NRRD files are not writing units in the header.

[muratmaga]

Summary

The units of the values of NRRD header space direction is specified in the space units field. Otherwise, it is up to the software to interpret the values as mm, cms, or ums. This is a data fidelity issue. We should support writing this space units explicitly to the NRRD file.

There is also a separate bug where if the space units are specified in the NRRD as "um" (micron), Slicer incorrectly multiplies these values by 1000.

There is a detail discussion of the forum: https://discourse.slicer.org/t/setting-up-units-in-slicer/34861