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We have proposed an ldquoOpenPETrdquo geometry, which consists of two axially separated detector rings, each of axial length W. A long and continuous field-of-view (FOV) including a 360-degree open gap G between two detector rings can be imaged through iterative image reconstruction. In addition to providing stress-less PET scanning and simultaneous PET/CT, the OpenPET is expected to lead to realization of in-beam PET. The OpenPET also extends the axial FOV with a limited number of detectors. However, the axial FOV is limited to 3 W because the maximum limit of G to obtain the axially continuous FOV is W. In this paper, therefore, we propose an alternative geometry to extend axial FOV even more without increasing the number of detectors. The proposed geometry consists of multiple detector rings separated by multiple gaps. By optimizing each width of the gaps based on a new concept of multiplex geometry of the OpenPET, the axial FOV can be theoretically increased to an unlimited extent without increasing the number of detectors. The multiplex OpenPET geometry was compared with the standard OpenPET and the conventional PET using numerical simulation data and experimental data. The results show that similar reconstructed images are obtained by three geometries. The proposed geometry is expected to help realize an affordable entire body PET scanner that enables whole body dynamic imaging.