Final answer:
Focal-spot blur in X-ray imaging is greatest towards the cathode end due to the geometry of the anode and the electron beam path, leading to greater absorption and a larger apparent focal spot size at the heel end. Positioning the electron filament at the focal point of the reflector can help minimize this blur.
Step-by-step explanation:
The question addresses the phenomenon of focal-spot blur in X-ray imaging, specifically the fact that focal-spot blur is greatest towards the cathode end of the X-ray beam. This effect arises because as the angle of the X-ray beam with respect to the anode increases, the effective focal spot size appears larger due to the geometry of the X-ray tube. The cathode is the source of electrons that when accelerated and strike the anode create X-rays. The anode typically has a beveled edge, causing the X-rays emitted at the heel (cathode side) to undergo greater absorption than those emitted at the anode side, contributing to the heel effect, where there is a variation in intensity and the focal spot size appears larger as one moves towards the cathode end. To minimize the blur and optimize image quality, the filament (source of electrons within the cathode) should be positioned at the focal point of the reflector, ensuring that the X-rays produced are as parallel as possible.