The present invention relates to an apparatus (10) for generating X-ray imaging data. It is described to position (210) a first grating between an X-ray source and a second grating. The second grating is positioned (220) between the first grating and a third grating. A third grating is positioned (230) between the second grating and an X-ray detector. An object is positioned (240) between the first grating and the third grating. At least one of the three gratings has a pitch attribute of having a constant grating pitch. At least one of the three gratings has a pitch attribute of having a varying grating pitch. Both gratings of an adjacent pair of gratings are bent such that a distance between the two adjacent gratings is constant as a function of fan angle. Both gratings of the adjacent pair of gratings that are bent have the same pitch attribute. An X-ray detector detects (250) at least some of the X-rays transmitted by the three gratings and the object.