Multi-centre assessment of HRRT image uniformity via 68Ge and 18F cylindrical and anthropomorphic phantoms

To date, the high resolution research tomograph (HRRT) still offers the highest resolution PET imaging capability of the human brain. Spatial uniformity of the images is of paramount importance, as many radioligand studies require either accurate regional dynamic quantification or, at the very least, accurate measurement of relative activities between regions. Often uniform cylinders, of ⁶⁸Ge or ¹⁸F, are used for quality control and calibration purposes. This work examines the spatial uniformity obtained in reconstructed images of uniform cylinders from seven different HRRT centres, using either ⁶⁸Ge or ¹⁸F, and also considering a head and brain phantom filled with ¹⁸F scanned at three of the centres. To account for varying activity levels used in each centre´s data acquisition, and possibly different image reconstruction parameters, a progressive smoothing method was used to analyze image uniformity, with metrics designed to assess non-uniformity in image planes as well as in randomly selected region of interest pairs. Unusually high image non-uniformities were found for germanium cylinders at two centres, but of greater concern was the fact that uniformity for the brain phantom was found to be notably inferior at two of the three centres which scanned the phantom. A change of mumap reconstruction parameters, and minor adjustment of a parameter used in the single scatter simulation, was shown to remedy the issue (reducing relative region errors to <;5%).