Scientists in The University of Edinburgh have developed a method of detecting a leaky BBB in living people. See

http://alzheimers.org.uk/site/script...documentID=217

"We have developed a method of demonstrating leak into the brain from the blood stream using magnetic resonance imaging and an intravenous injection of magnetic dye. In a pilot project, we compared 10 patients with maturity onset diabetes (which is known to be associated with cognitive impairment but for unknown reasons) with 10 controls and found evidence of leak into the brain and fluid around the brain in diabetics. The leak was also most prominent in those with small holes scattered in the brain (infarcts). The leak was subtle and would not have been seen without sophisticated processing of the images."

They now plan to extend it to Alzheimers (if they can get funds!!)
With luck (and funds) we might be able to get a group of PD patients tested. I will write to them.
They finished the project in 2004, but it is not clear when this was published. Google alerts have only just flagged it up.
Ron