A method of reconstructing an image based on a list mode comprising an axial position, comprising: receiving a master clock signal; (101, 701) determining coincidence events detected by a detector during a continuous incremental scanning in which a scanning bed continuously moves; measuring a time difference between a first timing at which the detector detects each of the coincidence events and a second timing for a rising edge of the master clock signal which is the nearest to the first timing; determining an actual axial position of the scanning bed at which the coincidence event is detected based on the master clock signal and the time difference corresponding to the coincidence event; (102, 702) determining an axial position for the coincidence event, wherein the axial position of the coincidence event is represented by the actual axial position of the scanning bed at which the coincidence event is detected; (103, 705) storing data for each of the coincidence events comprising the axial position in a list mode, wherein the data in the list mode includes: coincidence event N: [an axial slant angle index, a radial angle index, the axial position, and a radial position index], and N is a serial number of the coincidence event; (104, 706)sorting, the data for each of the coincidence events according to a spatial order; and (105, 707) obtaining an image by performing iterative reconstruction with the sorted data for each of the coincidence events. receiving a master clock signal; (101, 701) determining coincidence events detected by a detector during a continuous incremental scanning in which a scanning bed continuously moves; measuring a time difference between a first timing at which the detector detects each of the coincidence events and a second timing for a rising edge of the master clock signal which is the nearest to the first timing; determining an actual axial position of the scanning bed at which the coincidence event is detected based on the master clock signal and the time difference corresponding to the coincidence event; (102, 702) determining an axial position for the coincidence event, wherein the axial position of the coincidence event is represented by the actual axial position of the scanning bed at which the coincidence event is detected; (103, 705) storing data for each of the coincidence events comprising the axial position in a list mode, wherein the data in the list mode includes: coincidence event N: [an axial slant angle index, a radial angle index, the axial position, and a radial position index], and N is a serial number of the coincidence event; (104, 706)sorting, the data for each of the coincidence events according to a spatial order; and (105, 707) obtaining an image by performing iterative reconstruction with the sorted data for each of the coincidence events. coincidence event N: [an axial slant angle index, a radial angle index, the axial position, and a radial position index], and N is a serial number of the coincidence event; (104, 706)sorting, the data for each of the coincidence events according to a spatial order; and (105, 707) obtaining an image by performing iterative reconstruction with the sorted data for each of the coincidence events.