Metallic Foreign Bodies and Screening
A patient or individual with a history of being injured by a metallic foreign body such as a bullet, shrapnel, or other metallic object should be thoroughly screened and evaluated prior to admission to the area of the MR system. This is particularly important because serious injury may occur as a result of movement or dislodgment of the metallic foreign body as it is attracted by the powerful static magnetic field. In addition, excessive heating may occur, although this tends to happen only if the object is made from conductive material and has an elongated shape or forms a loop of a certain diameter.
The relative risk of injury is dependent on the ferromagnetic properties of the foreign body, the geometry and dimensions of the object, the strength of the static magnetic field, and the strength of the spatial gradient magnetic field of the MR system. Additionally, the potential for injury is related to the amount of force with which the object is “fixed” or retained within the tissue (i.e. counter-force or retention force from scarring or encapsulation may prevent migration of the metal) and whether or not it is positioned in or adjacent to a sensitive site of the body such as vital neural, vascular, or soft tissue structure.
The use of plain film radiography is the technique of choice recommended to detect metallic foreign bodies for individuals and patients prior to admission to the MR environment. This includes screening for the presence of metallic orbital foreign bodies (see Metallic Orbital Foreign Bodies and Screening). The sensitivity of plain film radiography is considered to be sufficient to identify any metal object with a mass large enough to present a hazard to an individual or patient in the MR environment.
Recent information from a report by Karacozoff and Shellock (2013) suggests that a special ferromagnetic detection system (Ferroguard Screener, Metrasens) may provide an additional means of guiding the decision making process regarding management of patients with metallic foreign bodies insofar as this device was successfully utilized to identify a ferromagnetic, armor-piercing bullet.
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