D K Mahoney | PubMed Health

PubMed1992

The effects of carbidopa administration on 6-[18F]fluoro-L-dopa kinetics in positron emission tomography.

Carbidopa (L-alpha-hydrazino-alpha-methyl-b-(3,4-dihydroxyphenyl) propionic acid is a known inhibitor of aromatic amino acid decarboxylase. In both humans and monkeys, we studied the effects of carbidopa on plasma and brain kinetics of 6-[18F]fluoro-L-DOPA (FDOPA), an analog of L-DOPA used for PET studies of the central dopaminergic system. Pretreatment with carbidopa resulted in increases in the plasma levels of FDOPA and 3-O-methyl-6-[18F]fluoro-L-DOPA (3-OMFD). Total striatal and cerebellar activities measured with PET were also increased. Furthermore, increases observed in the specific striatal activity (striatum minus cerebellum total activity) were correlated with increases in the plasma FDOPA curve. Carbidopa pretreatment did not affect the influx rate constant (K) for FDOPA from plasma to striatum in humans as determined by Patlak graphical analysis. Thus, an increase in measured striatal tomographic activity was secondary to the increase in plasma FDOPA levels rather than as a result of changes in the FDOPA influx rate constant.

Adult↗

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Quantitation in positron emission tomography: 8. Effects of nonlinear parameter estimation on functional images.

Functional images used in positron emission tomography (PET) have the advantage of presenting simultaneously the anatomical and functional information in cross-sectional body slices. However, the nonlinearity in parameter estimation, when combined with the finite image resolution, can cause systematic errors or biases in the estimated functional parameters. The effect of this error on blood flow images, which are commonly used in PET, is investigated in this study. Computer-simulated brain and heart phantoms of realistic configurations are used to examine the effect of various factors, such as imaging resolution, estimation nonlinearity, and structure configuration. The nonlinearity characteristics of six commonly used blood flow estimation techniques are simulated. Results show that structure boundaries on parametric images between tissues of different blood flows do not usually coincide with the true anatomical boundaries and would thus cause an apparent change in the cross-sectional size of the structures. The regional blood flow values as obtained from the blood flow images are usually lower than the true values. The severity of these effects is dependent on the characteristics of the flow estimation technique, the image resolution, and the size and shape of the structure. Although image resolution is a major factor in causing errors in the parametric images, its improvement, within the range examined in the present study [from 1.5 to 0.5 cm full width at half maximum (FWHM)], is not found to reduce drastically the underestimation of blood flow in brain phantom. The effect on boundary shift, however, is found to be in proportion to the FWHM of image resolution. Implications of these effects on generation, interpretation, and comparison of parametric/functional images are discussed.

Cerebrovascular Circulation↗