PET vs imaging

PET and imaging both are nouns.

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

As nouns, imaging is a hypernym of PET; that is, imaging 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)
imaging: (medicine) obtaining pictures of the interior of the body

Other hypernyms of PET include tomography.

PET (noun)	imaging (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
	the ability to form mental images of things or events