The invention relates to an X-ray apparatus (10) and a method (64) for a depth localization of a catheter (12) within an object of interest (22), arranged at an object receiving space (14) of the X-ray imaging apparatus. The X-ray imaging apparatus comprises an X-ray source (16) and an X-ray detector (20). Between the X-ray source and the X-ray detector, at least one interferometer (24) formed by a phase grating is arranged. The X-ray detector is configured to detect X-ray radiation, which has been influenced by the phase grating and the object of interest and the catheter, which may also be arranged at the object receiving space and/or the object of interest. The X-ray detector is configured to provide a detector signal s. A signal component of the detector signal relates to a phase-contrast detector signal component, which may be calculated on the basis of the detector signal and may represent a visibility loss. It has been found, that a repective visibility loss signal component of the detector signal depends on a distance between the catheter and the phase grating. Accordingly, depth information for the catheter may be calculated on the basis of the visibility loss signal component of the detector signal. Correspondingly, a three-dimensional position of the catheter at the object receiving space may be calculated. It is to be noted, that the catheter may just be one example of an interventional device, which may be arranged at the object receiving space. As an effect, a three-dimensional tracking of the interventional device, in particular of the catheter, may be enabled from the X-ray detector signal.
