A system and related method for signal processing. Interferometric projection data reconstructed into one or more images for a spatial distribution of a physical property of an imaged object. The interferometric projection data is derived from signals acquired by an X-ray detector (D), said signals caused by X-ray radiation after interaction of said X-ray radiation with an interferometer and with the object (OB) to be imaged, said interferometer (IF) having a reference phase. A reconstructor (RECON) reconstructs for the image(s) by fitting said data to a signal model by adapting fitting variables, said fitting variables including i) one or more imaging variables for the one or more images and ii), in addition to said one or more imaging variables, a dedicated phase variable for a fluctuation of said reference phase.