The alien was shot earlier, yet no bullet can be seen on the x-ray scan.
One can see the bullet wound patch that was placed on Sanders' chest. It's round and doesn't match the pigment of his skin.
After the doctor checks Sanders' wound there is a close up shot of his chest and the bullet hole in his shirt is missing.
The x-ray device appears to simply be a glass plate; far more equipment than that is needed to project onto film an image generated by x-radiation.
(at around 29 mins) As two security men from the Soviet diplomatic mission break into Dr. Serret's laboratory to rescue her, and one of the men runs toward the window, the camera pans to follow him: As it does so, it captures the reflection of two studio lights in the screen of the X-ray device.