Changes in images of brain functional activity that are produced by disease or by activation of various pathways in the normal brain can only be unambiguously interpreted if the rates of the physiological and biochemical processes that underlie the imaging method are quantified. In imaging modalities that use radioactive tracers, e.g. positron emission tomography (PET), quantification is carried out by means of a mathematical model that describes the rates of the biochemical reactions in the metabolic pathway of the tracer and traced molecules. Selection of the best kinetic model is critical as the use of an inappropriate model can lead to substantial errors in quantification and possible misinterpretation of results. Once a model is selected, numerical procedures that are efficient, robust, and require minimal assumptions about the errors in the measurements are required to estimate accurately the parameters. Additionally, powerful statistical tests are needed so that the data can be examined for significant differences among experimental groups. The objective of this project is to develop better techniques for addressing these interrelated mathematical and statistical issues; advances in the current year were made in the following areas: (1) Work continued on adapting the quantitative autoradiographic 14C-leucine method for determination of regional rates of cerebral protein synthesis (rCPS) for use in man with L-[1-11C]leucine and PET. Data from PET studies following injection L-[1-11C]leucine in rhesus monkeys were analyzed for estimation of lamda, the fraction of the precursor pool for protein synthesis that is derived from the arterial plasma, and for determination of rCPS. These data were found to be in excellent agreement with biochemical determinations of lamda and autoradiographic measurements of rCPS determined in separate series of experiments. Preliminary results were presented at the VIth International Conference on Quantification of Brain Function with PET (BrainPET03) in Calgary, Alberta, Canada. (2) Work was initiated to develop kinetic analyses for determination of regional rates of cerebral glucose metabolism (rCMRglc) in rats with 2-[18F]fluoro-2-deoxyglucose and a small animal PET scanner. An approach was developed to minimize sensitivity to the effects of functional tissue heterogeneity, i.e., inclusion of tissues with different rates of blood flow and metabolism within a single voxel or region of interest. Heterogeneity is an especially acute problem in these studies due to the limited spatial resolution of the PET scanner relative to the size of the rodent brain structures examined; failure to take it into account leads to errors in estimates of rCMRglc. Preliminary results suggest the rCMRglc is stable between approximately 45 and 60 min when determined with the single scan or Patlak approach. Findings were presented at the 50th annual meeting of the Society of Nuclear Medicine in New Orleans, LA and at the VIth International Conference on Quantification of Brain Function with PET (BrainPET03) in Calgary, Alberta, Canada. (3) Work was initiated to develop mathematical techniques for analyzing muscarinic receptor binding in rats studied with a small animal PET scanner following injection of [18F]FP-TZTP. Experiments are underway to compare total volumes of distribution of the tracer determined with either bolus injection or continuous infusion of the tracer, and to evaluate the reproducibility and robustness of these measures.
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