however Dat/Pet/Spect scan results are lights on an image and diagnosis based on interpretation of light is tricky and error prone.

To my knowledge primary diagnosis is based on clinical observation, ldopa responsiveness and Dat/pet/spect scans are supporting "tools".

l-dopa responsiveness is in the 70% - 80% range according to this study, (still leaves 20% to 30% non responsive though, a significant proportion).

http://cat.inist.fr/?aModele=afficheN&cpsidt=13807564

Neil.