[simonster]

This is possible and I'd be surprised if no one has done it. The methodology is to inject subjects with a mildly radioactive drug that has a lower binding affinity for a given receptor than the endogenous ligand (e.g. [11C]raclopride for dopamine receptor occupancy) and then measure how much of this drug is bound to the receptor using PET. If more of the drug is bound, then the level of the endogenous ligand is lower. I don't know much of the literature here, but the technique is well-established and should be substantially more accurate than anything that measures urine or saliva.