X-rays are used instead of visible light because x-rays have shorter than visible light, allowing them to produce images of much greater detail.

X-rays have shorter wavelength than visible light. But that's hardly the reason that they're used for medical imaging. xrays have much higher frequencies then visible light which means they have much greater penetrating ability. with xrays you can see inside the body. you can't do that with a visible flashlight no matter how bright and powerful it is.