Measurement of hepatic blood perfusion and of the exchange of substances between blood and
cells is a challenge. Long before the development of PET, multiple indicator data were analysed using
models based on elaborate capillary theories. Today, PET is a unique modality that allows external
quantitative measurements of the regional distribution of intravenously injected radiotracers and their
metabolites in tissues, but dynamic PET data are analysed using less physiologically based schemes. The
standard compartment model is an inlet equilibration model that does not naturally incorporate blood flow,
and a single-uptake model that does not allow substances to re-enter the capillaries. These deficiencies lead
to paradoxes when modelling fast blood-cell exchange. We have combined compartmental and capillary
theory and developed microvascular models that account for blood flow and concentration gradients in
capillaries. The microvascular models can be regarded as revisions of the input function which include more
physiological realism and provide a superior description and interpretation of dynamic PET data when
compared to the standard compartmental scheme.
Keywords: PET kinetics; Blood flow; Capillaries, Microcirculation.