ABSTRACT
Colloidal particles in the form of microspheres, emulsions, suspensions, and liposomes can be labeled with gamma-emitting radionuclides and thereby find use as radi-odiagnostic agents which are administered by a variety of routes. Colloidal particles can be used to study the lung either by intravenous administration or by nebulization. The simplest way of localizing a radiodiagnostic colloid is to provide retention of the agent in a tissue or body fluid space for a significant period of time. Thus, administration of a material in the gastrointestinal tract, the blood circulation, or the lymphatic system comes into this category. The blood capillaries range from about 7 to 10 µm in diameter, so that the injection of particles greater than this size will result in their being trapped in the first capillary bed that they meet. Endocytosis is the term used to describe the uptake into cells of extracellular material and it can be subdivided into two separate processes, phagocytosis and pinocytosis.
