Background Positron emission tomography (PET) uses radiopharmaceutical labelling with b⁺ emitting isotopes. ¹⁸F is the most commonly used radioisotope in PET and is produced by Medical Cyclotron. During bombardment of target with [¹⁸O]water to produce the radiopharmaceutical ¹⁸F-metil-choline, radionuclidic impurities are generated. For the European Pharmacopoeia, these impurities have to be checked before application for human use.

Purpose In this work, we set up accurate geometry for measurements with the HpGe spectrometer to assess radionuclidic impurities generated during the production of ¹⁸F-metil-choline.

Material and methods High resolution gamma spectrometry is the most appropriate method to determine gamma emitting radionuclides, but it needs the correct geometry for measurement. Samples from the different steps of the production process were collected: [¹⁸O] irradiated water, waste target water, Cromafix cartridge, waste Cromafix water, WCX cartridge, final waste water and ¹⁸F-FMeCh. Counting of samples was carried out after an appropriate period to allow for complete decay of ¹⁸F. Liquid samples were analysed by volumetrically diluting an appropriate quantity of each solution (2 mL) with distilled water to a volume of 15 mL. The cartridges Cromafix and WCX were measured by placing the samples directly over the detector, through a support. Counting efficiency was established using a certificated standard Amersham, containing ²⁴¹Am, ¹³³ Ba and ¹⁵² Eu (beaker Bertocchi 100 mL). We used Gespecor software to transfer the efficiency calibration from the geometry of standard to the geometry of the samples and the analysis was performed using the GammaVision analysis software.

Results The data showed the presence of gamma emitting ⁵¹Cr, ⁵²Mn,⁵⁴Mn, ⁵⁶Co, ⁵⁷Co, ⁵⁸Co, ^95mTc, ⁹⁶Tc, ¹⁰⁹Cd, ¹⁸⁴Re and ¹⁸⁶Re in the [¹⁸O] irradiated water. In the final ¹⁸F-FMeCh solution, the activity of the impurities was lower than the minimum detectable activity of the spectrometer.

Conclusion The software Gespecor has enabled us to determine radionuclide impurity with a single calibration source and to confirm the radiochemical purity of ¹⁸F-metil-choline. Contaminants were identified in all stage of the synthesis process but they were absent in the final product. The purification methods adopted are effective as requested by the patient’s radiation protection standards and European Pharmacopoeia.

References and/or Acknowledgements Technical staff at Cyclotron

No conflict of interest.