Hello,
 A reviewer disagreed with one of our conclusions and suggested we could prove our claim by creating Axial Diffusivity - Radial Diffusivity voxel based correlational maps.  I wanted to verify that this can be accomplished by using the AD data as an input and loading the RD data as an additional voxel-dependent EV and setting up the contrast as normal.

Hypothetically no other variables:
Design:
EV1
1
1
1
1           EV2 = RD data
...

Contrasts:

0 1 --> AD and RD positively correlate
0 -1 --> AD and RD negatively correlate

Thanks,
Jeremy