[MINC-users] mincblur with FWHM <= voxel dims

[Andrew Janke]

On 21 June 2012 13:53, Alex Zijdenbos <zijdenbos at gmail.com> wrote:
> Empirically, when you reduce the FWHM to below the voxel dimension, the voxel value of the blurred image increases. Moreover, for an image with 1mm isotropic voxels, the FWHM at which the voxel values of the result are about equivalent to those in the source image seems to be around 1.3, give or take. See this image:
>
> mni_icbm_00102_t1_blurs.png (http://cl.ly/2A1m3Y470P0M3S3J2Q0j)
>
> which shows in the top row the original, 1mm isotropic, volume, followed by the result of running mincblur on it with decreasing FWHM from 1.5 to 0.5mm. I used spectral with a fixed range across all rows to bring out the signal difference.
>
> So - is this expected? Surprising? Follows theory or bad behaviour of mincblur? After scribbling a few pages with normal distributions and looking through the mincblur code I still can't quite figure it out. Anybody can explain this to me?

I'd say expected (given how mincblur works) and I'd also say is the
correct behaviour.  If you think of the blur as fitting a Gaussian to
your noisy peak then as the width of this peak decreases, the height
of it increases to the same given amount of signal.

As for if it should happen when you (or a script) doesn't sanitise
inputs then you probably have an argument that mincblur should give a
warning. I would argue that the script should check the input sampling
though, certainly I remember doing this in a few scripts of mine. I
won't be so bold to say that I do check in all scripts I write though!



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