IE: If you have say a tumor (cancer) in the brain - a CT or MRI would be much better able to identify the structural abnormality of it's presence.
Again Positron Emission detects metabolic function -- thus if you are experiencing neurological issues that don't seem to stem from structural issues in the brain or spine.. then a PET scan could be quite useful in detecting if the blood flow is adequate throughout the brain, if there is an ischemic state anywhere in the brain (low oxygen), or other metabolic/cellular issues.
The first answer was right.. a PET scan can't detect IQ, Schizophrenia , etc.. But no test can detect these things.
I think it is easier to understand what a PET scan can and cannot 'find' by understanding how it works -- which I attempted to explain above.
Hope that helps,