1. A method for calibrating an image acquisition system with an analytic S-distortion model, said method comprising: (a) acquiring a set of images, wherein said images include patient anatomy and fiducial markers embedded within a calibration target; (b) processing said images to obtain measured fiducial markers shadow positions in imaging plane coordinates; (c) estimating image acquisition system intrinsic parameters, image acquisition system extrinsic parameters, and optical distortion parameters based on the measured fiducial markers shadow positions; (d) computing a set of model fiducial markers positions in an imaging plane based on the estimated image acquisition system intrinsic parameters and image acquisition system extrinsic parameters; (e) correcting for the S-distortion and pincushion distortion of the measured fiducial markers shadow positions; (f) computing the difference between the measured fiducial markers shadow positions and the model fiducial positions; (g) comparing the difference between the measured fiducial shadow positions and the model fiducial positions with a threshold value, if said difference is greater than the threshold value, the image acquisition system intrinsic parameters, the image acquisition system extrinsic parameters, and the optical distortion parameters are updated and used as input for step d) in the next iteration cycle; (h) if said difference is less than the threshold value, the optical distortion parameters are used for linearizing said set of acquired images.