Do phantom harmonization efforts translate into harmonized patient images?

Abstract

Scanners with different performance characteristics and reconstruction protocols can produce images that show large variations in uptake in small lesions. There have been a number of studies aimed at reducing this variability for clinical trials using phantom data to optimize the acquisition and/or reconstruction protocols. The underlying assumption is that the protocols that harmonize the phantom images will also result in reduced variability in patient images among scanners/sites. The goal of this study was to use patient data with embedded lesions of known uptake to test this assumption. The NEMA image quality (IQ) phantom with hot spheres (diam.: 10–37 mm) was scanned on two time-of-flight (TOF) PET scanners and reconstructed using a list-mode TOF ordered subsets expectation maximization (OSEM) algorithm. Patient FDG data were also acquired on these scanners and the data stored in list-mode format. List-mode events from spheres measured in air on these systems were embedded into the phantom and patient list-mode data to insert 5–6 spheres in the phantom background region and the patient liver and lung regions with a known (9.7:1) uptake with respect to the local uptake. The impact of applying post-reconstruction filtering (both resolution recovery and Gaussian smoothing) on the phantom and patient images was also studied to determine if changes measured with the phantom images using these post-filters would be the same as those measured with patient images. Contrast recovery coefficient (CRC) values measured from lesion embedding studies on the phantoms and patients agree with one another, although CRC max showed greater variability. Additionally, relative changes in CRC brought about by application of the reconstruction post-filters to the patient images were consistent with those observed for the phantom images. This study illustrates, therefore, that these methods may be used to harmonize patient studies by first optimizing (and harmonizing) the reconstruction approach with the NEMA IQ phantom.