There is provided methods and devices for estimating object scatter and/or internal scatter in a multi-level photon-counting x-ray detector (102, 103), as well as methods and devices for X-ray tomographic imaging of an object while correcting for object scatter and/or internal scatter. The X-ray detector has at least two layers of detector diodes mounted in an edge- on geometry, e.g. designed for1 ) estimating the object scatter contribution to the counts in a top layer of said at least two layers based on difference(s) in counts between the top layer and lower layer(s) under the assuption the object scatter has a slowly varying spatial distribution, and/or 2) estimating counts from reabsorption of photons that have Compton scattered inside the detector based on selectively blinding somedetector elements from primary radiation by placing a highly attenuating beam stop on top of the detector elements, in lower layer(s) or in both top layer and lower layer(s), and measuring the counts in those detector elements.