I'm sorry you're so worried about this symptom, and have had to go through so many tests. It's good that they've ruled out other possibilities. We have to be thankful for what we DON'T have!
My husband currently is going through the diagnostic process for probable PD. I haven't heard (yet!) of the scan you mention. But I have heard about the SPECT test--a brain scan using radio isotopes which apparently can measure the number of dopamine-producing neurons in the brain. Is that what you're talking about? It seemed to me to be a good thing to do, and would help with the diagnosis. But when I asked our general neurologist if it would be advisable in my husband's case, his answer was that this test is used in large studies, but would not be helpful in an individual. I don't know what he meant by that, and didn't ask, because this doctor is not very open to questions. We'll be seeing a movement disorder specialist this week. I will ask her the same question and get back to you.
I too am being watched for PD. 2 docs said I don't have it but an internationally known movement disorder specialist said there is a possibility. I also asked about the SPECT and he said there are so many false positives that he and his partners wonder why any dr. would prescribe it. Good luck!