The visible human data is not in DICOM format. It is distributes with “Z” compression, in the GE Signa format. This format was proprietary and pre-dates DICOM.
You can convert this format to NIfTI with the following steps
- Uncompress all of the images. From the command line, this would be
uncompress *.Z - Convert the images to NIfTI using dcm2nii (which pre-dates dcm2niix). This software was distributed with MRIcron - just get the 2016 version or earlier. The only benefit of this old software relative to dcm2niix is support for these archival proprietary formats. Be aware that the Signal format does not include much information about image spacing and orientation. Therefore, you may have to tune the resulting NIfTI header by hand.
The visible human scans are very old, and you should keep your expectations realistic. While the color photographs (particularly for the female) are unique, the MRI and CT scans are obsolete. Pressing the “Download Sample Data” button in Slicer will provide you access to far better MRI and CT images.