Dear Saad ,
thank you for your email. the two groups have the same resolution. also,
the mask in standard space is about a 1000 voxels, which translates into
about 100 voxels in the subject's fMRI space.
best,
paymon
Saad Jbabdi wrote:
>
> hi - it seems like your two groups do not have the same data resolution,
> is that right ?.
> if your mask in standard space is too small (size less than half a voxel
> in raw data space), then it will be interpolated to zero. the option -i
> decreases this threshold, so you get bigger masks at the end.
> b.t.w : i'm not sure you can compare two groups that do not have the
> same acquisition parameters.
>
> cheers,
> saad
>
>
> On 21 Dec 2006, at 16:37, Paymon Hosseini wrote:
>
>> dear FSL users,
>>
>> i am having trouble with the masks used for featquery.
>>
>> i have two groups of subjects, a young group, and an old group. in
>> order to do an ROI analysis, i defined a mask in the standard space.
>> when i run featquery, the masks in the featquery folders of the young
>> subjects are close to the size of the original mask and at the correct
>> location. the masks in the featquery folders of old subjects, however,
>> are either zero everywhere, or only a 3-4 voxels and slightly off the
>> target area.
>>
>> i have checked the registrations from each subject to the standard
>> space (12 DOF) and they look correct. when i reduce the mask threshold
>> in featquery (with the -i option) to .3 for older subjects, the size
>> of the masks increase by a few voxels.
>>
>> why are the masks for older subjects often zero every where. is this a
>> registration problem? is reducing the mask threshold the correct
>> solution here?
>>
>> thank you very much for any advice,
>> paymon
>
> ---------------------------------------------------------------------------
> Saad Jbabdi,
> Postdoctoral Research Assistant,
> Oxford University FMRIB Centre
>
> FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
> +44 (0) 1865 222545 (fax 222717)
> [log in to unmask] <mailto:[log in to unmask]>
> http://www.fmrib.ox.ac.uk/~saad
> ---------------------------------------------------------------------------
>
>
>
>
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