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We can monitor individuals who work with certain beta emitters with a urine bioassay if there is reason to believe we need to look for internal contamination. A urine sample measured on a liquid scintillation counter can indicate whether the worker has taken some radioactive material internally. Nearly 100 percent of some beta emitters are excreted in the urine; for others it may be just a few percent. The excretion of the radionuclide into the urine is dependent upon the chemical form of the radionuclide, how it is metabolized, and the half-life of the radionuclide. Internal contamination with some gamma emitters can also be measured in this manner; however, the next two paragraphs discuss other techniques more commonly used for gamma emitters.


Thyroid counting is performed for determination of internal radioactive iodine uptake. This is a noninvasive test. Radioactive iodine, when taken into the body, seeks out the thyroid gland and deposits there. A thyroid count generally consists of the placement of a scintillation detector in front of the thyroid gland (the instrument is outside the body at the neck level) for one or more minutes with the resultant count indicating the presence or absence of radioactive iodine.


Whole-body counting is performed to detect the presence of medium- to high-energy gamma-emitting radionuclides within the body. This is noninvasive. Whole-body counting can also be used to determine the distribution of a gamma emitter throughout the body. A whole-body counter may be a chair, bed, or standing type. In nearly all cases, individuals being counted have a series of detectors near them (usually scintillation detectors) that will detect the gamma photons being emitted from the radionuclide source in their body.