PET vs tomography

PET and tomography both are nouns.

Word	Noun	Adjective	Verb	Adverb
PET	Yes	No	No	No
tomography	Yes	No	No	No

As nouns, tomography is a hypernym of PET; that is, tomography is a word with a broader meaning than PET:

PET: using a computerized radiographic technique to examine the metabolic activity in various tissues (especially in the brain)
tomography: (medicine) obtaining pictures of the interior of the body

Other hypernyms of PET include imaging.

PET (noun)	tomography (noun)
using a computerized radiographic technique to examine the metabolic activity in various tissues (especially in the brain)	(medicine) obtaining pictures of the interior of the body