Abstract: The invention relates to an X-ray cassette for a medical X-ray apparatus. The X-ray cassette has a mutually hinged top and bottom to define a housing structure, which includes opposite end faces and narrow side faces therebetween. Inside the housing structure is a cavity for accommodating a receiving means. The X-ray cassette is adapted to be placed in a cassette holder present in the X-ray apparatus for the duration of an Imaging session, said cassette holder including an X-ray cassette receiving slot in a shape substantially complementary to the X-ray cassette. The X-ray cassette comprises an one of its narrow side faces protruding guide elements offset from the longitudinal center line of the side face to lie closer to one of the main faces, said guide elements being adapted to fit in a complementary guide groove or guide recess formed in the receiving slot of a cassette holder, such that the X-ray cassette is insertable in the cassette holder in a specified position only.