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OMG this is a good catch. We wrestled with this a long time ago when setting up Slice Geometry. The correct way to set slice spacing (pixel spacing in z) from a DICOM is to use the slice position info (0020,1041 or 0020,0032) and divide by (stack size - 1). It can be out by not much or quite a bit.

A closer read of this paper makes it seem that some of the scans were done in chunks of a few slices each, and that the chunks had different slice spacing from each other. That's a bit of a mess: making a composite assuming equal z-spacing throughout the volume will lead to distortions.