Which method is used to measure radionuclide distribution inside a living body, such as whole-body or organ counting?

• A. In vivo counting
• B. In vitro analysis of urine
• C. Air monitoring
• D. Soil sampling

Answer: (A)

Measuring radionuclide distribution inside a living body is done with in vivo counting. This technique uses detectors placed outside the body to detect radiation emitted by radionuclides inside tissues or organs, providing information on how the radioactive material is distributed and the overall body burden. Whole-body counters or organ-specific counters (like thyroid or chest detectors) are calibrated to estimate activity in different regions and to assess internal dose. In vivo counting is noninvasive and reflects true biokinetic distribution, unlike analyzing urine samples, which is an in vitro analysis of excreted material and shows elimination rather than internal distribution. Environmental methods like air monitoring or soil sampling measure external contamination and exposure, not the radionuclide distribution inside the body.