1. A method for calculated image fusion in medical X-ray imaging, which comprises the steps of: merging absorption data, differential phase contrast data and small-angle scattering data of images of an underlying sample by a fusion algorithm into one single, grey-level image with enhanced details; basing the calculated image fusion on a multiple-resolution framework, where an original image is decomposed into several sub-images containing information of the original image at different spatial frequencies; obtaining the absorption data, the differential phase contrast data and the small-angle scattering data of the images from X-ray investigations based on grating-based interferometry, analyzer-crystal-based imaging, or coded aperture imaging, and using an arrangement for X-rays for obtaining quantitative X-ray images from a sample, the arrangement for X-rays containing: an X-ray source; at least two gratings; and a position-sensitive detector with spatially modulated detection sensitivity having a number of individual pixels; transforming the absorption data, the differential phase contrast data and the small-angle scattering data of the images into a multiple-resolution (MR) domain consisting of multiple levels (s denotes a level index), each level containing several sub-bands (o denotes a band index); processing and merging sub-band images in the MR domain by performing intra-band processing, inter-band processing or both the intra-band processing and the inter-band processing, wherein a fused sub-band F l ({circumflex over (t)}) can be generally expressed by F l ⁡ ( t ^ ) = ∑ X ∈ [ A ⁢ ⁢ C , DPC , DFC ] ⁢ wn X , l · wb X , l · w X , l ⁡ ( t ^ ) · D X , l ⁡ ( t ^ ) , where X represents one of possible image types: an absorption contrast (AC), a differential phase contrast (DPC) or a dark-field contrast (DFC); D X,l ({circumflex over (t)}) represents an unprocessed coefficients of band l; l denotes a pair (s,o), indicating a certain band at level S and of band o, oε{HL,LH,HH} when a wavelet transform is used for a decomposition; {circumflex over (t)}=(m,n) denotes 2D coordinates in a sub-band image: the intra-band processing, which is represented by w X,l ({circumflex over (t)})·D X,l ({circumflex over (t)}), assigns a weighting factor w X,l ({circumflex over (t)}) to each coefficient within the band l to increase a local signal-to-noise ratio; the inter-band processing gives a global weighting factor wn X,l ·wb X,l to each band for selecting useful information according to image characteristics and constraining a global noise level; and wb X,l denotes a band selection weighting factor and wn X,l denotes a noise constraint weighting factor; and reconstructing a merged image by an inverse multiple-resolution transform.