Methods, devices, systems and apparatus for arranging detector sub-modules in a medical device are provided. In one aspect, a detector includes a housing and a plurality of detector modules arranged in parallel along a direction on the housing and configured to detect rays emitted from a radiation source and attenuated by a subject. Each of the plurality of detector modules includes a support extending in the direction and a plurality of detector sub-modules arranged on the support along the direction. A top surface of each of the plurality of detector sub-modules is tangent to a respective spherical surface of a corresponding target sphere of at least two target spheres having different radiuses, and a respective sphere center of each of the at least two target spheres is substantially overlapped with a focal spot of the radiation source.
