[18F]altanserin has been used to label serotonin 5-HT2A receptors, which are 
believed to be important in the pathophysiology of schizophrenia and depression. 
The purpose of this study was to test the feasibility of a constant infusion 
paradigm for equilibrium modeling of [18F]altanserin with PET. Kinetic modeling 
with [18F]altanserin may be hampered by the presence of lipophilic 
radiometabolites observed in plasma after intravenous administration.
METHODS: Eight healthy volunteers were injected with [18F]altanserin as a bolus 
(208+/-9 MBq [5.62+/-0.25 mCi]) plus constant infusion (65+/-3 MBq/h 
[1.76+/-0.08 mCi/h]) ranging from 555 to 626 min (615+/-24 min) after injection. 
PET acquisitions (10-20 min) and venous blood sampling were performed every 
30-60 min throughout the infusion period.
RESULTS: Linear regression analysis revealed that time-activity curves for both 
brain activity and plasma [18F]altanserin and metabolite concentrations 
stabilized after about 6 h. This permitted equilibrium modeling and estimation 
of V3' (ratio of specific uptake [cortical-cerebellar] to total plasma parent 
concentration after 6 h). Values of V3' ranged from 1.57+/-0.38 for anterior 
cingulate cortex to 1.02+/-0.39 for frontal cortex. The binding potential V3 
(ratio of specific uptake to free plasma parent concentration after 6 h, using 
group mean f1) was also calculated and ranged from 169+/-41 for anterior 
cingulate cortex to 110+/-42 for frontal cortex. From 6 h onward, the rate of 
change for V3' and V3 was only 1.11+/-1.69 %/h.
CONCLUSION: These results demonstrate the feasibility of equilibrium imaging 
with [18F]altanserin over more than 5 radioactive half-lives and suggest a 
method to overcome difficulties associated with lipophilic radiolabeled 
metabolites. The stability in V3 and V3' once equilibrium is achieved suggests 
that a single PET acquisition obtained at 6 h may provide a reasonable measure 
of 5-HT2A receptor density.