Dear,

If we use the ‘Extract DW Information’ tool from the diffusion toolbox, we find a discrepancy in the number of unique gradient directions.

We feed 12 image-files to the script containing 4 unique directions and 3 different b-values, using following options:

B-value tolerance: 10

Gradient direction tolerance: 1

Distinguish antiparallel gradient directions: No

The result shows however 5 unique gradient directions.

SPM5: dti_extract_dirs (v1.7)                      15:44:46 - 09/02/2010

========================================================================

Reading diffusion information

# unique b values:            3

# unique gradient directions: 5

extracted information saved into workspace variable 'res'

      allb: [12x1 double]

      allg: [12x3 double]

     bfile: [1x96 char]

     gfile: [1x96 char]

        ub: [3x1 double]

       ubi: [3x1 double]

       ubj: [12x1 double]

       ugi: [5x1 double]

       ugj: [12x1 double]

        ug: [5x3 double]

    dirsel: [12x12 logical]

         b: [12x1 double]

         g: [12x3 double]

If we look into the ‘res’ variable, we see that the same gradient direction is present twice in the ‘res.ug’ variable.

-0,901242247736658       -0,419113115207296       -0,110030030245444

0,109966984346586        0,663801905510129        0,739779894694929

0,109966984346586        0,663801905510129        0,739779894694929

-0,166021035584538       0,986122211362992        0

-1                                           0                                             0

Can you clarify this and what are the consequences of this discrepancy?

Kind regards,

Geert De Groof