The present invention relates to positioning of an X-ray source. In order to provide an improved user interface for facilitating the X-ray imaging positioning, a transportable handheld planning device (10) for visualizing projected X-ray radiation for medical X-ray imaging is provided, that comprises a first structure (12) for representing a central axis (14) of a projected X-ray radiation (16), and a second structure (18) for representing a cross-sectional area (20) of the projected X-ray radiation. The first structure is manually positionable by a user in relation to an object to be examined. The second structure is adjustable by a user such that the size and proportions of the cross-sectional area are adjustable in relation to the object to be examined. Further, a current spatial position of the first structure and a current size and proportions of the cross-sectional area are detectable by a measurement arrangement (22).