MRI for metallic foreign bodies?

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In their excellent study, Lakits et al 1Lakits A. Prokesch R. Scholda C. et al.Multiplanar imaging in the preoperative assessment of metallic intraocular foreign bodies. Helical computed tomography versus conventional computed tomography.Ophthalmology. 1998; 105: 1670-1685Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar clearly demonstrate the need for intricate preoperative imaging methods for actual or presumed intraocular metallic foreign bodies. Helical computed tomography (CT) and its resulting three-dimensional reconstruction are an elegant means to assess the location and damage done by a metallic foreign body. However, we wonder why magnetic resonance imaging (MRI) is usually regarded to be of limited value in these cases. If a metallic foreign body does not have ferromagnetic properties, MRI offers quite a few advantages over any CT scan technique.2Journeaux S.F. Cummins B.H. Lewis T.T. Wild A.M. Paraplegia following a rivet gun injury the use of magnetic resonance imaging in the management.Br J Neurosurg. 1992; 6: 75-78Crossref PubMed Scopus (4) Google Scholar, 3Ebraheim N.A. Savolaine E.R. Jackson W.T. et al.Magnetic resonance imaging in the evaluation of a gunshot wound to the cervical spine.J Orthop Trauma. 1989; 3: 19-22Crossref PubMed Scopus (17) Google Scholar There is no radiation dose delivered to the lens. Bone windows in CT minimize streak artifacts due to the metal but do not allow good soft tissue differentiation. Soft tissue windows in CT, in contrast, have huge streak artifacts and impede good localization of the foreign body. Both problems do not exist in MRI, which offers excellent soft tissue resolution and no metal-induced streak artifacts. So if you want to use the enhanced imaging properties that are offered by MRI, how do you exclude ferromagnetic properties of the foreign body that could cause the metal to move during examination?4Kelly W.M. Paglen P.G. Pearson J.A. et al.Ferromagnetism of intraocular foreign body causes unilateral blindness after MR study.AJNR Am J Neuroradiol. 1986; 7: 243-245PubMed Google Scholar, 5Teitelbaum G.P. Metallic ballistic fragments MR imaging safety and artifacts.Radiology. 1990; 177 ([letter].): 883Crossref PubMed Scopus (15) Google Scholar It is often possible to find either a part of the metallic foreign body that entered the orbit or locate a comparable one, as for example in shot gun injuries when the projectile is known. In addition to offering a relatively easily accessible comparsion item, many projectiles are not ferromagnetic because they mostly consist of lead with sources of antimony, bismuth, and copper. The fact that there are some projectiles that contain steel or iron should, however, not be disregarded. If an available comparison item, identical in composition to the one that caused the patient’s injury, is placed on the examiner’s hand into the coil of the MRI during an examination and it does not move, the foreign body in question will not move during the patient’s examination. A shot gun injury in which the refined soft tissue resolution of MRI helped to localize the projectile in question as outside of the globe illustrates the point (Figure 1, Figure 2).Figure 2Sagittal T1-weighted MRI SE sequence displaying the same metallic foreign body without any artifacts. Localization of foreign body outside the muscle cone is clearly demonstrated. Note absence of associated soft tissue damage.View Large Image Figure ViewerDownload Hi-res image Download (PPT) In summary, if proper caution is used to distinguish ferromagnetic properties of the foreign metallic body before examination, MRI offers superior localization possibilities and additional information regarding associated soft tissue damage than even multiplanar CT imaging. MRI for metallic foreign bodies?: authors’ replyOphthalmologyVol. 106Issue 7Preview Full-Text PDF