MRI Screening for Orbital Foreign Body

Screening Patients and Individuals with Metallic Foreign Bodies

All patients and individuals with a history of being injured by a metallic foreign body such as a bullet, shrapnel, or other type of 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 magnetic field of the MR system. In addition, heating may occur, although this tends to only happen if the object forms a resonant conductive loop. 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 of the MR system.

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. In the past, any individual with a suspected orbital foreign body was required to have film radiographs of the orbits to determine the presence of a metallic fragment prior to exposure to the MR environment. Thus, screening plain films of the orbits were deemed necessary for every individual with a history of a known intraocular or periorbital foreign body or when a patient had a history of exposure to potential metallic ocular injury (e.g., welders, grinders, metal workers, sculptors, etc.). This was considered the standard of care to prevent serious injuries to the eye associated with the MR environment. However, based on an investigation by Seidenwurm et al., new guidelines for radiographic screening of patients with suspected metallic foreign bodies have been proposed and implemented in the clinical MR setting. However a study by Seidenwurm et al. decided if a patient reports injury from an ocular foreign body that was subsequently removed by a doctor or that resulted in negative findings on any examination, we perform MR imaging. Those persons with a history of injury and no subsequent negative eye examination are screened radiographically. Of note is that Seidenwurm et al. performed approximately 100,000 MRI procedures under this protocol without incident. Thus, an occupational history of exposure to metallic fragments, by itself, is not sufficient to mandate radiographic orbital screening. Therefore, current practice guidelines for foreign body screening should be altered in consideration of this new information and because radiographic screening before MRI procedures on the basis of occupational exposure alone is not cost effective. Furthermore, it is not clinically necessary.

Clinical Screening Protocol

This involves asking patients whether they have a high-risk occupation and whether they have had an ocular injury. If they sustained an ocular injury from a metallic object, they are asked whether they had a medical examination at the time of the injury, and whether they were told by the doctor, "It's all out." If they did not have an injury, if they were told their ophthalmologic examination was normal, and/or if the foreign body was removed at the time of the injury, then they proceed to MR imaging, as scheduled.

Radiographic Screening Protocol

Based on the results of the clinical screening protocol, patients are screened radiographically if they sustained an ocular injury related to a metallic foreign object and they were not told their post-injury eye examination was normal. In these cases, the MR examination is postponed and the patient is scheduled for screening radiography.

Shellock R & D Services, Inc. and Frank G. Shellock, Ph.D.