Objective:: The study performs a comprehensive analysis of image metrics to objectively support the reduction of injected activity in pediatric oncology 18F-FDG PET/MR (18F-fludeoxyglucose PET/MR) examinations. Contrast-to-Noise Ratio (CNR), Normalized Noise (NN), tumor burden, and standardized uptake value (SUV) parameters stability were investigated to robustly define the acceptable reduced activity level that preserves the clinical utility of images, considering different PET applications.
Methods:: 21 PET/MRI examinations performed on a 3-Tesla Biograph mMR scanner were analyzed. Tracer activity reduction was stimulated by decreasing the count statistics of the original list-mode data (3 MBq kg-1). In addition to the already studied SUV metrics and subjective scoring on lesion detectability, a thorough analysis of CNR, NN, Metabolic Tumor Volume (MTV), and Total Lesion Glycolysis (TLG) was performed.
Results:: SUVmax and SUVmean increased more than 5% only in 0.6 MBq kg-1 reconstructed images (+10% and +9%, respectively), while SUVpeak was almost unaffected (average variations < 2%). The quantified CNR, NN, MTV, and TLG behavior with the decrease of the injected activity clearly defines 1.5 MBq kg-1 as a threshold of activity after which the quality of the image degrades. Subjective and objective analyses yielded consistent results. All 56 lesions were detected until activity of 1.2 MBq kg-1, whereas five lesions were missed on the 0.6 MBq kg-1 image. Perceived image quality (IQ) decreased in Lower Tracer Activity (LTA) images but remained acceptable until 1.5 MBq kg-1.
Conclusion:: Results about the stability of image metrics beyond the semi-quantitative SUV parameters and subjective analysis, rigorously proves the feasibility of the reduction of injected activity to 1.5 MBqkg-1 for pediatric patients aged between 7 and 17 years.
Advances in knowledge:: This is the first report on the quantitative evaluation of the effect of activity reduction on image quality in pediatric PET/MR. The findings offer objective corroboration to the feasibility of a significant dose reduction without consequences on clinical image reading and tumor burden metrics.