[Observation of relation between object size or length and count rate for 3D PET]

We obtained the concentration range of better S/N in 3D than 2D acquisition by measuring count rate for phantoms of different size and length. We used noise equivalent count rate (NECR) for evaluation of S/N of images. For 20 cm diameter 15 cm length cylindrical phantom, NECR of 2D was higher than that of 3D more than 1.2 microCi/ml. For 30 cm diameter 15 cm length cylindrical phantom, NECR of 2D was higher than that of 3D more than 0.7 microCi/ml. For 14 cm diameter 25 cm length cylindrical phantom, NECR of 3D was lower than that of 25 cm length phantom. With these results, though 3D is useful when administered dose is small, image quality of 2D becomes better than 3D more than a certain concentration. For the larger diameter or longer object, the advantage of 3D is diminished at lower concentration.