The present disclosure relates to systems and methods for determining rotation angles. The systems may perform the methods to: obtain a rotation speed of a radioactive scanning source in a CT scanner; 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; 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; and determine a plurality of modified rotation angles corresponding to the plurality of projection samples by modifying the plurality of original rotation angles.