It's the MRI (Magnetic Resonance Imaging) I thought just the patient needed to be free of any metallic objects. But I just found this interesting tidbit of information: Metal objects can become dangerous projectiles if they are taken into the scan room. For example, paperclips, pens, keys, scissors, hemostats, stethoscopes and any other small objects can be pulled out of pockets and off the body without warning, at which point they fly toward the opening of the magnet (where the patient is placed) at very high speeds, posing a threat to everyone in the room.