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TEMPORAL SAMPLING EFFECTS ON PARAMETER ESTIMATION ACCURACY IN KINETIC POSITRON COMPUTED TOMOGRAPHIC (PCT) STUDIES

Kinetic studies with PCT produce parameter estimates of physiologic models (e.g., glucose metabolism with F-18 FDG and blood brain barrier diffusion with Ga-68 EDTA). We and others have previously shown that blood volume effects can not only affect these estimates but can also be included in the compartmental models as an adjustable parameter. This permits estimation of regional blood volume and increases the accuracy of the other parameter estimates. Because plasma or blood tracer concentrations change rapidly after a bolus injection, we investigated the effect of varying scan intervals on the parameter estimates. Using FDG kinetics in the brain as a model, we simulated initial scan intervals ranging from 12 seconds to 3 minutes on tissue time activitly curves generated from rate constants typical of normal gray matter (including noise) with a CBV of 6cc/100gm. Arterial plasma FDG measurements from an actual patient were used in the simulation. FDG rate constants (k1*, k2*, k3* and k4*) and a fifth parameter representing CBV were iteratively generated for each scan sequence. The estimated CBV was nearly constant (5.7–6.2cc/100gm) over the range of scan intervals tested although the fractional standard error of the CBV estimate increased with longer scan times. This error ranged from about 1% with 12 second scans to about 3% with 3 minute scans. A similar stability in the estimates of k1*, k2*, k3* and k4* was also found. This indicates that initial scan intervals of 1–2 minutes as typically used with NeuroECAT kinetic PCT studies permits accurate estimations of parameters including those with a relatively rapidly changing component such as blood volume in bolus injection studies.