A number of new concepts are presently appearing for large size X-ray imaging. The almost perfect MTF obtained with photoconductors read by an a-Si matrix is undoubtedly the best as regards signal, but the associated aliasing effects force the total photon noise power into the useful spatial frequency range, which degrades the signal to noise ratio. At present, an imager consisting of a CsI:Tl scintillator layer coupled to an array of a-Si pixels is the most attractive concept because it combines (1) A strong X-ray absorption together with a good MTF, (2) The best overall X-ray to electrons conversion factor, necessary to overcome electronic noises, (3) A low pass filtering effect which strongly attenuates photon noise beyond the Nyquist limit. The variation of DQE with spatial frequency is the right factor of merit for an X-ray imager. It not only accounts for the spatial resolution, but also contains the effects of all noises in the detection process, including aliasing. Whenever a digital imager is compared with an analogue detector such as the screen/film, the effect of the position of the object with respect to the pixel lattice (or phase difference between the sampling pitch a and the spatial frequency of interest f) must be taken into account. This results in an additional sinc2(fa) factor applied to the DQE(f).