1. A method for calibrating tracking systems in imaging systems and for prospective motion correction of a moving object being imaged or of a moving object being imaged in MRT, in IMRT or in CT, the method comprising the steps of: a) providing a tracking system having a tracking coordinate system; b) providing at least one imaging system having an imaging coordinate system; c) providing at least one first marker that is stationary relative to the imaging system as a reference marker having a position and orientation; d) cross calibrating the tracking coordinate system and the imaging coordinate system; e) measuring a position and orientation of the first marker in the tracking coordinate system; f) calibrating the position and orientation of the first marker in the imaging coordinate system; g) monitoring a position and orientation of the first marker in the tracking coordinate system; h) correcting the cross calibration of step d) using the results of step g), wherein the correction in the cross calibration is carried out using quaternion algebra; i) disposing at least one second marker on the moving object; j) detecting a position and orientation of the second marker in the tracking system coordinate system during imaging; k) converting, following step h), the position and orientation of the second marker detected in step j) into the coordinate system of the imaging system using quaternion algebra; and l) performing, following steps g) through k), a prospective motion correction such that an imaging volume remains stationary with respect to the moving object during imaging.