Yesterday, a 60-year-old grandfather came to my office and asked me to remove the many metal crowns in his mouth, on the grounds that the patient had a brain infarction and needed to undergo an MRI, and many tertiary hospitals required him to remove the metal crowns in his mouth, otherwise he would not be examined. This incident touched me a lot, because now the crowns that have no effect on MRI are all-ceramic crowns, which are generally more expensive; while the price of metal-containing crowns (excluding precious metals) is generally easier to be accepted by universal patients. Therefore, in the clinic, the use of metal-containing crowns is still very widespread. Whenever we are asked by patients how much impact metal crowns have on MRI, I believe that many doctors will have a weak heart because, if we are not professional in radiology and materials, our knowledge base of how much impact each kind of metal for oral use has on MRI is still very lacking. Therefore, after reviewing the relevant literature, I came across the following interesting findings. First, the principle of MRI: MRI is a biomagnetic spin imaging technique, in which the “nucleus” is the nucleus of a hydrogen atom. Because the human body is composed of about 70% water, MRI examinations use hydrogen atoms throughout the body to be excited by radiofrequency pulses in an applied strong magnetic field, resulting in the phenomenon of nuclear magnetic resonance, and after spatial encoding technology, the detector detects and accepts the MRI signal in the form of electromagnetic discharge, which is input into a computer, and after data processing and conversion, the form of the human body tissues is finally formed into images for diagnosis. Second, the principle of metal-influenced MRI: the artifacts caused by metal materials are divided into ferromagnetic metal artifacts and non-ferromagnetic metal artifacts, and the size of the artifacts produced is related to the magnetization rate and magnetic field strength of the metal materials, the larger the magnetization rate and the higher the magnetic field strength, the larger the artifacts produced. Metal restorations in the oral cavity have effects involving the cranial, cervical, facial, and inner ear areas during MRI. What are the differences between the various metals used in the oral cavity and their effects on MRI? A study showed that gold alloys had the least impact, soft cobalt-chromium alloys the second and hard cobalt-chromium alloys the greatest for the same imaging sequence. The same metal produces different artifacts in different imaging sequences. It is recommended that when only metal crowns can be used for oral fixed restorations, priority should be given to precious metal crowns. When cobalt-chromium alloy crowns must be chosen, the MRI examiner needs to choose a reasonable imaging sequence (choose a spin-echo sequence and avoid using a planar echo sequence). Gold, platinum alloys, silver, and silver amalgam have minimal effect on MRI; pure titanium also produces relatively small artifacts, while cobalt-chromium and nickel-chromium alloys have a greater effect on MRI. The position and size of the metal restoration in the mouth also have an effect on the MRI artifacts. The artifact size is twice as large as the proximal and distal median diameter of a metal crown and bridge and four times as large as its buccolingual diameter. Therefore, it is recommended that when the oral fixed restoration, priority be given to all-ceramic restorations, and when metal crown and bridge have to be used, priority be given to precious metals (gold alloy, gold-platinum alloy, etc.), followed by pure titanium metal, followed by titanium-containing alloys, and finally cobalt-chromium and nickel-chromium alloys. Even if you choose precious metals, it is still appropriate to have individual single crowns and avoid multi-unit crown and bridge restorations.