A scanning X-ray imaging device (100) is proposed. The device may be specifically adapted for mammography applications. The device comprises an X-ray source (110), an X-ray line detector (150), a field limiter arrangement (170) and an observation volume (135). The field limiter arrangement (170) comprises at least two variable shield plates (174, 175) enclosing an opening (176) and a displacing mechanism (177). The shield plates may be displaced with respect to a centre axis of an X-ray beam (115) by means of the displacing mechanism such as to vary at least one of a cross-section and a location of the opening (176) between the field plates (174, 175) through which at least a part of the X-ray beam (115) is transmitted. The shield plates may be displaced independently from each other by means of the displacing mechanism (177). Such device (100) may be operated to perform a scanning motion such as to scan at least portions of the X-ray beam through a region of interest within the observation volume (135) and, at the same time, control the displacing mechanism (177) such that the opening (176) between the field plates (174, 175) is first successively opened to a larger cross-section and then successively closed again to a smaller cross-section during the scanning motion. Thereby, an X-ray dose level and an X-ray exposure time may be kept constant throughout an image acquisition procedure and an overall dose level may be minimized.