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Activity measurement principles


A correct measurement of activity is fundamental in all nuclear medicine applications including radiation protection. All instruments for activity measurements should be calibrated against national or international standards (e.g. primary or secondary standards). The SI-unit for activity is 1 Bq defined as 1 disintegration per second of a radionuclide. In some places the older unit of curie (Ci) is still used (1Ci = 3.7*1010 Bq).

Important principles

Many detectors can be used to measure radioactivity. However, for precise and accurate measurements, the proper selection of an instrument suitable for the specific task is fundamental. Important factors to take into account are: the radionuclide being measured (energy and type of radiation), the sensitivity of the instrument, its geometric efficiency, background radiation and properties of the associated electronic equipment, such as any dead-time losses or environmental factors. Equipment must be operated only within the limits and conditions established in the technical specifications and subject to a regular QC-programme. For in-house calibration of instruments a set of suitable standard sources should be used.

In an ordinary nuclear medicine department there should be a radionuclide calibrator to be used to determine the activities administered to the patients. For measurements of biological samples a well-counter is required and in the case of iodine therapy a scintillation detector for uptake measurements should be available. Estimation of the activity in vivo should be done with the gamma cameras, SPECT scanners or PET scanners.

Introduction to references

The properties and principles of operation of a radionuclide calibrator as well as other instruments for activity measurements are found in the IAEA document and on the IAEA web-site Radiation Protection of the Patient. The UK National Physical Laboratory designed an e-Learning Course to support the effective use of radionuclide calibrators in a clinical setting. It delivers the right skills and strategies when operating them, and provides a methodological approach when implementing quality assurance tasks, and identifying sources of uncertainty.