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Methods to test the practical performance of the Philips Gemini PET/CT system during clinical imaging are described and results presented. The test methods used were based on the NEMA standards and those suggested by a task group of the Commission of European Communities (EEC). These were performed using the EEC emission phantom, and phantoms provided by Philips. Scans were acquired using the routine clinical protocols. Images were reconstructed using a 3D row action maximum likelihood algorithm (RAMLA). Attenuation correction (AC) was applied using CT data (CTAC) and Cs-137 source transmission scans (CsAC). Images can be acquired with the PET and CT gantries closed or open. Results of tests on the gantry alignment show that the static offsets varied by a maximum of 4.5 mm when the system was first installed, but as the floor has settled these variations have decreased to a maximum of 2.5 mm. The mean negative nonuniformity was and for CsAC and CTAC respectively and the mean positive nonuniformity was and . Scatter fraction measurements were made in the EEC body phantom. The reconstructed scatter fractions were 0.18 and 0.16 with and without out-of-field-of-view activity respectively ( OFOVA) for both CsAC and CTAC images. The sinogram-derived (NEMA NU 2-1994) scatter fraction was 0.44 and 0.43 OFOVA. At the centre of the field of view (FOV) the transverse spatial resolution (SR) was 5.0 mm in air (5.0 mm in water) and the axial SR was 6.0 mm (6.8 mm). 10 cm away from the centre the transverse radial SR was 5.4 mm (5.3 mm), the transverse tangential SR was 5.2 mm (5.4 mm) and the axial SR was 6.6 mm (7.2 mm). The recovery coefficients demonstrate the partial volume effect. The AC accuracy was similar in water (both ) and Teflon [ (CTAC) and (CsAC)], for both types of AC, but very different in air [ (CTAC) and (CsAC)]. If the transmission maps were not segmented prior to AC these gave much higher and very different residual AC errors. The peak noise equivalent count rate was 45.2 kcps at - 184 MBq and 25.2 kcps at 127 MBq OFOVA. The system sensitivity to true and scattered counts was 34.1 kcps/(kBq/cm ) and to true counts was 19.4 kcps/(kBq/cm ).