Experimental assessment of PET/CT repeatability

Purpose: In longitudinal oncological and brain PET/CT studies it is important to understand the repeatability of quantitative PET metrics in order to assess change in tracer uptake or binding. The present studies were performed in order to assess the aspects of PET/CT repeatability related to reconstruction settings, analysis methods, scan duration (or image noise) and position in the field of view. Procedures: Multiple (repeated) scans have been performed using a uniform Ge phantom, a NEMA image quality (IQ) phantom and a 3D Hoffman brain phantom filled with F solutions. Studies were performed with and without repositioning the phantom and all (12 replicate for IQ phantom and 10 replicate for Hoffman brain phantom) scans were performed using equal count statistics. For the Ge phantom, the coefficient of variation (COV%) and variance of the voxel values across the phantom were studied as function of scan statistics and reconstruction settings. For the NEMA IQ phantom we studied the maximum, peak and mean recovery coefficients (RC) in each sphere as function of experimental conditions (noise level, reconstruction settings and phantom repositioning). For the 3D Hoffman phantom the mean activity concentration was determined within several volumes of interest and activity recovery and its precision was studied as function of experimental conditions. Results: For all phantom studies voxel noise (expressed by variance and COV) and SUVmax or SUVmean repeatability depended on reconstruction settings and frame duration, as expected. When exploring SUVmax, SUVpeak or SUVmean of the spheres in the NEMA IQ phantom, it was observed that repeatability depended on phantom position with SUVmax being most and SUVpeak least sensitive to phantom repositioning. Moreover, the impact of phantom positioning on SUV metrics depended on sphere size. For the brain phantom, regional average SUVs were derived and these were only minimally affected by phantom repositioning. Conclusion: The repeatability of quantitative PET metrics depends on the combination of reconstruction settings, data analysis methods and scan duration (scan statistics). Moreover, repeatability was also affected by phantom repositioning but its impact depended largely on the data analysis method being used. The study suggest that for oncological PET studies use of SUVpeak may be preferred over SUVmax because SUVpeak metric is likely less sensitive to patient positioning/tumor sampling effects.