THE ECAT EXACT HR - PERFORMANCE OF A NEW HIGH-RESOLUTION POSITRON SCANNER

Objective: The ECAT EXACT HR is a newly designed CTI-Siemens PET scanner with high spatial resolution. Its physical performance with respect to resolution, count rate efficiency, and scatter was investigated and evaluated with phantom studies. Materials and Methods: The new tomograph consists of three rings of 112 BGO block detectors (50 mm x 23 mm x 30 mm deep) each, covering an axial field of view of 15 cm with a patient port of 56 cm diameter. Each block is sawed into an 8 x 7 matrix giving 24 detector rings with 784 crystals each. Results: Total sensitivity for a 20 cm cylinder phantom is 177 kcps/mu Ci/ml in two-dimensional (2D) mode and increases to 1.46 Mcps/mu Ci/ml in 3D mode. Count rate performance was investigated for different low energy discriminator thresholds. Smaller detector blocks improve noise equivalent counts by similar to 50% compared with the EXACT system both in 2D and in 3D mode. Scatter fractions vary in 2D from 0.09 to 0.13 for energy thresholds from 450 to 250 keV for line sources in a 20 cm diameter phantom. In 3D mode an increase of scatter by a factor of 3 is observed. Transaxial spatial resolution varies from 3.6 mm full width at half-maximum (FWHM) at the center to 4.5 mm FWHM tangentially and 7.4 mm FWHM radially at R = 20 cm. Average axial resolution changes from 4.0 mm FWHM at center to 6.7 mm FWHM at R = 20 cm. Conclusion: Due to its special properties, the EXACT HR can be equally applied to routine clinical brain and whole-body imaging and to noninvasive experimental studies of regional tracer concentrations in medium-sized animals