Mathematical And Statistical Analysis Techniques

National Institute Of Mental Health

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Abstract

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) We have validated a method for quantitative determination of regional rates of cerebral protein synthesis (rCPS) with C-11 leucine and PET (see project MH000889). The method uses a kinetic modeling approach to estimate lambda, the fraction of the precursor pool for protein synthesis derived from arterial plasma. We have recently undertaken a study to examine the feasibility of performing voxel-wise kinetic modeling analyses and to compare results with region of interest (ROI) analyses. Preliminary results indicate that greater than 98% of voxels yield valid parameters values. Regional estimates of the kinetic parameters are lower when estimated using voxel-based compared to ROI-based methods. rCPS and lambda are robustly estimated by both methods. (2) We have undertaken a study to use detailed distributional data from C-14 leucine autoradiographic studies to estimate the precision of PET measurements with C-11 leucine. We are constructing a three-dimensional volume from C-14 leucine autoradiograms of one hemisphere of a monkey brain and are computationally matching the resolution of this volume to that of the PET scanner. We are developing analysis methods to use this database to estimate the sensitivity of the PET scanner for detecting specific changes in regional rates of cerebral protein synthesis. (3) Work continued on the development of kinetic analyses for determination of regional rates of cerebral glucose metabolism (rCMRglc) in rats with 2-[18F]fluoro-2-deoxyglucose (FDG) 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 estimates of kinetic model rate constants and rCMRglc that decline with time and approach their true values only after the free FDG in all tissues within the region have equilibrated with the FDG in the arterial plasma. (4) Development of techniques for analyzing muscarinic receptor occupancy in rats studied with a small animal PET scanner following injection of 18F labeled 3-(3-(3-fluoropropyl)thio)-1,2,5,thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine ([18F]FP-TZTP) was completed. A manuscript detailing the findings is in preparation. (5) Development of techniques for assessing changes in serotonin 5HT1A receptor occupancy in rats by use of a small animal PET scanner and a continuous infusion of the 5HT1A receptor antagonist 4-[18F]-fluoro-N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyrimidinyl)benzamide ([18F]FP-WAY) was completed. A manuscript detailing the findings is in preparation.