1. A radiation imaging system comprising: a radiation imaging device which includes at least one grating provided in an emission axis direction of radiation and obtains a plurality of periodic pattern images by performing fringe scanning with a subject placed at a subject placement position provided on an emission path of the radiation while moving the grating; and an image processing device including: a reconstruction unit which generates at least two reconstructed images of a differential phase image, an absorption image and a small-angle scattering image based on the periodic pattern images; and a detection unit which (i) performs regression analysis on signal values of at least two images of (a) the differential phase image, a differential absorption image generated by differentiating the absorption image, and a differential small-angle scattering image generated by differentiating the small-angle scattering image, or (b) a phase image generated by integrating the differential phase image, the absorption image, and the small-angle scattering image, the at least two images being a target of the regression analysis, (ii) calculates an indicator value of an indicator indicating a relationship between the at least two images, and (iii) detects image quality deterioration due to change in relative position of the radiation imaging device and the subject based on the calculated indicator value.