A method implemented on a computing device having one or more processors and one or more storage devices, the method comprising: obtaining a rotation speed of a radioactive scanning source in a computed tomography (CT) scanner in a time period and a sample count of the plurality of projection samples in the time period; obtaining 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 in the CT scanner; determining information of a plurality of standard rotation angles based on information of a standard angle difference, wherein the standard angle difference is determined by the sample count of the plurality of projection samples in the time period and the rotation speed of the radioactive scanning source; determining information of 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 determining a plurality of modified rotation angles corresponding to the plurality of projection samples by modifying the plurality of original rotation angles, wherein the modifying the plurality of original rotation angles includes: comparing the information of each of the plurality of original rotation angles with the information of a corresponding one of the plurality of corresponding standard rotation angles to generate a comparison result; and modifying each of the plurality of original rotation angles based on the information of the plurality of standard rotation angles and the comparison result. obtaining a rotation speed of a radioactive scanning source in a computed tomography (CT) scanner in a time period and a sample count of the plurality of projection samples in the time period; obtaining 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 in the CT scanner; determining information of a plurality of standard rotation angles based on information of a standard angle difference, wherein the standard angle difference is determined by the sample count of the plurality of projection samples in the time period and the rotation speed of the radioactive scanning source; determining information of 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 determining a plurality of modified rotation angles corresponding to the plurality of projection samples by modifying the plurality of original rotation angles, wherein the modifying the plurality of original rotation angles includes: comparing the information of each of the plurality of original rotation angles with the information of a corresponding one of the plurality of corresponding standard rotation angles to generate a comparison result; and modifying each of the plurality of original rotation angles based on the information of the plurality of standard rotation angles and the comparison result. comparing the information of each of the plurality of original rotation angles with the information of a corresponding one of the plurality of corresponding standard rotation angles to generate a comparison result; and modifying each of the plurality of original rotation angles based on the information of the plurality of standard rotation angles and the comparison result. <EOS>