The present disclosure relates to a system for imaging. The system may include a supporting assembly and a detector assembly. The supporting assembly may include a detection region to accommodate a subject. The detector assembly may surround the detection region. The detector assembly may be configured to detect radiation rays emitted from the subject located within the detection region. The detector assembly may include a plurality of detector rings. Each detector ring may include a scintillator array and a plurality of photosensors. The plurality of detector rings may be arranged on the supporting assembly in an axial direction of the supporting assembly to form an axial field of view (FOV) having a length no less than 0.75 meters.