A comparison of the imaging properties of a 3- and 4-ring biograph PET scanner using a novel extended NEMA phantom

The imaging quality of a Siemens biograph PET/CT camera was evaluated before and after an axial field of view (FOV) upgrade from 16.2 cm (TruePoint, 3-rings) to 21.6 cm (TrueV, 4-rings). A series of five measurements were performed with the original 3- and with the upgraded 4-ring system using the original, as well as a newly developed modified IEC/2001-NEMA phantom with thoracic and abdominal lucite extensions. A scan protocol comprising five different measurements was used; 1) the pure IEC phantom positioned at the center of FOV 2) IEC, positioned 100 mm off-axis, 3) IEC, with thoracic slabs, positioned at the center, 4) IEC, with abdominal slabs, positioned at the center, 5) the same as in 1) but with approximately half the 18F-activity. All measurements were performed and reconstructed according to the standard whole-body PET/CT protocol. For hot spheres, the average contrast was improved (between 8 and 23%) for all spheres of scan 1 and 2 with the upgraded system. With extension slabs, the hot sphere contrast gave no clear-cut results. The contrast impaired for the smallest spheres but increased for the two largest spheres with the upgraded system. For cold spheres, no contrast improvement was observed in any of the scans with the upgraded camera. Instead there was up to 14% reduction in cold sphere contrast in the phantom measurements with extension slabs. The reduced cold sphere contrast in the phantom with extension slabs may be due to difficulties in correcting for the increased scatter fraction.