[Bone-seeking radioactive substances in nuclear medicine]

The concept of bone affinity of a radioactive tracer is developed on theoretical grounds and is discussed on the basis of the various substances used in nuclear medical diagnosis of bone disease. On the basis of results on the uptake of very short lived nuclides and extremely large molecules, evidence is provided that the incorporation of the tracer in the apatite crystal is not a primary criterion of bone affinity since incorporation cannot take place on timelimiting grounds in the former case and on spatial grounds in the latter. The fixation on bone is therefore more likely the result of non-specific adsorption processes. The utility of a radioactive substance in practical application in nuclear medicine depends on the radioactive characteristics of the nuclide and on its behavior in the organism. In this context the quality of the scintigram is particularly dependent on the mode and rate of elimination of that fraction of the tracer that is not bound by the skeleton. The various mechanism which cause differences in the uptake of tracer by healthy and pathological bone tissue are discussed with special regard to the role of blood flow.