Physical characteristics of the ECAT EXACT3D positrontomograph

Abstract

The `EXACT3D' positron tomograph, which is now in routine clinical research use,was developed with the aim of achieving unprecedented sensitivity, high spatialand temporal resolution and simplicity of design using proven detectortechnology. It consists of six rings of standard detector blocks (CTI/SiemensEXACT HR+) with 4.39 mm × 4.05 mm × 30 mm elements,giving an axial field of view (FOV) of 23.4 cm. This extended FOV and theabsence of interplane septa and retractable transmission rod sources has allowedgreatly simplified gantry and detector cassette design. Operation in exclusive3D mode requires an alternative to the conventional coincidence method fortransmission scanning, and a single photon approach using a hydraulically driven137Cs point source has been implemented. The tomograph has no other movingparts. A single time frame of data without any compression is very large (>300 Mbyte) and two approaches are employed to overcome this difficulty:(a) adjacent sinograms can be summed automatically into different combinations and(b) listmode (event-by-event) acquisition has been instituted, which is bothstorage efficient (particularly for acquisition of sparse data sets) andmaximizes temporal resolution. The high-speed I/O and computing hardware canmaintain a sustained acquisition rate of about 4 million coincidence events persecond. A disadvantage of the large axial FOV in 3D is the increased sensitivityto activity outside the coincidence FOV. However, this can be minimized byadditional side shielding. The mean spatial resolution is 4.8±0.2 mm FWHM(transaxial, 1 cm off-axis) and 5.6±0.5 mm (axial, on-axis). Its absoluteefficiency is 5.8% for a line source in air (just spanning the axial FOV) and10% for a central point source (with thresholds of 350-650 keV). For a uniform20 cm diameter cylinder, the efficiency is 69 kcps kBq-1 ml-1 (aftersubtraction of a scatter fraction of 42%). Sensitivity relative to the EXACTHR+ (with four rings of blocks) is 2.5 (3D) and 12 (2D) times respectively.The rate of random events in blood flow studies in the brain and body, using15O-labelled water, can be controlled by limiting the administered dose andinserting additional side shielding.