Hi -
PixelSpacing should always give the in-plane sizes in mm. Be sure it is in a valid dicom format of two DS values (basically decimal strings separated by backslash).
If you use dcmdump from dcmtk it should look something like this:
(0028,0030) DS [1.000000e+00\1.000000e+00] # 26, 2 PixelSpacing
The out of plane spacing is defined by the distance between the ImagePositionPatient of adjacent slices, not by the SliceThickness, which can be different for a variety of reasons.
If Slicer (or ITK) can’t parse what your file contains you could end up with spacings of 1,1,1.
It’s a slightly different context, but this blog post gives you some background and context:
HTH,
Steve