Quantifying capillary transit time distribution: a novel tracer kinetic model accounting for leakiness, back diffusion, and perfusion in MRI and PET imaging—the outflow model

BACKGROUND: F]fluoroglucose. The temporal resolution was optimized (1-2 s) to capture the bolus passages. Tissue curves were derived from the putamen, thalamus, frontal white matter, and regions of malignancy for comparative analysis. For the PET data the Outflow model was compared with a conventional 3-compartment model for validation. RESULTS: F]fluoroglucose obtain from the basal ganglia, capillary transit time heterogeneity was around 2 s, extraction fraction 40.2%, and cerebral metabolic rate of glucose consumption was 44 µmol/100 ml/min. Both the outflow model and the 3-compartment model gave excellent fit to data and for the comparable metrics the two models gave results in reasonable agreement. CONCLUSION: This proof-of-concept study demonstrated that calculating capillary transit time heterogeneity in the brain via the proposed new tracer kinetic model is feasible for both MRI and PET data. The model adeptly addresses scenarios where tracers or contrast agents undergo bidirectional transport across the BBB.