For the purposes of this question, I have limited "dangerous" to refer to the attraction/movement of metal objects by the main magnetic field (Bo). Interaction effects of metals with gradient and RF magnetic fields due to electrical conductivity are discussed elsewhere.
Iron and its alloys are the most commonly used ferromagnetic materials. Although the Latin root "ferro-" refers to "iron", several non-iron metals can also possess ferromagnetism — cobalt, nickel, chromium, manganese, and various rare earth elements.
- Martensitic (ferritic) stainless steels are highly ferromagnetic due to their body-centered cubic crystal structure. Typified by the 400 Series, these hardened stainless steels are widely used for medical and surgical equipment.
- Austenitic stainless steels contain 8-14% nickel, producing a face-centered cubic crystal structure, rendering them essentially "non-magnetic". In 1990 the US Food and Drug Administration (FDA) required all steel medical implants to be made of austenitic 300 Series stainless steel. The low-carbon type 316L is commonly used for stents, wires, plates, clips, and screws.
Permanently magnetized components are found in certain implants, including dentures, maxillofacial prostheses, hearing devices, tissue expanders, artificial sphincters, and orthopedic rods. Because of their strength-to-size ratio and biocompatibility, nearly all of these are "rare-earth" magnets, either Samarium-cobalt (SmCo5) or Neodymium alloy (Nd2Fe14B). Some dental and facial devices utilize older and less powerful Alnico (Al-Ni-Co) magnets. Permanent magnets will experience torque and/or displacement in the MRI field. The danger associated with magnets, as with all ferromagnetic materials, depends on how well they are affixed to the surrounding tissues.
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Food and Drug Administration. Safe Medical Devices Act (SMDA). Section on: orthopedic, prosthetic, and surgical appliances and supplies. Rockville, MD: National Press Office; 1990. (SIC 3842)
Hermawan H, Ramdan D, Djuansjah JRP. Metals for biomedical applications. In: Fazel R (ed). Biomedical Engineering - From Theory to Applications, Intechopen.com, 2011: 411-430. [DOI Link]
International Stainless Steel Forum. The Stainless Steel Family. Brussels, Belgium. Downloaded from www.worldstainless.org on 1/1/20.
Jackson DP. Dancing paperclips and the geometric influence on magnetization: a surprising result. Am J Phys 2006; 74:272-279. [DOI Link]
"Magnetic Susceptibility." Wikipedia, The Free Encyclopedia.
Schenck JF. Safety of strong, static magnetic fields. J Magn Reson Imaging 2000; 12:2-19. [DOI Link]
Schenck JF. The role of magnetic susceptibility in magnetic resonance imaging: MRI magnetic compatibility of the first and second kinds. Med Phys 1996;23:815-850. (Slightly dated, but an excellent and enduring explanation of susceptibility from a pioneer in MRI and first inductee in GE's Genius Hall of Fame). [DOI Link]
What is magnetic susceptibility?
What is ferromagnetism?
I don't understand how an object's shape affects its magnetization.
Is the MRI field strong enough to magnetize a piece of metal?
If steels and permanent magnets are potentially dangerous, which metals are "safe" around the MRI field?