Simulation Study on Sensitivity and Count Rate Characteristics of “OpenPET” Geometries

An "OpenPET" geometry, which consists of two axially separated detector rings, has been proposed to image a 360-degree open gap. An open gap could be less stressful for patients and would be suitable for simultaneous PET/CT, whole-body PET and in-beam PET. It has been shown that OpenPET geometries can maintain high spatial resolution using a depth-of-interaction detector. However, when the PET scanner is used for an extended axial field-of-view (FOV), several characteristics of oblique lines-of-responses, such as decreased sensitivity due to smaller solid angle fraction and increased scatter and attenuation, should be carefully discussed. It is expected that these characteristics are dependent on the gap width and the target object size. In this work, we studied sensitivity and count rate characteristics for the OpenPET geometry using GATE simulation. This scanner has a 30.7-cm open gap and 93.7-cm axial FOV. When the gap is extended, the axial FOV is extended at the cost of decreased sensitivity on the slopes of the main peak. Maximum sensitivity decreases by 25% due to the decreased solid angle fraction. The noise equivalent count rate (NECR) test utilized four activity distributions that simulated whole-body and in-beam PET applications. For whole-body PET simulation, the loss of the peak NECR with the open gap decreases by 15% for the scanner. For in-beam PET simulation, the NECR values of the scanner are approximately 70% of those without the open gap. The OpenPET geometry promises to provide a large axial FOV with the open space and to have sufficient performance values. But, the open gap decreases sensitivity and NECR. Whole-body PET focuses to extend to the axial FOV, while keeping sufficient performance. In-beam PET only images within the open gap and heavy ion beams limit the minimum length of the open gap.