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Detection of plastic BBs on CT scanning of the orbit

Published:December 12, 2017DOI:https://doi.org/10.1016/j.jcjo.2017.10.019
      A 25-year-old male was referred to our ophthalmology service after accidentally shooting himself in the right periocular area with a BB pellet gun. According to the patient, the gun had fired a single spherical, red plastic BB pellet approximately 5 mm in diameter. He experienced immediate decreased vision and floaters and was initially seen in a peripheral emergency department. He did not recover the BB. A computed tomography (CT) scan of the orbits performed at the peripheral hospital was reported as negative for a foreign body.
      On ophthalmological assessment, the patient had a small entry wound through the superomedial eyelid above the level of the eyelid crease. Visual acuity was 20/30 in the affected eye. A microhyphema was present, as well as retinitis sclopetaria in the superonasal retina. The remainder of the examination was normal.
      Films from the CT were reviewed with our on-site neuroradiologist, who determined that the suspicious periorbital hypodensities at the level of the wound were in keeping with air trapped under the eyelid rather than plastic on the basis of Hounsfield units (Fig. 1A). Nonetheless, due to the concerning history and clinical findings, we opted to explore the wound and were able to extract a red 5-mm plastic BB approximately 1 cm deep in the wound (Fig. 1B). The patient was followed closely and made an excellent recovery.
      Fig. 1
      Fig. 1(A) Axial computed tomography (CT) of the right orbit using thin 0.7 mm slices at the level superior orbit (site of entry wound), demonstrating multiple hypodensities (white arrows) with Hounsfield units thought to be most consistent with air (approximate averages of −600 HU). (B) Extracted red plastic BB pellet approximately 1 cm deep to the wound (wound shown with white arrow). (C and D) Sagittal and axial CT scans of multiple BB pellets submerged in a cup of water. Note the black hypodense core representing air (green arrow; approximately −600 HU) and dark gray outer plastic shell (white arrow; approximately −50 HU).
      Common intraorbital foreign bodies include metal, glass, plastic, and wood. A high index of suspicion is crucial in the setting of projectile trauma. CT scan of the orbits with thin slices is the preferred initial investigation and can detect most foreign bodies >1 mm with the exception of wood or other organic material, which may be more apparent on magnetic resonance imaging (contraindicated unless a metallic foreign body is ruled out).
      • Fulcher T.P.
      • McNab A.A.
      • Sullivan T.J.
      Clinical features and management of intraorbital foreign bodies.
      • Javarashid R.
      • Golamian M.
      • Shahrzad M.
      • et al.
      Visibility of different intraorbital foreign bodies using plain radiography, computed tomography, magnetic resonance imaging, and cone-beam computed tomography: an in vitro study.
      The Hounsfield unit is a quantitative numerical scale to describe CT radiodensity and may help to differentiate between different types of foreign bodies and normal anatomy. Hypodense materials include air (−1000 HU), wood (−150 to −464 HU), and plastic (−47 to −100 HU) and hyperdense materials include iron (>3000 HU) and glass (947–1300 HU).
      • Lagalla R.
      • Manfre L.
      • Caronia A.
      • Bencivinni F.
      • Duranti C.
      • Ponte F.
      Plain film, CT and MRI sensibility in the evaluation of intraorbital foreign bodies in an in vitro model of the orbit and in pig eyes.
      • Choi H.J.
      • Lee H.J.
      • Kang S.G.
      The clinical significance of Hounsfield number of metallic and non-metallic foreign bodies in the soft tissue.
      The BB pellet in our patient was later found to be composed of a thin plastic shell with a hollow core. Using a CT scanner at our institution, we examined the CT characteristics of the BB pellets in an artificial setting submerged in water (Fig. 1C and D). The outer shell of the pellets measured approximately –50 HU, consistent with plastic. Of note, fat has a similar radiodensity, ranging from –100 to 0 HU.
      • Vogl T.
      • Reith W.
      • Rummeny E.J.
      The hollow core measured approximately –600 HU, approximating air. The BB pellet in our patient may have been embedded in preseptal fat, with the plastic shell blending in imperceptibly with the surrounding tissue while the hollow core mimicked air trapped in the fornix (a normal finding in 22% of CTs).
      • Patel B.C.
      • Elahi E.
      • Anderson R.L.
      • Harnsberger H.R.
      “Pneumofornix” (air under the eyelid): a normal finding.
      This particular appearance would have made it difficult to detect the BB on CT scanning, despite a rather large size of 5 mm, especially in the presence of normal fornix air pockets.
      This case highlights the importance of a thorough history and clinical examinations in evaluating patients with suspected intraorbital foreign bodies. Importantly, foreign bodies may be undetectable with specific imaging techniques, depending on their size, shape, and composition. Classically, CT scanning is thought to be adequate for detecting BB pellets in the orbit; however, the case described illustrates that specific BB designs with central hollow cores may appear as air and be missed. Although not all foreign bodies in the orbit require removal, it is important from a medicolegal perspective to detect and document foreign bodies at presentation so that appropriate referral and/or follow-up can be performed.

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