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The Orbit and Optic Nerves

Donald William Chakeres¹, Eric C. Bourekas¹

¹Ohio State University College of Medicine and Public Health, Columbus, Ohio

Publication Name:

Current Protocols in Magnetic Resonance Imaging

Unit Number:

Unit A7.5

DOI:

10.1002/0471142719.mia0705s00

Online Posting Date:

May, 2001

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Abstract

Magnetic Resonance Imaging (MRI) of the orbits and optic nerves has developed into the “gold standard” of imaging modalities for the evaluation of many soft tissue abnormalities of the orbit. Computed tomography (CT) remains the modality of choice for evaluation of the bony structures of the orbit. MRI is also more flexible, allowing for multiplanar imaging that is not possible with CT. This unit presents a basic protocol for imaging of the orbit. An alternate protocol is presented for the case of dedicated high-resolution surface coil orbital study.

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Unit Introduction
Basic Protocol: Imaging of the Orbits and Brain
Alternate Protocol: Dedicated High-Resolution Surface Coil Orbital Study
Commentary
Literature Cited
Figures
Tables

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Materials

Basic Protocol: Imaging of the Orbits and Brain

Materials

Normal saline (0.9% NaCl), sterile
Extravascular contrast agent (e.g., Magnevist, Omniscan, or Prohance)

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Figures

Figure A7.5.1

Magnetic susceptibility artifacts. Image (A) is a transverse T₂-weighted image. Image (B) is a transverse proton density image. Both images demonstrate magnetic susceptibility artifact from mascara distorting the globes. There is loss of signal and “smearing” of the image.

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Figure A7.5.2

Sagittal T₁-weighted image phase encoded artifacts (ghosting). This is a sagittal image of the orbit demonstrating a choroidal melanoma. Note the phase encoding artifact (arrows) of the muscle cone. Since the globe cannot be fixed, motion artifacts are common. This patient demonstrates a sclera band and an invasive melanoma of the choroids along the superior posterior portion of the globe.

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Figure A7.5.3

High-resolution transverse T₁-weighted enhanced image. This is a 3-D sequence and can be reformatted in any orientation since the image thickness is only 1.5 mm and the matrix is 512. This sequence is used to create high-quality-resolution images. The higher the initial quality of the image, the better the reconstruction. This sequence is optimal for evaluation of cranial nerves intracranially, prior to their entering the orbit. The image is similar to a T₁-weighted spin echo sequence, but much higher resolution. Note the good definition of the gray and white matter, the cerebrospinal fluid (CSF) spaces, and the blood vessels.

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Figure A7.5.4

Transverse T₁-weighted contrast enhanced image of the head with fat suppression. Note the intense normal contrast enhancement of the extraocular muscles and the lack of signal from the orbital fat which allows for evaluation of the orbital contents. The suppression of the fat increases the signal intensity differences between the contrast enhanced structures and the other orbital structures, particularly the optic nerve. Without fat saturation the contrast enhancement is “lost” in the fat signal. Imaging the entire head also allows for the evaluation of intracranial structures, which may account for vision changes (occipital lobe lesions).

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Figure A7.5.5

Coronal T₁-weighted fat saturation images obtained with a surface coil. Image (A) is centered just behind the globe. Note the lack of enhancement of the optic nerve, in contrast to the extraocular muscles. Image (B) is centered through the globe and allows detailed imaging of the globe. Note the intense enhancement of the nasal cavity mucosa.

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Literature Cited

Literature Cited
	Amano, Y., Amano, M., and Kumazaki, T. 1997. Normal contrast enhancement of extraocular muscles: Fat-suppressed MR findings. AJNR Am. J. Neuroradiol. 18:161-164.
	Anzai, Y., Lufkin, R.B., Jabour, B.A., and Hanafee, W.N. 1992. Fat-suppression failure simulating pathology on frequency-selective fat-suppression images of the head and neck. AJNR Am. J. Neuroradiol. 13(3):879-884.
	Atlas, S.W., Bilaniuk, L.T., and Zimmerman, R.A. et al. 1987. Orbit: Initial experience with surface coil spin-echo imaging at 1.5 T. Radiology 164:501-509.
	Barakos, J.A., Dillon, W.P., and Chew, W.M. 1991. Orbit, skull base and pharynx: Contrast enhanced fat suppression MR imaging. Radiology 179:191-198.
	Herrick, R.C., Hayman, L.A., Taber, K.H., et al. 1997. Artifacts and pitfalls in MR imaging of the orbit: A clinical review. Radiographics 17(3):707-724.
	Kelly, W.M., Paglen, P.G., Pearson, J.A., et al. 1986. Ferromagnetism of intraocular foreign body causes unilateral blindness after MR study. AJNR Am. J. Neuroradiol. 7:243-245.
	Lagouros, P.A., Langer, B.G., Peyman, G.A., et al. 1987. Magnetic resonance imaging and intraocular foreign bodies. Arch. Ophthalmol. 105:551-553.
	Mafee, M.F., Ainbinder, D., Afshani, E., and Mafee, R. 1996. The eye, imaging of the globe, orbit, and visual pathways. Neuroimaging Clin. North Am. 6:29-59.
	Otto, P.M., Otto, R.A., Virapongse, C., et al. 1992. Screening test for detection of metallic foreign objects in the orbit before magnetic resonance imaging. Invest Radiol 27:308-311.
	Sacco, D.C., Steiger, D.A., Bellon, E.M., et al. 1987. Artifacts caused by cosmetics in MR imaging of the head. AJR Am. J. Roentgenol. 148:1001-1004.
	Shellock, F.G. 1996. Pocket Guide to MR Procedures and Metallic Objects. Lippincott-Raven, Philadelphia.
Key References
	Mafee et al., 1996. See above.
This reference is a general review article focused on imaging of the orbit and visual system.