Dr. Wang: Yesterday, I accompanied my father, who was hospitalized, to have an MRI. While waiting in line, the doctor in charge suddenly remembered that my father had been shot during the war and still had shrapnel in his body. After hearing this, the MRI doctor refused our examination. Why was this? Reader Lee: The doctor refused to let your father have an MRI because of the presence of a bullet in his body. The MRI machine is like a big magnet, and the magnet generates a magnetic field. For an MRI, the body needs to be placed in a strong magnetic field. When metal is close to it, it may be attracted by the large magnet and change the position it was in. At the same time, the metal may heat up, increasing the local temperature and burning the tissue around it. Therefore, when a patient has metal or metal implants (most commonly shrapnel, iron sand, contraceptive rings, artery clips, artificial joints, screw caps, etc.), the metal may move under the action of a strong magnetic field, cutting through normal tissue and damaging the body, even causing life-threatening injuries. In addition to posing a threat to the patient’s body, metal objects also have an impact on the quality of the images obtained from the examination. Due to the presence of metal, the signal used to form the image is disturbed, which can lead to unclear images and make it difficult for the doctor to make a correct diagnosis.