Abstract: In a scanning radiographic apparatus in which the object of the scanning is to project the radiation through a substantially planar region of a body along many linear paths, means is provided for monitoring the progress of the scanning and producing timing signals indicative thereof. The timing signals are used to determine, at least in part, the widths of the aforementioned beams and a characteristic, for example the frequency in the event that the signals comprise pulses, is changed to permit the effective width of the paths to be changed. This can be done by means of graticules having differently pitched markings, for example, and each graticule may be associated with a respective range of body sizes.