I. Some General Problems in Connection with the Measurement of Radioactivity in Patients

Isotopes, both radioactive and stable, have proved to be very valuable research tools when used as tracers in the study of many of the dynamic processes, both physical and chemical, which occur in the living organism. In many cases, where a particular disease is associated with a characteristic disturbance in one of these processes, the isotope technique could provide a useful diagnostic method were it not necessary to take serial samples of a particular tissue, or samples from various tissues, for analysis. However, in the case of the radioactive isotopes, it is often possible to avoid this difficulty by using what have now come to be known as in vivo counting techniques. The present discussion is confined to what is probably the most generally useful type of measure-ment in this category, namely, the detection and measurement of a radioactive isotope within the patient's body by means of externally placed instruments which are sensitive to γ radiation. Although such measurements account for only a small proportion of the reported applications of radioactive isotopes to date, sufficient work has been done to indicate their great potential value for diagnostic purposes. In fact, they have been applied to such diverse medical studies that it would appear that each application presents unique experimental problems.