HI - from the Featquery manual:
"
If you have selected a mask image in standard or highres space, this
will get transformed into lowres space as described above. This
involves interpolation; at the edges of the mask there will be a
continuous range of values from 1 down to 0. In order to get back to a
binary mask, this must be thresholded at some value - the default is
0.5. However, if you want the mask to be slightly more or less
inclusive than that default, you can Change post-interpolation
thresholding of mask - for example, by reducing the value to 0.3, the
final lowres mask will be slightly larger.
"
So - there is no "right" answer wrt what the correct threshold is -
for example, it is not possible to pre-set a threshold value that will
maintain the volume of binarised voxels, as that depends on the actual
mask as well as the start and end resolutions. Hence we have a default
of 0.5, which you can change if you need to.
Steve.
On 5 Feb 2009, at 11:57, Daniel Shaw wrote:
> Hi,
>
> I am currently using Featquery to extract the number of voxels/mean
> z values within a
> number of functionally defined binarised masks specified in MNI
> (1x1x1mm) space.
>
> I notice that when Featquery registers the masks back to native
> (4x4x4mm) space they
> seem to lose a degree of their (proportional) spatial extent. I
> discovered this by manually
> registering the binarised masks to native space (using reg/
> standard2example_func.mat)
> and finding a large discrepancy when comparing the number of voxels
> within the
> manually registered masks to that calculated within the
> corresponding mask by
> Featquery.
>
> I see now that, despite being originally binarised, the edges of the
> masks have values
> ranging from 0-1 when I manually register them to native space.
> Although I can change
> the interpolation value in Featquery to preserve the size of the
> original masks, would you
> be kind enough to explain to me where this alteration in mask values
> comes from so that
> I don't overcompensate.
>
> Any assistance would be greatly appreciated.
>
> Many thanks in advance,
>
> Dan.
>
---------------------------------------------------------------------------
Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
---------------------------------------------------------------------------
|