Metabolic brain imaging. Direct regional measurement of transfer coefficients and lumped constant

In the deoxyglucose method of measuring brain glucose utilization, the Michaelis-Menten formalism implies 1) symmetrical transport across the blood brain barrier, 2) identical ratios between forward and backward transport across the blood brain barrier for substrates of the same carrier, and 3) constant transport (tau) and phosphorylation (phi) ratios between native substrate and tracer. Incorporation of these assumptions into the fluoro-deoxyglucose method in humans allowed us to replace the brain blood transfer (k2*) and phosphorylation (k3*) coefficients by the coefficients of net (K*) and undirectional (K1*) transfer in the operational equation of the method, and to calculate the lumped constant directly. The reduction of the number of essential parameters amounts to a constraint of the freedom of the transfer coefficients to assume any value. On this basis, we were able to improve the estimates of the transfer coefficients. Seven healthy, older subjects were studied by this method. The values of K1*, k2* and k3* changed in parallel to K*. The values of the regionally calculated lumped constant ranged from 0.48 to 0.73 with a mean of 0.58.