Unit for slice thickness

[neurolabusc]

The BIDS microscopy BEP031 defined the tag SliceThickness with the unit of micrometers ("um"). I do worry that for years dcm2niix has populated the tag SliceThickness with the unit "mm", corresponding to the DICOM Slice Thickness (0018,0050) tag. Note that for MRI, the slice thickness will be different from the NIfTI pixdim[4] if there is a slice gap. While it is great to see BIDS fill niches in multiple domains, I do worry this will cause confusion when one expects different units in different modalities. I realize that MR SliceThickness was not a formal attribute of the BIDS specification, but rather a useful but ad hoc attribute added by dcm2niix and dicm2nii. However, I do fear opportunity for unintended consequences.

[Remi-Gau]

Technically SliceThickness is only an official BIDS microscopy metadata. For now. So in theory nothing prevents making it official for MRI and at the same time specifying a different unit for this datatype.