In broad terms all diagnostic nuclear medicine methods are tracer studies. However, the term is generally used for those methods not utilizing imaging equipment but instead based on measurement of whole body retention or collection and measurement of biological samples such as plasma, urine etc.. Tracer studies are also an important tool in biomedical research, where tracer studies can be used to develop kinetic models, which in turn provide quantitative analysis.
The principle of a tracer study is that it allows the analysis of a substance and its interactions in the body through the labeling of the substance with a radionuclide in a manner that does not alter the substances original properties. The main steps in a tracer study include selection and administration of the radiotracer, sampling, sample preparation and measurement. In diagnostic applications the tracer substance (radiopharmaceutical) is selected by its appropriateness for the study being applied, and the procedures for performing the study should be in accordance with the recommendations from professional organizations (international or national). Samples (e.g. blood, urine, …) are collected and prepared in accordance with the study objectives. The selection of measurement method and instrument should be based on the properties of the radioactive sample such as the radionuclide, expected activity, sample volume etc. Sample preparation may include careful dilution and measurement of volume and/or weight. The result of the measurement is generally expressed in terms of count-rate, which many times has to be corrected for background registrations, dead-time losses, decay of the radionuclide and the sensitivity of the instrument in order to calculate absolute activity or relative to a standard. A regular programme for quality control of the instruments should be established.
The most commonly used equipment for sample measurements include the gamma-counter and the liquid scintillation counter. Also external probes (e.g. for thyroid uptake measurement, surgical probes for detection of sentinel node), the gamma-camera (bile malabsorption) and the PET-camera (biomedical research in general) are used.
Introduction to references
The web-book Basic Physics of Nuclear Medicine includes some basic information on compartment modeling. Details on the instrumentation used for measurements as well as the content of a quality control programme are presented in an IAEA document: Quality control of nuclear medicine instruments.