THE ASSESSMENT OF THE NONEQUILIBRIUM EFFECT IN THE PATLAK ANALYSIS OFFDOPA PET STUDIES

'Patlak analysis' is a common approach used in Fdopa PET studies to ca lculate the uptake constant (K-i) of the tracer. It is assumed in the Patlak analysis that the reversible compartment of tissue radioactivit y used is in effective equilibrium with the tracer in plasma. Therefor e, using the data prior to equilibrium is in conflict with the assumpt ion, and its effect on the estimate thus needs to be examined. In this study, we used simulations to investigate the errors due to the viola tion of the equilibrium assumption. Two factors affecting the estimate of K-i were examined-the eigenvalue of the model response function an d the shape of the input function. The K-i estimate obtained from the Patlak analysis was found to be markedly biased because the system is not in equilibrium during the first 2 h post Fdopa injection. The magn itude of the bias was found to be dependent on the time interval used in the analysis (10% difference when comparing results from intervals 30-120 min and 60-120 min) and on the eigenvalue of the response funct ion (10% change in K-i when the eigenvalue was changed by 20% around t he nominal value). The estimates are also affected by the intersubject variations in the plasma time-activity curves. Therefore, Patlak anal ysis users should interpret their results with caution, particularly w hen examining small intersubject differences and small changes due to physiological or pharmacological interventions.