An initial investigation of open PET geometries

A long patient port of PET scanner tends to put stress on patients, especially patients with claustrophobia. It also prevents doctors and technicians from taking care of patients during scanning. In this paper, we propose an "open PET" geometry, which consists of two axially separated detector rings. Long and continuous field-of-view (FOV) including 360-degree opened gap space between two detector-rings can be visualized by 3D image reconstruction from all the possible lines-of- response (LORs). Based on the redundant characteristics of 3D PET, oblique LORs between two separated detector-rings compensate the missing LORs in the gap. In order to evaluate imaging performance of the open PET geometries, we simulated dual HR+ scanner (ring diameter=827 mm, axial length=153 mm x 2) separated by a variable gap. 3D OS-EM with geometrical system modeling was applied. The gap of 153 mm was the maximum limit for the simulated scanner to have axially continuous FOV of 459 mm though the maximum diameter of FOV at the central slice was limited to 414 mm. The results show that high resolution images can be obtained with the gap up to 153 mm, though missing LORs increase a little but almost invisible artifacts. We also tested open PET geometries using experimental data obtained by the JPET-D4, a prototype brain scanner. The JPET-D4 has 5 rings of 24 detector blocks. Therefore we simulated open PET data with a gap of 65 mm by eliminating 1 block-ring from experimental FDG-PET data. Although some artifacts were seen at both ends of the gap, very similar images were obtained with and without the gap. The open PET geometry is expected to enable in-beam PET, which is a method for an in situ monitoring of charged particle therapy, by letting beams though the gap. The open PET geometry also enables a simultaneous PET/CT scanner to measure the same PET FOV as the CT FOV at the same time, in contrast to conventional PET/CT scanners where each FOVs are separated by several tens centimeters.