is defined that employs projections between these dyadic tensors, T
he optimal eigenvalue assignment within an ROI maximizes this overlap. Bias
in the estimate of the mean and of the variance of the eigenvalues and of
their corresponding eigenvectors is reduced in DT-MRI experiments and in Mo
nte Carlo simulations of such experiments. Improvement is most significant
in isotropic regions, but some is also observed in anisotropic regions. Thi
s statistical framework should enhance our ability to characterize microstr
ucture and architecture of healthy tissue, and help to assess its changes i
n development, disease, and degeneration. Mitigating these artifacts should
also improve the characterization of diffusion anisotropy and the elucidat
ion of fiber-tract trajectories in the brain and in other fibrous tissues.
Published 2000 Wiley-Liss, Inc.(dagger).