Metal blocks some x-rays but not all. The more x-rays you have, the more pass through. When doctors are xraying people, they use as little xrays as they need

Metal blocks some x-rays but not all. The more x-rays you have, the more pass through. When doctors are xraying people, they use as little xrays as they need to get a good image of the fleshy bits, so metal objects appear opaque relative to that.