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; determining binary images of the marker for respective ones of the projection images; and constructing 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 a three-dimensional shape of the marker, wherein the act of constructing the three-dimensional model is performed using a processing unit.