Can pet scans tell if a person is having a psychosis?

Not so clearly : Functional brain scans are being researched, but are not yet reliable enough to be a clinical tool. Talking with the person is much more helpful; seeing what s/he has to say & how it's said. How language is; thoughts expressed; what is the emotional tone or lack of it. Is s/he oriented to person, place, time, or not? Are there neurological signs that need further examination, etc. Human contact.

PET scan in Psychosi: There are ongoing studies that looked at pet scans in people claiming to be hearing voices, and showed activation in primary auditory cortex or parts of the brain involved in the perception & understanding of speech.