A method for reconstructing an Emission Computed Tomography image implemented on at least one machine each of which has at least one processor and storage, the method comprising: obtaining ECT projection data, the ECT projection data being four-dimensional data containing time domain data and three dimensional spatial data and corresponding to a plurality of voxels that correspond to a plurality of spatial points of a subject; determining a plurality of gate numbers for the plurality of voxels, the plurality of gate numbers relating to motion information of the plurality of voxels; determining a feature relating to the motion information of the plurality of voxels based on the plurality of gate numbers; and reconstructing an ECT image based on the ECT projection data and the feature, wherein the determining a plurality of gate numbers for the plurality of voxels comprises: generating an intermediate image by gating the ECT projection data with a uniform gate number, the intermediate image containing the time domain data; for each of the plurality of voxels, determining a difference between a voxel at a first time point in the intermediate image and a corresponding voxel at at least one second time point in the intermediate image, the at least one second time point being different from the first time point, and the voxel at the first time point and the corresponding voxel at the at least one second time point corresponding to a same spatial point of the subject; determining a difference image based on the determined differences of the plurality of voxels; and determining the plurality of gate numbers for the plurality of voxels based on the difference image. obtaining ECT projection data, the ECT projection data being four-dimensional data containing time domain data and three dimensional spatial data and corresponding to a plurality of voxels that correspond to a plurality of spatial points of a subject; determining a plurality of gate numbers for the plurality of voxels, the plurality of gate numbers relating to motion information of the plurality of voxels; determining a feature relating to the motion information of the plurality of voxels based on the plurality of gate numbers; and reconstructing an ECT image based on the ECT projection data and the feature, wherein the determining a plurality of gate numbers for the plurality of voxels comprises: generating an intermediate image by gating the ECT projection data with a uniform gate number, the intermediate image containing the time domain data; for each of the plurality of voxels, determining a difference between a voxel at a first time point in the intermediate image and a corresponding voxel at at least one second time point in the intermediate image, the at least one second time point being different from the first time point, and the voxel at the first time point and the corresponding voxel at the at least one second time point corresponding to a same spatial point of the subject; determining a difference image based on the determined differences of the plurality of voxels; and determining the plurality of gate numbers for the plurality of voxels based on the difference image. generating an intermediate image by gating the ECT projection data with a uniform gate number, the intermediate image containing the time domain data; for each of the plurality of voxels, determining a difference between a voxel at a first time point in the intermediate image and a corresponding voxel at at least one second time point in the intermediate image, the at least one second time point being different from the first time point, and the voxel at the first time point and the corresponding voxel at the at least one second time point corresponding to a same spatial point of the subject; determining a difference image based on the determined differences of the plurality of voxels; and determining the plurality of gate numbers for the plurality of voxels based on the difference image.