Influence of Outside Field of View Activity on the Quality of High Resolution Research Tomograph (HRRT) Brain studies

Outside field of view (FOV) activity might influence quantification of PET brain scans, as it results in increased random coincidences, dead time and scatter fractions. Furthermore, it deceases noise equivalent count rates (NECR) and may result in quantification errors of PET brain studies. In this study, the effects of outside FOV activity on studies acquired with the High Resolution Research Tomograph (HRRT) are studied. The HRRT is a dedicated scanner with a high resolution especially designed for human brain studies and animals. So far, effects of outside FOV activity on HRRT scans have not yet been investigated. To investigate the impact of outside FOV activity on dead time, scatter fraction and NECR two phantom studies were acquired on the HRRT. The first study was performed with a cylindrical Ge-68 phantom with a constant activity in the FOV and different radioactivity concentrations F-18 outside FOV. The second study was performed with a Hoffman brain phantom with different inside/outside FOV activity ratios. Outside FOV activity seems to result in somewhat increased dead time (which was slightly overcorrected with at most 5% during reconstruction), decreased NECR, increased scatter fraction and 10 to 30% quantification errors. NECR decreased with about 30% at 151 MBq outside the FOV. This study showed that outside FOV has relatively small to moderate effects on dead time and scatter fraction. However, outside FOV activity has a somewhat larger effect on NECR and it may potentially affect quantification of HRRT PET studies, especially for tracers showing low brain uptake or in `early' frames of dynamic scan protocols. Use of an additional extern shield ('neuro-insert') might therefore be useful to improve NECR of HRRT studies.