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To scan or not to scan. (Technical Query).

Magnetic resonance (MR) imaging is a state-of-the-art, noninvasive imaging modality widely used in clinical diagnosis. During an MR examination, the patient is exposed to 3 different forms of electromagnetic radiation: radiofrequency (RF), static magnetic field and gradient magnetic fields. Generally, the damaging biological effects of these fields are thermal, with localized tissue heating, and nonthermal, such as muscle and nerve stimulation. Both effects are considered negligible. (1) However, if a patient has intraocular foreign bodies with ferromagnetic properties, these fields may cause adverse bioeffects that may result in serious ocular injury and even blindness. This case demonstrates the importance of obtaining a thorough patient history and, when indicated, additional radiographs or computed, tomography (CT) scans to rule out the presence of a foreign body before an MR study.

Problem

A patient presented to the Fort Harrison Veteran Administration Hospital for a scheduled MR appointment. From the patient history questionnaire, it was noted that the patient had a previous history of occupational welding and metal grinding exposure. The radiology department policy requires a radiograph (Waters method) of the skull to evaluate the orbits for possible metal fragments. On reviewing the radiograph, a radiopaque foreign body was noted in the left orbit. (See 1.)

[FIGURE 1 OMITTED]

Additionally, a lateral projection was ordered to verify the position of the foreign body. The lateral projection was not useful for pinpointing the object's location; thus, a limited CT scan of the orbits was deemed necessary.

CT scans were obtained in both the axial and sagittal planes. Axial CT images revealed a foreign object within the left orbit. (See Fig. 2.) However, the sagittal images showed that the metallic fragment was embedded in the patient's left eyelid, and not within the eye itself. (See Fig. 3.)

[FIGURES 2-3 OMITTED]

On physical examination by the radiologist, the metal fragment could be seen through the skin of the upper eyelid. Once the location of the fragment was determined, the next question was whether to perform the MR scan.

Solution

The potential risk of intraocular damage was avoided due to the fragment's location in the skin of the eyelid as seen in the sagittal images. (See Fig. 2.) The patient was able to have his MR examination without any complication. Potential risks for patients with eyelid metallic foreign bodies include local tissue heating and foreign body fragment migration. Fortunately, previous studies on animal subjects with carbon steel implants in the eyelids have shown no measurable displacement or migration, as well as no adverse tissue effects when subjected to the magnetic field of 1.5 Tesla systems. (3)

References

(1.) Shellock FG. Bioeffects and safety considerations. In: Atlas SW, ed. Magnetic Imaging of the Brain and Spine. New York, NY: Raven Press; 1991.

(2.) Bowman RW, Ta CN. Hyphema caused by a metallic intraocular foreign body during magnetic resonance imaging. Am J Ophthalmol. 2000:129 (4):533-534.

(3.) Marra S, Leonetti JP, Konior RJ, Raslan W. Effects of magnetic resonance imaging on implantable eyelidweights. Ann Otol Rhinol Laryngol. 1995;104(6):448-452.

Technical Query is a troubleshooting column that covers image acquisition and processing. This issue's column is by Krys Geissler, M. Ed., R.T.(R)(M). Ms. Geissler is employed by the Brookwood/Leeds Family Health Center in Leeds, Ala.

Submissions for the "Technical Query" column are welcomed and should be mailed to Radiologic Technology, c/o "Technical Query," 15000 Central Ave. SE, Albuquerque, NM 871233917.
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Author:Geissler, Krys
Publication:Radiologic Technology
Date:Jan 1, 2003
Words:568
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