A phase contrast imaging apparatus (MA) and related image processing method. The imaging apparatus includes a movable arm (AR) that carries a detector (D) and one or more interferometric gratings (G0, G1, G2). The imaging apparatus includes a rigidizer (RGD) to control the rigidity of at least the arm (AR) or a mounting (GM) for the gratings (G0, G1, G2). This allows controlling a drift of a Moiré pattern as detected in a sequence of readouts. A phase of the so controlled Moiré pattern can be used to calibrate the imaging apparatus by using the image processing method.
