When it comes to X-ray for medical imaging purposes, it is important to mention that, as dick_freebird stated above, part of the emission generated (those of low energy) that does not surpass the human body towards the receiver is useless because they are absorbed by the body, and so they must be eliminated before reach the patient (or rather the 'victim').
In the case of bulb-based devices, for example, aluminum plates are often used, being the thickness of which proportional to the mass of the part of the body to be x-rayed, and is tabulated to turn easy the choice of the suited plate for each case. These plates have to be affixed to the front side of the equipment head, being easily distinguishable the groove where these plates have to be coupled; that is, when being X-rayed, instead of one seeing a glass surface, one would be seeing a metal surface.
However, it is notable that, at least here where I live, a significant part of the radiographic operators, do not use such accessories, and some of them do not even know what the purpose of such parts.