As far as traditional scans they often don't show mets, if they are micro sized.
We were told by our Urolo. to have RT for my husbands rising psa post surgery.
Everything I read said it (The tumor) has probably already escaped the capsule. We had traditional CT scans and MRI to abdomen and pelvis that showed nothing as they often aren't reliable.
I took it upon myself to take my husband to Stanford for a second opinion and he was put in a trial. That's where we had the F-18 pet scan, which showed 1 isolated micro-met to his spine.
I am not privey to the perametric MRI, so others may know more about that test.
You seem to be eager for info as I was, but stay calm and try not to compare your stats to others.
You could talk to several Dr's and often they aren't in agreement.
We also went to UCSF and Stanford and UCSF had differing opinions on what to do about my husbands met.
Do get 2nd opinions.