The glucose metabolic rate of the human brain can be measured with labeled deoxyglucose, using positron emission tomography, provided certain condi tions are fulfilled. The original method assumed irrevers ible trapping of deoxyglucose metabolites in brain during the experimental period, and it further requires that a conversion factor between deoxyglucose and glucose, the "lumped constant," be known for the brain regions of interest. We examined the assumption of irreversible trapping of

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... deoxyglucose metabolites in brain of four patients in 365 normal and 4 recently infarcted re gions. The average net, steady-state rate of fluorode oxyglucose (KD) accumulation in normal regions of the four patients was 0.025 ml g-I min -I . We also examined the variability of the lumped constant. We first confirmed that methylglucose is not phosphorylated in the human brain. We then estimated the lumped constant from the regional distribution of labeled methylglucose in brain. The average (virtual) volume of distribution of labeled methylglucose in the normal regions was 0.46 ml g-I and was the same in both gray and white matter structures. The average brain glucose content corresponding to this value was 1.3 f-lmol g-I , assuming a Michaelis constant (Kt) of 3.7 mM for glucose transport across the blood brain barrier. The lumped constant varied insignificantly between 0.4 and 0.5 in most regions, with an overall av-