Development and characterization of a compact gamma-ray spectrometry system for quality control of PET radiopharmaceuticals

Introduction: In this work we characterized and validated a very compact, USB powered, CdZnTe (CZT) detector for gamma-ray spectrometry applications in Radiopharmacy. This type of detector operates at room temperature, i.e., without need for expensive and cumbersome cooling equipment. The detector was calibrated in energy and efficiency for different geometries and all the acquisition chain was developed for application to quality control of positron emission tomography (PET) radiopharmaceuticals. Materials and Methods: a GR1 model (by KromekTM , Sedgefield, UK) a 1 cm3 CdZnTe crystal with all the electronics enclosed in a 25x25x63mm containment, was utilized. The detector was placed in a 5 cm thick lead shielding; a plastic sample holder was used to secure a reproducible acquisition geometry. Acquisitions were recorded using the software driver KSpect 1.1.6. Energy and efficiency calibrations were performed using certified standard sources in three different geometries: point source, 1 cm3 tube and 5 cm3 vial, all at 6 cm distance. Samples of [18F]FDG and [68Ga]-DOTANOC were measured to assess radionuclidic purity and results were compared with those of an HPGe detector ( relative efficiency 15% , resolution 1.8 keV @ 1332 keV). Results and Conclusions: The energy resolution obtained for the Kromek GR1 was 1.5% (10.3 keV) at 662 keV and to 1.1% (14.5 keV) at 1332 keV, while the efficiency response at the same energies was 7E-3 % and of 2E-3 %, respectively, in 1 cm3 tube geometry. The energy resolution obtained is fully adequate for QC of PET radiopharmaceuticals, even though, at energies lower than 100 keV, peak resolution is suboptimal. Analysis of [18F]FDG and [68Ga]-DOTANOC samples proved to be comparable to those obtained using a HPGe detector (max activity deviation 2%). The order of magnitude of the minimum detectable activity (MDA) at 511 keV was of 2 Bq for annihilation photons of 18F and 68Ga. For impurities like 57Co, 60Co, 52Mn and 54Mn the MDA varies from 40 Bq to 120 Bq, whereas for 51Cr it is about 3500 Bq. In conclusion, the performance of small sized CZT detectors seems promising; its reduced dimensions and the energy resolution intermediate between those of NaI(TI) and HPGe detectors, make this type of devices an extremely attractive tool for QC applications in Radiopharmacy.