1. A method of determining a model of a marker, comprising: obtaining projection images, each of the projection images having an image of a marker that indicates a shape of the marker, the marker being a physical object implanted inside a patient; determining binary images of the marker for respective ones of the projection images; and constructing, using a processing unit, a three-dimensional model of the marker using the binary images, the three-dimensional model comprising a set of voxels in a three-dimensional space that collectively indicates an actual three-dimensional shape of the marker.