Physical characteristics of the ECAT EXACT3D positron tomograph

Abstract

The 'EXACT3D' positron tomograph, which is now in routine clinical research use, was developed with the aim of achieving unprecedented sensitivity, high spatial and temporal resolution and simplicity of design using proven detector technology. It consists of six rings of standard detector blocks (CTI/Siemens EXACT HR+) with 4.39 mm x 4.05 mm x 30 mm elements, giving an axial field of view (FOV) of 23.4 cm. This extended FOV and the absence of interplane septa and retractable transmission rod sources has allowed greatly simplified gantry and detector cassette design. Operation in exclusive 3D mode requires an alternative to the conventional coincidence method for transmission scanning, and a single photon approach using a hydraulically driven 137Cs point source has been implemented. The tomograph has no other moving parts. 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 both storage efficient (particularly for acquisition of sparse data sets) and maximizes temporal resolution. The high-speed I/O and computing hardware can maintain a sustained acquisition rate of about 4 million coincidence events per second. A disadvantage of the large axial FOV in 3D is the increased sensitivity to activity outside the coincidence FOV. However, this can be minimized by additional 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 absolute efficiency is 5.8% for a line source in air (just spanning the axial FOV) and 10% for a central point source (with thresholds of 350-650 keV). For a uniform 20 cm diameter cylinder, the efficiency is 69 kcps kBq(-1) ml(-1) (after subtraction of a scatter fraction of 42%). Sensitivity relative to the EXACT HR+ (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, using 15O-labelled water, can be controlled by limiting the administered dose and inserting additional side shielding.

Absolute Cerebral Blood Volume and Blood Flow Measurements Based on Synchrotron Radiation Quantitative Computed Tomography

“…Thus, the presence of high-bloodvolume areas (e.g., big vessels, highly perfused regions) induces errors in the CBV values. PET is used for CBV and CBF in vivo measurements (Huang et al, 1983;Phelps et al, 1982;Ter Pogossian and Herscovitch, 1985) but offers a limited spatial resolution (Spinks et al, 2000), leading to important partial volume effects and systematic errors in absolute functional parameters assessment (Iida et al, 2000). Moreover, the very short half-life of the radioisotopes requires the proximity of a cyclotron.…”

Abnormal prefrontal activation directly related to pre-synaptic striatal dopamine dysfunction in people at clinical high risk for psychosis

“…Images were acquired using a 3-D PET scanner (ECAT/EXACT3D; Siemens/CTI, Knoxville, TN, USA), which has a spatial resolution of 4.8±0.2 mm and a sensitivity of 69 cps/Bq/ml. 30 High resolution images of the whole brain were reconstructed from 95 planes with a slice spacing of 2.425 mm. All subjects received 150 mg carbidopa and 400 mg entacapone orally 1 h before scanning to reduce the formation of radiolabelled metabolites, 31,32 which may potentially cross the blood-brain barrier.…”

Effects of Citalopram Infusion on the Serotonin Transporter Binding of [11C]DASB in Healthy Controls

“…The radiotracer [ 11 C]DASB was synthesised as previously described All PET scans were performed on the high-sensitivity Siemens/CTI scanner ECAT EXACT3D with an axial field of view of 23.4 cm and 95 reconstructed transaxial image planes (Spinks et al, 2000). To reduce the effect of activity outside the direct field of view in brain scans, the tomograph was equipped with annular side shielding (Spinks et al, 1998).…”