Model analysis of transcapillary exchange: the role of cyclotron produced isotopes in the study of the capillary permeability of substances

The purpose of this paper is to review different models used in the interpretation of capillary permeability studies using tracer dilution methods, with special emphasis on the role of cyclotron-produced isotopes. The physical model is a simplified description of the biological system, and the mathematical formalism is a device by which the model and experimental data can be compared. If there is a correlation and the solution is unique and consistent, the model is considered acceptable. Capillaries--the end organs of the circulatory system--are physiologically important because they are the site for the exchange of materials (oxygen, glucose, waste products etc.). The positive and negative findings in radionuclide imaging studies are caused by changes in capillary permeability. The indicator diffusion methods have many advantages over the conventional compartmental analyses. The diffusion models take into account the normal histological structure of the tissue and it is possible to get information about both capillary and cellular physiology. Short-lived, cyclotron produced positron emitting radionuclides, carbon-11, nitrogen-13, oxygen-15 and fluorine-18, are suitable for the study of transcapillary exchange.