Studies of skeletal tracer kinetics. I. Digital-computer solution of a five-compartment model of [18F] fluoride kinetics in humans.

We have developed a new model of short-term fluoride kinetics in humans and have solved the model on a digital computer using the SAAM-25 program. The solution accords well with available data. About 60% of intravenously administered [18F] fluoride is taken up by bone. Evaluation of the rate constants of tracer egress from blood indicates that about 17% of the cardiac output is distributed to the skeleton. When the model was perturbed to simulate changes in systemic or skeletal blood flow, we found that the system behaves in a nonlinear manner; even a five-fold increase in systemic or skeletal blood flow did not appreciably increase the amount of fluoride taken up by bone 1--2 hr later, the time when scans are usually made. A simulated increase in bone extraction rate, however, had a marked effect on bone-fluoride uptake. These findings suggest an important homeostatic role for bone in the regulation of blood calcium concentration and have considerable bearing on the interpretation of bone scans.