Gradient effects in extravascular water determination using 15O-labelled water under steady state conditions: theory and error sensitivity.

Steady state tracer measurements with short-lived isotopes allow the determination of dynamic and static physiological functions and parameters. Both types of measurements are affected by the gradient effects which result from the decay of the tracer through the volume observed. Using 15O-labelled compounds and positron emission tomography (PET), dynamic functions like flow are quite sensitive to variations in extraction fraction and distribution coefficient. In the operational equation for the determination of distribution volumes the gradient effect can be reduced to correction terms which are governed by the mean transit time of the tracer through the observed volume. Because mean transit times can be measured independently they allow a reasonable correction of gradient effects in distribution volume determinations, even if deviations of extraction fraction and distribution coefficient from unity must be considered.