A Monte Carlo estimation of effects of activity outside field of view in O-15 cardiac 3D-PET

3D-PET (Positron Emitter Tomography) is widely used for quantification of myocardial blood flow. Quantification of myocardial blood flow is very important for enhancement of the sensitivity and the specificity. In the case of an O-15 cardiac PET, the heart is placed in the center of a field of view (FOV) and the liver which can be main background is placed outside FOV (OFOV). In a phase, the liver can have ~1.5 times larger activity than that of the heart. If large activity exists in OFOV like an O-15 cardiac PET, are scatter events corrected accurately? Is the quantification of myocardial blood flow assured? The purpose of this study is to figure out effects of OFOV activity on the quantification, and evaluate a scatter correction by using Monte Carlo simulation (Geant4) and fundamental phantom experiments. First, in order to validate the simulation, we made a comparison of a scatter fraction between simulations and experiments. The scatter fraction is measured according to NEMA-NU2007. We found out that, in the simulation, to construct structures like a bed and a front shield in OFOV is better to reproduce results of the experiment. Next, fundamental phantom experiments of 4 cases are performed. Case 1 has a decentering phantom placed in FOV. Case 2, 3, and 4 have an additional cylindrical phantom in OFOV, which has larger activity than the phantom placed in FOV by 1, 2, and 5 times. FOV phantoms have equivalent initial activity in all cases. Meanwhile, simulations are also performed with same configurations of these cases. Both the experiment and the simulation overestimate quantification of ROI (region of interest) values of FOV phantom in Case 2, 3 and 4. Finally, a realistic simulation using a numerical human model was performed aimed at an O-15 cardiac PET. We also obtained overestimations of ROI value in the heart due to the activity of the liver placed in OFOV. But quantifications of myocardial blood flows remained unchanged. Because myocardial blood flows are calculated from the clearance of O-15. When large activity exists in OFOV like the liver has, the scatter correction is insufficiency, which however dose not effects on the quantifications of myocardial blood flow in the O-15 cardiac PET.