Results of a nationwide phantom based PET-Survey in Switzerland before harmonization

Aim: To gain an overview about the variability of PET measurements with diverse PET/CT-systems used in Switzerland, and to provide an inventory on acquisition protocol adherence in a multi-centre setting. Materials and methods: A total of 28 PET/CT sites in Switzerland was invited to participate in a PET phantom survey. Based on obligatory semiannual phantom measurements as prescribed by the Federal Office of Public Health in Switzerland, a detailed study protocol defined PET acquisitions of two different durations with a homogeneous cylinder phantom (activity concentration (AC) 10 kBq/ml) and a fillable “hot spheres” (18F) phantom (3.5 kBq background AC; sphere to background ratio 5:1). At least 7 image reconstructions for each acquisition (28 per study site) were requested. Reconstructions included sets of standard (4mm, isotropic) and high resolution (2mm, isotropic) reconstructions, using filtered back-projection (FBP), ordered subset expectation maximization (OSEM) and vendor specific point spread function (PSF) based reconstructions. Data of all sites underwent automatic quality control (QC) utilizing an in-house developed multi-paradigm software and were analyzed with regard to comparability of measured AC. Results: Phantom data was provided by 14 of 28 PET sites (5/5 university hospitals, 7/13 cantonal hospitals, 2/9 private hospitals). From 18 PET devices, 459/504 expected datasets (92%) were received and underwent QC. Of these, 6 datasets were not readable, and 31 datasets were not accepted (e.g. no attenuation correction), resulting in 422 evaluated datasets. 257/422 (61%) datasets were of restricted use for quantification because of protocol violations (e.g. strongly anisotropic voxels, filter parameters or AC out of proposed range, faulty exposure and axial or radial offset of phantom spheres), and/or actual background AC was not provided in 160/422 datasets (38 %). We found high variations in prepared actual or presupposed phantom AC and in recovery of AC, latter depending on reconstruction methods and reconstruction parameters (e.g. isotropy of voxels). Device-specific acquisition parameters - in terms of exposure per reconstructed voxel size - had a major influence on inter-site comparability of quantitative PET measurements. Underexposed PET acquisitions lead to faulty quantification of AC. Conclusions: As expected from comparable studies, the nationwide Swiss phantom survey also found considerable variation in PET AC quantification. In the context of multi-centre trials, protocol compliance may not be assumed. Thus, a throughout QC of all datasets appears to be mandatory to assure data quality and comparability. In addition, quantitative PET data may only be used after check of adequate image exposure.