Influence of TOF information in OpenPET image reconstruction

We have proposed an ¿OpenPET¿ geometry, which consists of two axially separated detector rings. The central point of the field of view, where the highest sensitivity is obtained, is opened. The OpenPET mainly has three applications, namely, simultaneous PET/CT, extension of the axial FOV, and in-beam PET. In our previous report, we showed that axial spatial resolution, which is degraded with the extended gap due to the parallax error, can be recovered by use of depth-of-interaction (DOI) detectors. On the other hand, image reconstruction of the OpenPET is an incomplete problem because projection data do not satisfy Orlov´s condition. Low-frequency components are missing in oblique LORs, i.e., LORs with large ring differences. The gap would suffer from loss of low-frequency components because the gap, where direct LORs (i.e., LORs in the same ring) do not exist, is imaged only from the oblique LORs. In this paper, we focused on time-of-flight (TOF) information, which is expected to compensate for the loss of low-frequency components in the gap. We investigated influence of TOF information in the OpenPET image reconstruction through numerical simulations. Simulated OpenPET scanner had dual detector rings (827.0 mm in diameter and axial length of 153.6 mm each) separated by a gap of 153.6 mm. We supposed that the detectors had a DOI capability of 8 layers and had a TOF capability of 400 ps FWHM resolution. For the non-TOF case, hot-spot objects, which are commonly seen in cancer diagnosis with use of FDG, were imaged without any artifacts, but objects containing more low-frequency components suffered from strong distortion. However, these artifacts were effectively reduced by using TOF information. These results showed that TOF information can compensate for low-frequency components missing in the gap.