Band Aid Covers the Bullet Hole - S2-E21
Corrected entry: Derek tells Meredith to do an MRI on the lady that has a fork stuck in her neck. But an MRI-machine is actually a huge magnet; that's why there's absolutely no metal allowed inside of it. Doing an MRI on someone with a fork stuck in her neck is extremely dangerous.
Correction: Not all metal is magnetic so "absolutely no metal allowed" is incorrect. People with metal hardware in their bodies have MRI's all the time. Cutlery is sometimes magnetic but this could be checked with a small magnet first. An MRI would then be the best type of scan for this situation if it was not magnetic.