Systems and methods for determining rotation angles are disclosed. The systems may perform the methods to: obtain a rotation speed of a radioactive scanning source(115) in a CT scanner(110)； obtain a plurality of original projection acquisition times corresponding to a plurality of projection samples, the plurality of projection samples being associated with rotation of the radioactive scanning source(115)； determine a plurality of original rotation angles corresponding to the plurality of projection samples based on the plurality of original projection acquisition times and the rotation speed of the radioactive scanning source(115)； and determine a plurality of modified rotation angles corresponding to the plurality of projection samples by modifying the plurality of original rotation angles.
