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Radioactivity calibrators

Introduction

The levels of activity in radiopharmaceuticals that are administered clinically are governed primarily by the need to balance the effectiveness and the safety of the medical procedure by choosing the minimum radiation dose delivered to the patient needed to achieve the required objective (e.g. diagnostic image quality or therapeutic outcome). Thus, accurate and reproducible measurement of radioactivity in nuclear medicine applications is vital to ensure these objectives. The need to maintain a high degree of confidence in those measurements requires a regular quality control of the instrument carried out in accordance with national and international standards. The most commonly used equipment for activity determination is a well-type gas ionisation chamber, where the activity is measured in terms of the ionisation current. The chamber is sealed under pressure and the gas is usually a noble gas with a high atomic number in order to increase the probability of photon interaction.

Important principles

The calibration of the instrument is generally made at the place of manufacture using standard sources with activities that are traceable to a standard laboratory. A certificate should accompany the instrument. The objective of a regular quality control programme is to maintain the original function of the instrument, and assure its correct use. The quality control programme generally consists of daily, weekly, monthly and yearly checks and should include: test of precision and accuracy, test of linearity of activity response, test of reproducibility and check of background. The sources needed to perform the tests should include a sealed reference source certified to less than +/-5% overall uncertainty. Examples of suitable sources are Co-57, Ba-133, Cs-137 and Co-60.

Introduction to references

The IAEA document on Quality Assurance of Radioactivity Measurement in Nuclear Medicine covers all aspects of a QA-programme. More specific recommendations regarding a quality control programme for the radionuclide calibrator is found in IAEA-TECDOC-602 and also on the IAEA Radiation Protection of the Patient website. A general overview of quality control of nuclear medicine instrumentation is provided in the article by Zanzonico. The European Association of Nuclear medicine has published general recommendations on routine testing of equipment including the radionuclide calibrator. The American National Standards Institute (ANSI) has published a document on calibration and usage of the dose calibrator. International recommendations covering different aspects of the instrument are found in the publications from the International Electrotechnical Commission (IEC).