Abstract
PurposeTo assess the applicability of the Fluorine-18 performance specifications defined by EANM Research Ltd (EARL), in Gallium-68 multi-centre PET-CT trials using conventional (ordered subset expectation maximisation, OSEM) and advanced iterative reconstructions which include the systems’ point spread function (PSF) and a Bayesian penalised likelihood algorithm (BPL) commercially known as Q.CLEAR. The possibility of standardising the two advanced reconstruction methods was examined.MethodsThe NEMA image quality phantom was filled with Gallium-68 and scanned on a GE PET-CT system. PSF and BPL with varying post-reconstruction Gaussian filter width (2–6.4 mm) and penalisation factor (200–1200), respectively, were applied. The average peak-to-valley ratio from six profiles across each sphere was estimated to inspect any edge artefacts. Image noise was assessed using background variability and image roughness. Six GE and Siemens PET-CT scanners provided Gallium-68 images of the NEMA phantom using both conventional and advanced reconstructions from which the maximum, mean and peak recoveries were drawn. Fourteen patients underwent 68Ga-PSMA PET-CT imaging. BPL (200-1200) reconstructions of the data were compared against PSF smoothed with a 6.4-mm Gaussian filter.ResultsA Gaussian filter width of approximately 6 mm for PSF and a penalisation factor of 800 for BPL were needed to suppress the edge artefacts. In addition, those reconstructions provided the closest agreement between the two advanced iterative reconstructions and low noise levels with the background variability and the image roughness being lower than 7.5% and 11.5%, respectively. The recoveries for all methods generally performed at the lower limits of the EARL specifications, especially for the 13- and 10-mm spheres for which up to 27% (conventional) and 41% (advanced reconstructions) lower limits are suggested. The lesion standardised uptake values from the clinical data were significantly different between BPL and PSF smoothed with a Gaussian filter of 6.4 mm wide for all penalisation factors except for 800 and 1000.ConclusionIt is possible to standardise the advanced reconstruction methods with the reconstruction parameters being also sufficient for minimising the edge artefacts and noise in the images. For both conventional and advanced reconstructions, Gallium-68 specific recovery coefficient limits were required, especially for the smallest phantom spheres.
Highlights
Standardisation is a key aspect when conducting PET multi-centre clinical trials in order to minimise the sources of variation between participating sites
A Gaussian filter width of approximately 6 mm for point spread function (PSF) and a penalisation factor of 800 for Bayesian penalised likelihood algorithm (BPL) were needed to suppress the edge artefacts. Those reconstructions provided the closest agreement between the two advanced iterative reconstructions and low noise levels with the background variability and the image roughness being lower than 7.5% and 11.5%, respectively
The lesion standardised uptake values from the clinical data were significantly different between BPL and PSF smoothed with a Gaussian filter of 6.4 mm wide for all penalisation factors except for 800 and 1000
Summary
Standardisation is a key aspect when conducting PET multi-centre clinical trials in order to minimise the sources of variation between participating sites. It improves the robustness of the performed analyses, especially when quantification is of importance. The importance of standardisation has been acknowledged by the scientific community especially in trials using 18F-fluoro-2-deoxy-D-glucose (FDG) This is mainly due to the radiotracer’s well-established role in oncology and its availability in most PET centres, making it logistically easier to conduct multi-centre trials. Huizing et al reported greater variation between sites for Gallium-68 and an 11% decrease in recovery coefficient compared to Fluorine-18 when scanning the NEMA Image Quality phantom as per the EARL guidelines [11]. Soderlund et al reported an inverse relationship between recovery and positron range when comparing phantom acquisitions reconstructed with the same parameters, indicating the need for tracer specific limits [13]
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