1. A method comprising: registering first image data of a first stream with second image data of a second stream, the first image data and the second image data comprising image information about a person or animal including image information about at least one organ of the person or animal, wherein a shape and/or position of the organ is changing thereby performing a cyclic change of the shape and/or position, and wherein each of the first stream and the second stream comprises a series of images of the at least one organ at a plurality of different discrete stages of the cyclic change of the shape and/or position so that each of the images corresponds to one of the discrete stages of the cyclic change of the shape and/or position; defining the stages of the cyclic change of the shape and/or position using a phase variable and assigning a phase value of the phase variable to each of the images within each of the first and second stream thereby defining unambiguously the discrete stage of the cyclic change of the shape and/or position represented by the image instead of a time during a specific cycle when the image was taken; and determining a registration comprising location-to-location associations of image values, for a pair of images comprising a first image of the first stream and a second image of the second stream, wherein the first image and the second image of the pair are selected by choosing or interpolating an image from the first stream and by choosing or interpolating an image from the second stream to which a same phase value is assigned.