A method for generating an object 3D point cloud in a medical imaging system comprising a machine table and a scanning device comprises: extracting a valid 3D point cloud in a valid region where the machine table is located from a global 3D point cloud based on a current height of the machine table and boundary information of the machine table, wherein the global 3D point cloud comprises 3D point clouds of an object and a surrounding environment thereof, and the object 3D point cloud is comprised in the valid 3D point cloud; and removing an environment 3D point cloud of the surrounding environment from the valid 3D point cloud to obtain the object 3D point cloud, wherein the surrounding environment comprises at least part of the machine table. In some embodiments, a moving path of the machine table in the medical imaging system is planned using the object 3D point cloud. In some embodiments, collision prediction of the object is performed in the medical imaging system using the object 3D point cloud.