Performance of a new 3D-only PET scanner-the EXACT3D

Characteristics of currently the highest sensitivity 3D-only PET scanner (CTI/Siemens 966/EXACT3D) have been studied. The device has 48 rings (82 cm diameter) of BGO detector elements (4.0×4.1×30 mm, 8×8 block) and an axial FOV of 23.4 cm. Attenuation correction is carried out with a point source of ¹³⁷Cs which moves under hydraulic pressure in a helical tube. Transaxial resolution is 4.7 mm 1 cm from the centre of the FOV increasing, in the radial direction to 6.3 mm and 7.3 mm at 10 cm and 15 cm. At a lower threshold of 350 keV, the scatter fraction is 40%. With scatter subtracted, the efficiency (20 cm cylinder) is 6.9×10⁴ cps/kBq/ml (maximum ring difference of 40); the absolute sensitivity is 5.8%. In the current configuration, the maximum total coincidence event rate is limited to about 3×10⁶ per sec. The maximum true rate is about 850 kcps (90 MBq in the FOV). List mode acquisition has been implemented to maximise temporal resolution and optimise data storage. The more open geometry of this device gives a broader singles FOV. Hence administered doses and/or additional side-shielding need to be carefully considered to optimise noise-equivalent counts