An x-ray system (100) comprises a gantry (102) on which an x-ray source (104) and an x-ray detector (106) are mounted. A control unit (110) comprises means (114) for effectuating a wiggling motion of the gantry, wherein an axis (116) connecting the x-ray source and the x-ray detector traces a surface (128) of a cone (118). The x-ray source and the x-ray detector have a fixed position with respect to the axis. The control unit comprises means (120) for acquiring a series of x-ray images during the wiggling motion of the gantry. An object recognition unit (122) detects an object (124) appearing in the series of x-ray images to obtain a tracked path. A depth estimation unit (126) uses the tracked path for estimating a depth parameter indicative of a position of the object in a direction substantially parallel to the axis (116).