DICOM
General description
DICOM (Digital Imaging and Communications in Medicine) is far and away the most widely-used (and probably the oldest) electronic file format in medical imaging. Nearly every device that acquires medical images -- ultrasound, CT, PET, and MRI -- acquire DICOM images in normal operation. There's a 20-part specification detailing the file format and its ecosystem. The IANA has assigned TCP and UDP port 104 to DICOM-related traffic.
It's kind of a big deal.
However, as with any sufficiently-adopted standard, there are splinter factions. The most common format is 2-dimensional images or "slices" that can be formed into a 3-dimensional image; however, some manufacturers have extended the standard to save 3 or even 4-dimensional images in a "mosaic" format.
Software
Software that reads DICOM files is pretty much everywhere. Most neuroimaging analysis packages have some way of importing DICOMs and turning them in to a higher-dimensional file; open-source stand-alone libraries abound, as well:
