Fluorine-18-fluorodeoxyglucose PET to determine regional cerebral glucose utilization: a re-examination.

ofthe various factors that go into the computation better; it also segregates the least reliable and error-prone model-dependent components into terms that can be identified and minimized. Regional cerebral glucose utilization is calcu lated as: IT C@†• (T) †" K'I' I C@(t) e@ +k@)(Tt) dt JO (IfTC(t) fTC@(t) e@ +k@)(Tt) dt)-@-dt-i; 0 @ 0 Eq.! where C@' represents total concentration of ‘4C in a homoge neous tissue in brain; C and C@are the [‘4C]deoxyglucose and glucose concentrations in arterial plasma, respectively; T is the duration ofthe experimental period following the pulse of tracer and t is the variable time; K'@' and k@are the rate constants for carrier-mediated transport of [‘4C]DG from plasma to tissue and back again, respectively; k@ is the rate constant for phosphorylation of [‘4C]deoxyglucose by hexokinase; A equals the ratio ofthe distribution spaces of deoxyglucose and glucose in the tissue; C1 equals the fraction of glucose which, once phosphorylated, continues down the glycolytic pathway; Vm and K,@, represent the maximal velocity and Michaelis-Menten constant of hexokinase for glucose and V@ and K@,are the equivalent kinetic constants of hexokinase for 2-deoxyglucose. The numerator of the equation equals the total tissue concen tration oflabeled products of [‘4C]DG phosphorylation formed in the tissue during the experimental period; this is determined as the difference between total radioactivity measured by quantitative autoradiography (Cr) less a term that represents the estimated free, unmetabolized [‘4C]DGstill remaining in the tissue at the end of the experimental period. In order to minimize the free [‘4C]DG in the tissueand also the effectsof errors in its estimation on the value of the numerator, a long period of tracer circulation after a pulse of [‘4C]DGwas adopted;the total tissue‘4C concentrationmeasuredautoradio graphically then represents mainly labeled products of['4C]DG phosphorylation. In the denominator of the operational equa tion, the integrated specific activity (ratio of['4C]DG to glucose concentrations) in the tissue precursor pool is computed by subtracting from the integrated specific activity measured in the plasma, a term that corrects for the lag in equilibration of the tissue precursor pool behind the plasma. The magnitude and, therefore, effect ofthis correction term is also minimized by the use of the long period of tracer circulation after the pulse. Because the total ‘4C concentration in the tissue and the integrated plasma specific activity are increasing while the The[18F}fluorodeoxyglucose ([‘8F]FDG) method for quanti tative measurement of regional cerebral glucose utilization (rCMRglc) was first developed …