I will. Thanks Bruce.
On Oct 7, 2009 9:07am, Bruce Fischl <fischl@nmr.mgh.harvard.edu> wrote:
> why don't you try doing a within-group (ideally control) across-sequence study and see what the effect is.
>
> On Wed, 7 Oct 2009, James Root wrote:
>
>
>
>
> In regard to group differences, roughly 1/3 of the scans in one group match
>
> the voxel size and orientation parameters in the other. The remaining scans
>
> are uniformly different from each other. Not knowing the specific effects
>
> that comparing unequal parameters will have in a Freesurfer analysis, is
>
> there a threshold or rule of thumb for when not to compare groups with
>
> different parameters? Are there specific parameter differences to avoid (x,
>
> y, z, size; orientation)? Thanks again for your help.
>
>
>
> On Tue, Oct 6, 2009 at 12:14 PM, Bruce Fischl fischl@nmr.mgh.harvard.edu>wrote:
>
>
>
>
> Hi James,
>
>
>
> it certainly can. If one group had all one scan type and another all a
>
> different one then you wouldn't be able to tell if the effects you find are
>
> due to the group membership or the scan differences.
>
>
>
> cheers,
>
> Bruce
>
>
>
>
>
> On Tue, 6 Oct 2009, James Root wrote:
>
>
>
> I am planning an analysis using freesurfer to compare 2 groups. The SPGRs
>
>
> that will be used vary in regard to slice thickness (from 0.86 to 1.5 in
>
> each axis), number of slices acquired (100-256), and orientation (axial;
>
> coronal) due to the fact that these were collected over a number of years
>
> on
>
> the same scanner. In another posting I read that this might be a problem
>
> but wanted to revisit this to see if any specific heterogeneity in the
>
> parameters should be particularly avoided. Also, if the groups differ
>
> such
>
> that one group has largely similar scan parameters and the second group is
>
> more heterogeneous, will this introduce any systematic bias in cortical
>
> thickness estimates, parcellation, or segmentation values? Thanks for
>
> any
>
> advice on this issue.
>
>
>
>
>
>
>
>
>