1. A method for determining distortions in an image comprising the steps of: arranging a subject in an imaging volume of an imaging system and obtaining an image of said subject in said imaging volume, with a first region of said imaging volume appearing undistorted in said image and a second region of said imaging volume appearing distorted in said image; prior to obtaining said image of said subject, obtaining an image, using said imaging system, of at least three markings having a known spatial position relative to each other, with a first and a second of said three markings being disposed in said first region and a third of said three markings being disposed in said second region; determining respective positions of said markings in said image of said markings; determining an ideal position of said third marking in said image of said markings from the known spatial position of said third marking relative to said first and second markings; and determining a positional difference of the image of the third marking in said image of said markings from said ideal position, and employing said positional difference as a criterion for distortion.