The bullet effect may occur if there is metal on the patient’s body when doing MRI, because most of the MRI instruments used in hospitals have a magnetic field strength of 0.5, 1.5 or 3.0 Tesla. 1 Tesla is equivalent to 10,000 gauss, which means that the magnetic field strength of the MRI instrument is about 30-60,000 times that of the earth’s magnet, which is equivalent to a large magnet. The powerful attraction will suck the metal on the patient’s body toward the instrument, and objects with ferromagnetic properties will be projected onto the MRI instrument like a bullet flying fast, which will cause serious equipment damage to the machine on the one hand, and may endanger the personal safety of the patient on the other. Therefore, patients must remove all the metals from their bodies before entering the MRI. If there are some metals that cannot be removed such as pacemakers, such patients are not able to do the MRI.