1. A calibration method for an imaging system comprising an imaging source and an image detector, comprising: determining a first sub-matrix of a projection matrix, the projection matrix describing a geometrical relationship between points of a three-dimensional coordinate system of the imaging system and points of a two-dimensional coordinate system of the image detector; determining a second sub-matrix of the projection matrix, where the first sub-matrix and the second sub-matrix comprise a decomposition of the projection matrix; converting a first point of the two-dimensional coordinate system to a first point of the three-dimensional coordinate system based on the first sub-matrix and the second sub-matrix; determining an updated first sub-matrix of an updated projection matrix according to a first calibration schedule, where the updated projection matrix describes a second geometrical relationship between points of the three-dimensional coordinate system and points of the two-dimensional coordinate system, and where the updated first sub-matrix and the second sub-matrix comprise a decomposition of the updated projection matrix; and converting a second point of the two-dimensional coordinate system to a second point of the three-dimensional coordinate system based on the updated first sub-matrix and the second sub-matrix.