Abstract

X-ray source assembly includes an X-ray tube comprising a filament for emitting an electron beam, a grid electrode for focusing the electron beam from the filament and a rotating anode-target on which the electron beam impinges thereby to generate an X-ray. The X-ray tube is installed in a housing having an X-ray radiation port for emitting the X-ray emanated from the rotating anode-target. To the X-ray radiation port there is attached a collimator which is secured to a support arm. The housing is supported by the support arm and is made swingable about an actual focal spot formed of the electron beam on the anode-target. When the housing is swung, the axis of the collimator swings about the actual focal spot relative to the axis of the X-ray tube in the plane including the axis of the X-ray tube and the axis of the X-ray radiation port, thereby changing the angle between the axis of the collimator and the axis of the X-ray radiation port. The size of the actual focal spot is varied by the grid electrode. Thus, said angle and the size of the actual focal spot determine the size of an effective focal spot, i.e. the actual focal spot as seen in the axial direction of the collimator.