Abstract
Background[11C]methionine (MET) has been used to monitor amino acid metabolism in tumors, the pancreas, liver, and myocardium. The aim of the present study was to standardize [11C]MET positron emission tomography (PET) by optimizing the timing of initiation of the scan and applying correction to the plasma concentrations of neutral amino acids (NAAs), where necessary.MethodsSequential whole-body MET PET/computed tomography (CT) was performed in 11 normal adults after they had fasted for at least 4 h. After whole-body CT for attenuation correction and intravenous bolus injection of MET, the subjects were scanned from the parietal to the groin. The scanning was repeated six to seven times. Decay of radioactivity during the PET scan was corrected to the time of initiation of the first scan. The standardized uptake values (SUVs) were evaluated in various organs by setting regions of interest on the tomographic images. Plasma concentrations of NAAs were examined in relation to the SUV values.ResultsThe SUVs in the pancreas reached their plateau from 6.5 to 11 min after the MET injection, and in the brain, lung, and myocardium, they reached their plateau from 19.6 to 24.1 min. The MET uptake in the spleen and kidney peaked early after the injection and steadily decreased thereafter. The SUVs in the liver and stomach wall rapidly increased during the first 0 to 4.5 min and gradually elevated thereafter during the scan period. Urinary radioactivity in the bladder reached its plateau from 26.1 to 30.6 min after the MET injection. There were no correlations between the plasma concentrations of NAAs and the maximal SUV in any organs.ConclusionsThe present study revealed the times taken to reach the plateau of MET uptake in various important organs, and little effects of the plasma neutral amino acid concentrations on the SUVs in PET studies conducted after the patients had fasted for at least 4 h. In the MET PET study, 4 h fasting period before MET administration and the scan initiation 20 min after MET administration provide the SUV values independent of scan initiation time and the plasma neutral amino acid concentrations.
Highlights
It has been reported that the extent of tumor cell invasion can be detected more clearly by MET positron emission tomography (PET) than by computed tomography (CT) or MRI [2,3,4,5,21]
We examined the effects of the neutral amino acids (NAAs) concentrations in the plasma on the standardized uptake values (SUVs) in the MET PET study
In the 2-[18F]fluoro-2-deoxy-D-glucose (FDG) PET study, while the SUVs decline in normal tissues in the postinjection period, those in tumors generally increase, indicating that the SUVs of normal and tumor tissues are dependent on the time of scan initiation after infusion
Summary
L-Methyl-[11C]methionine (MET) is a useful radiotracer in positron emission tomography (PET) conducted for the diagnosis of tumors [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]. The study protocol for MET PET has not yet been standardized. The concentrations of neutral amino acids (NAAs) in the plasma were not measured, despite the possibility of these affecting the tracer MET uptake by tissues in a competitive way. The NAA concentrations in the plasma were found to be influenced when measured after a meal, a fasting period before MET PET study has not been suggested. The purpose of this study was to confirm previous published protocols and standardize the scan initiation times of MET PET for various organs. We examined the effects of the NAA concentrations in the plasma on the standardized uptake values (SUVs) in the MET PET study
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