I don't think a sophisticated technology will produce an implant that can be easily discovered by X ray. I came up with a much better idea for an alien implant.
My idea of the implant is a millions of nanomachines. They look like silver serum in naked eyes. They are injected through the blood vein and they swim up to the brain. Once they reach the brain they swim to the right spots and attatch themselves on neurons. They are scattered around the brain like cloud. Once they are 'installed', they communicate each other with electro-magnetic wave and obtain the power from the body temperature. So, they work like a one massive cluster of microscopic computers. They can augment the brain function and even help the person communicate others with the same kind of implant.
I am no science expert. But someone could have come up with this before.
