Activity standardisation of 18F and ionisation chamber calibration for nuclear medicine

Primary activity standardisations were performed on solutions of 18F using 4πβ–γ coincidence counting and liquid scintillation counting (LSC) according to the CIEMAT/NIST method. A β+-emission probability of 96.86% was used for both methods. The various standardised 18F solutions were measured in ionisation chambers of the Physikalisch-Technische Bundesanstalt (PTB) and compared by determining radionuclide calibration factors. Already in 2001 an 18F solution had been standardised at the PTB and compared with the results of nine national metrology institutes (NMIs), using the ISOCAL IV secondary radionuclide calibrators of the National Physical Laboratory (NPL) as transfer instruments and a 68Ge check source solution. These results were linked to the International Reference System (SIR) at the Bureau International des Poids et Mesures (BIPM) by aliquots of solutions sent by the Laboratoire National Henri Becquerel (BNM-LNHB) and the NPL. Further on, in 2005, PTB sent an aliquot of an 18F solution to the SIR for ionisation chamber measurements. A value of the equivalent activity was determined and included in the key comparison database (KCDB). The recent PTB value of the equivalent activity of the SIR is in good agreement with the key comparison reference value determined from five NMIs. These results confirm that the standardisation of 18F solutions can be achieved with the accuracy required for use in nuclear medicine and, in particular, for applications in positron emission tomography (PET).