Disclosed herein is a method comprising: obtaining a third image from a first X-ray detector when the first X-ray detector and a second X-ray detector are misaligned; determining, based on a shift between a first image and the third image, a misalignment between the first X-ray detector and the second X-ray detector when the first and second detectors are misaligned; wherein the first image is an image the first X-ray detector should capture if the first and the second detectors are aligned.