The shrapnel was apparently from an M79 grenade that Kerry launched a little too close to the boat. If the grenade was made from steel or iron, he couldn't be in the same room as an MRI if there were shapnel in him.
If memory serves, the shrapnel from a 40mm M-79 grenade consists mainly of steel wire.
Was that the first or third purple heart? The first caused a piece of rock to come back and strike him. The third supposedly was a piece of rice although that is a theory. I don't know about the second or what really happened with the third, so I just offered a comment with no context as to the actual events.