According to the description of the Reddit user who posted this amazing image:
The upper left corner has the actual X-Ray tube. The lower right is the imager – it’s what basically is the digital “film” for the CT scanner. The box in the upper right is probably the motor that makes the whole assembly rotate. Oh, by the way… all this machinery rotates around you at a very high rate of speed. That’s why it has to be level and balanced, because if it’s offset or off-balance, it starts making some really nasty noises, and eventually tears up the bearings. I used to work on these things about 5-6 years ago. This one is more modern than the ones I worked on, but it’s the same concept.
CT scanners produce bi-dimensional slices of human bodies (or anything else) using X-rays. These cross-sections can be examined by doctors both in their original form or as part of three-dimensional models created using the data from each of the slices together. [Reddit]