Siemens Diffusion Images
The Siemens Trio may writes out several images from a single diffuction run. These are (your suffixes may vary):
<prefix>_DTI_64 - the diffusion image itself, a 4D dataset with the 4th dimension being diffusion directions
<prefix>_DTI_64_ADC - the apparent diffusion coefficient. This is derived from the calculation of the diffusion tensor from the diffusion image, and can be thought of as proportional to the speed of diffusion along the main diffusion axis at each voxel.
<prefix>_DTI_64_TRACEW - Trace weighted image - calculated. Quoting from the document below, 'The trace-weighted image is obtained by taking the geometric average of the individual diffusion weighted images from the DTI scan'.
<prefix>_DTI_64_FA - the fractional anisotropy map, calculated from the diffusion image, where values in the image represent deviations, per voxel, of the diffusion in that voxel from isotropy (equal diffusion in all directions).
See http://www.medical.siemens.com/siemens/sv_SE/gg_mr_FBAs/files/brochures/DTI_HT_FINAL_HI.pdf for more detail.