We have proposed a new “OpenPET” geometry consisting of two detector rings of axial length W each separated by a gap G. For obtaining an axially continuous field of view (FOV) of 2W + G, the maximum limit for G must be W. However, two valleys of sensitivity appear on both sides of the gap. Setting a more limited range for the gap as G < W, which is desirable for filling in the sensitivity valleys, results in not only a shortened gap, but also a shortened axial FOV. In this paper, we propose an alternative method for improving the uniformity of sensitivity by shifting two detector rings axially closer or further apart at the same velocity to each other. In addition, image reconstruction of the OpenPET is an incomplete problem, and low-frequency components are missing in the gap. Therefore, the proposed method is also expected to improve the conditions for the inverse problem. We simulated an OpenPET scanner which measures events simultaneously by shifting the detector rings. The results showed that the right and left peaks of the sensitivity approach each other upon shifting of the detector rings, and these valleys of sensitivity are effectively recovered. The results also showed that distortion, which is observed for objects containing low-frequency components, is reduced. Larger detector shifts allow a more uniform axial distribution of sensitivity and a higher image quality, but at the cost of a smaller minimum gap. Therefore, an appropriate detector-shifting pattern should be determined based on the desired scanner application.
