1. An extra-oral dental X-ray imaging system comprising: an X-ray source exposing x-rays to an object to be imaged; an X-ray imaging device configured for outputting multiple consecutive frames, during at least part of the exposure; at least one rotational axis around which at least one of the X-ray source and the imaging device rotates along a spline, the axis being located between a focal point of the X-ray source and the X-ray imaging device; wherein the X-ray imaging device has an active area with a long dimension m and a short dimension n and wherein m/n>1.5; and a processing device for taking data of a single exposure comprising the outputted multiple frames to compose selectively at least a predetermined dental panoramic layer image, and one of i) a transverse slice to a selected part of a dental panoramic layer image, and ii) a 3-D reconstruction of a volume corresponding to some part of a dental panoramic layer, wherein the processing device, from the data of the single exposure composes both i) the predetermined dental panoramic layer image and ii) the transverse slice to the selected part of the dental panoramic layer image.