Postinjection L-phenylalanine increases basal ganglia contrast in PET scans of 6-18F-DOPA.
The sensitivity of 18F-DOPA positron emission tomography for imaging presynaptic dopamine systems is limited by the amount of specific-to-nonspecific accumulation of radioactivity in brain. In rhesus monkeys, we have been able to increase this ratio by taking advantage of the lag time between 18F-DOPA injection and the formation of its main metabolite, the amino acid 18F-fluoromethoxydopa, the entrance of which into brain is responsible for most of the brain's nonspecific radioactivity. By infusing an unlabeled amino acid, L-phenylalanine, starting 15 min after 18F-DOPA administration, we preferentially blocked the accumulation of 18F-fluoromethoxydopa by preventing its entrance into brain through competition at the large neutral amino acid transport system of the blood-brain barrier. This method appears as reliable as the original and more sensitive, as demonstrated by the comparison of normal and MPTP-treated animals under both conditions.
Doudet
D J
DJ
Section on Clinical Brain Imaging, National Institutes of Mental Health, Bethesda, Maryland 20892-1000.
McLellan
C A
CA
Aigner
T G
TG
Wyatt
R
R
Adams
H R
HR
Miyake
H
H
Finn
R T
RT
Cohen
R M
RM
D000818:Animals; D001479:Basal Ganglia / Q000000981:diagnostic imaging*; D002230:Carbidopa / Q000008:administration & dosage; D004295:Dihydroxyphenylalanine / Q000031:analogs & derivatives* / Q000037:antagonists & inhibitors; D005260:Female; D005462:Fluorine Radioisotopes; D008253:Macaca mulatta; D008297:Male; D010649:Phenylalanine / Q000008:administration & dosage*; D013997:Time Factors; D014055:Tomography, Emission-Computed
D005462:Fluorine Radioisotopes; C055151:3-O-methyl-6-fluoro-dopa; C043437:fluorodopa F 18; D010649:Phenylalanine; D004295:Dihydroxyphenylalanine; D002230:Carbidopa