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Metrological Infrastructure for Activity Measurements of the Radionuclide 64Cu which is Important for Nuclear Medicine

29.05.2009

Activity determinations of radionuclides used in nuclear medicine rank among the important tasks of PTB in the field of radioactivity. For some of the mostly ephemeral radionuclides, PTB annually provides activity standards. It is, however, also possible to submit sources whose activity is then determined at PTB.

For quality assurance and to guarantee international acceptance of the PTB calibrations, comparison measurements with other national metrology institutes are of great importance. For a great number of radionuclides, especially if these have been used in nuclear medicine for a short time only or if their use is planned only for the future, a sufficient metrological infrastructure is often lacking, i.e. activity measuring procedures have not been established, there are no comparison measurements, and important nuclide data are often not known with sufficient accuracy and are frequently based on older work.

To improve these circumstances, PTB has decided to develop primary standard procedures for the activity determination of radionuclides for which an urgent need has been determined. As one of the first isotopes, 64Cu was chosen which is - due to a positron branch - suitable for being used in positron emissions tomography (PET) and represents an important complement to 67Cu in radio-immune therapy. This work is performed within the scope of the EURAMET project 1085.

The activity was determined at PTB with the aid of liquid scintillation counting and 4πβ-γ coincidence counting. The activity determined in this way was used to determine calibration factors for ionization chambers (in nuclear medicine often referred to as "activimeters"). These factors can later also be used to perform secondary standard measurements at PTB with little effort; it was, however, also possible to determine calibration factors for commercial ionization chambers which are used in practical nuclear-medical applications. It was also possible to use the ionization chamber measurements to determine the half-life (12.704(5) h). In addition, the measurements were supplemented by gamma-spectrometric investigations which allowed photon-emitting impurities to be determined or ruled out and the photon emission probabilities of the 511 keV annihilation quanta and the 1346 keV gamma radiation to be determined. Details regarding the measurement procedures and the results will soon be presented to the public [1].

Furthermore, PTB has - as the first national metrology institute - sent an ampoule with a 64Cu solution of known activity to the Bureau International des Poids et Mesures (BIPM) in Paris to face up to a comparison within the scope of the International Reference System (SIR). The activity value determined with a relative standard measurement uncertainty of only 0.47 % is in good agreement with a value calculated by the BIPM. Other national institutes have announced that they, too, will send in 64Cu solutions to the SIR so that a direct comparison and the determination of a key comparison reference value (KCRV) will probably be possible already at the end of 2009.

Literature

  1. C. Wanke, K. Kossert, O. J. Nähle, O. Ott:
    Activity Standardization and Decay Data of 64Cu.
    International Conference on Radionuclide Metrology and its Applications (ICRM), Bratislava, Slovakia, September 2009, in preparation.