Importance of gated CT acquisition for the quantitative improvement of the gated PET/CT in moving phantom

The aim of this study was to investigate the utility of gated PET/CT and CT attenuation correction (AC) for the quantitation of radioactivity. An ellipse phantom containing six spheres, ranging from 10 to 37 mm in diameter, was filled with 36.7 kBq/mL of F-18. The respiratory motion was simulated by a motor-driven plastic platform to move the phantom with a displacement of 2 cm in the craniocaudal direction at a frequency of 15/min. With the phantom at rest, PET/CT data were acquired and used as a standard (nonmotion). With the phantom in motion, PET data were acquired in both the static and gated modes (sPET and gPET, respectively). Helical CT (HCT), slow CT (SCT), average CT (ACT), and four-dimensional CT (4DCT) were acquired and used to correct attenuation. On both PET and CT images, the maximum radioactivity, dimensions, and CT numbers were measured on the central slices. In nonmotion, recovery coefficients whose spheres were 22 mm or smaller gradually decreased. Regarding motion, the PET counts of the spheres in the static acquisition were lower than those acquired in nonmotion with either type of CTAC (sPET-HCT: -43.8%, sPET-SCT: -51.4%, sPET-ACT: -49.5%). Gated acquisition of PET significantly improved the PET counts (gPET-HCT: -30.1%) (p < 0.05), while additional gated acquisition of CT significantly improved them further (gPET-4DCT: -15.2%) (p < 0.01). The dimensions of sPET were overestimated, but those of gPET were close to the standard values. The SCT significantly overestimated the dimensions, and the water density area decreased (p < 0.01). The 4DCT images were similar to the HCT images. In respiratory motion, PET acquisition in the static mode underestimated the radioactivity and overestimated the dimensions. Neither SCT nor ACT improved these errors. Although PET acquisition in the gated mode improved the quantification of PET/CT images, the additional gated CT acquisition using 4DCT is required for further improvement.