It must be worrying not having a clear diagnosis. havent heard borderline PD used as a label but i can imagine being told the results were borderline. I have never heard of that scan either, could you mean SPECT scan?
One of the difficulties with PD is that there is no test that proves conclusively if we have Parkinsons. Alot of research is focused on this problem.
You could try a google search on Diagnosing PD. This is the link to the PUK information on it.http://www.parkinsons.org.uk/sites/default/files/publications/download/english/fs48_diagnosingparkinsons.pdf