CALCULATION OF THE FDG LUMPED CONSTANT BY SIMULTANEOUS MEASUREMENTS OF GLOBAL GLUCOSE AND FDG METABOLISM IN HUMANS

The lumped constant defined as the conversion factor between the net u ptake of fluoro-2-deoxy-D-glucose (FDG) and glucose was calculated fro m global CMRglc and from positron emission tomography (PET) using FDG as tracer (CMRFDG), Fifteen healthy, normal volunteers (mean age 24 +/ - 4 years) were studied. Global CBF and CMRglc were measured with the Kety-Schmidt technique using Xe-133 as tracer, and values were correct ed for errors from incomplete diffusion equilibrium for inert gas trac er between brain tissue and cerebral venous blood. Measurements of CMR FDG were obtained with PET using the dynamic and single-scan methods a nd the K-1-k(3) model. Measurements with the Kety-Schmidt technique an d PET-FDG were performed simultaneously. Global CBF was 47.1 +/- 8.0 m L.100 g(-1).min(-1), and CMRglc was 22.8 +/- 4.1 mu mol.100 g(-1).min( -1). No difference in CMRFDG was found with the two methods (17.8 +/- 1.6 and 18.2 +/- 1.3 mu mol.100 g(-1).min(-1), dynamic and single scan methods, respectively). Accordingly, the lumped constant ranged from 0.80 +/- 0.16 to 0.82 +/- 0.15, with a mean value of 0.81 +/- 0.15. Th e mean ratio between phosphorylation of FDG and glucose (k(3)/k(3)) w as 0.39 +/- 0.15. The discrepancy between the lumped constant determin ed in this study and previously obtained values can be explained partl y by methodologic problems, and we conclude that most of the discrepan cy results from previous overestimation of global CBF.