The invention relates to a general composite compartmental model and a compartmental analysis procedure to extract non-invasively the concentration (Cp) of the imaging agent in plasma (301), in metabolites (304, 504) and in blood elements (303) (like red cells, platelets, plasma protein etc.) from time signal curves measured within a reference tissue region (200). This is made possible by deploying an injection function (SINJ(t)) as input which models the amount of imaging agent administered to the patient as a function of time. The invention allows the presentation of the plasma input function to the medical practitioner without the need for invasively drawing blood samples.