[Pingu]

What are you using voxels for?

I want to specify a small spherical region of brain centered on a set (xyz) of coordinates in millimeters.

That will happen if all of the voxels are cubes. Have you tried different shapes?

Well, no, because I'm using cartesian coordinates.

The issue is that I have a location-of-interest derived from fMRI, on a series of scan using a 3mm grid (i.e. 3 x 3 x 3 mm voxels). Usually, if you want to define a spherical region (a "Region Of Interest" ROI) with radius, say 10mm, of a specific voxel, you just include all voxels within 10mm (Euclidian distance) of the coordinates and you get a set of sphericalish looking things of approximately the same size i.e. same number of voxels.

But I am going to an image that is much more grainy - the voxels are 8mm. So I wanted to standardise my "sphere" so I have the same shape and size ROI for each coordinate. But for a small "sphere" there's a lot of variance in shape and size.

It's a bit like trying to copy a Lego design in Duplo.

Interestingly, for a 10mm radius, most come out as 8 voxels, one as 12 - because the location I'm interested in is not dead centre of any 8mm voxel, so instead of finding a nice symmetrical 7 voxel object, it finds a lumpy thing.

But they are OK. And probably better than finding symmetrical "spheres" with a centre in the in the wrong place.

Do you use mango?

No, we use SPM mostly.