Here is OsiriX for Mac OS X 10.4+ (Universal binary). This is an
excellent DICOM (medical image/CT-scan) viewer, analyzer, and
exporter. It can do surface/volume generation from DICOM sets (and
possibly other types of data if you're creative) and export to several
3D formats.

Old versions are becoming hard to find online, e.g. all gone from
SourceForge now, and the link in [this 2018 thread][1] is dead.
[There's a Github mirror][2], though.

This was originally an open source project partly funded by donations.
Version 2.7.5 is distributed under the GNU General Public License
(GPL) and was also advertised as "freeware". Nevertheless, it is a
very polished app.

The sample screens show a juvenile chimpanzee skull CT-scanned at
Kyoto University, Japan.

Compatibility
Architecture: PPC x86 (Intel:Mac)

OSX 10.4+ (PPC and Intel)

OsiriX 2.7 or higher requires OS X 10.4 and (officially) a minimum of
1 GB RAM, with 2 GB recommended.

However, 512 MB is more than enough to work with smaller DICOM sets.

  [1]: http://macintoshgarden.org/forum/osirix
  [2]: https://github.com/pixmeo/osirix