Beware of artifacts in orbital magnetic resonance imaging.

Abstract

Orbital magnetic resonance imaging (MRI) is plagued by many artifacts, that sometimes closely mimic pathology.[1] We review the most common artifacts in clinical practice.[2] 2.1 Motion artifacts Optic neuritis is characterized by T2 hyperintensity of the optic nerves [Fig. 1a]. Similar changes can also be produced by motion artifacts [Fig. 1b], which are recognizable by the associated blurring of nerve outlines and ghosting [Fig. 1b and c]. Repeating the scan with the patient fixing the gaze removes these artifacts [Fig. 1d].Figure 1: (a) Bilateral optic neuritis, (b) Eye motion during scanning causes motion artifacts, (c) Motion also produces ghosting and blurring, (d) Repeat MRI of the patient in Figure 1b during eye fixation2.2 Partial volume artifacts Signal changes similar to optic neuritis [Fig. 2a] can occasionally be seen in normal optic nerves [Fig. 2b]. This occurs when the nerves are tortuous and the MR section includes adjacent fat, which artifactually increases signal in the nerve [Fig. 2c]. Fat suppressed sequences [Fig. 2d] are immune to this artifact.Figure 2: (a) Right optic neuritis, (b) Similar changes in both optic nerves produced by partial volume artifact, (c) Genesis of partial volume artifacts, (d) Fat saturated scan shows normal optic nervesSimilar phenomena can affect the extraocular muscles. Extraocular muscles are enlarged and show T2 hyperintensity in disorders such as orbital myositis [Fig. 3a] and thyroid ophthalmopathy. However, even normally, the lateral rectus muscles can appear bulky and hyperintense to rest of the muscles on coronal images [Fig. 3b]. Oblique orientation of lateral rectus muscles to the coronal plane makes them more susceptible to this phenomenon [Fig. 3c].Figure 3: (a) Orbital myositis, (b) Bulky, hyperintense lateral rectus muscles in a normal individual, (c) Obliquity of lateral rectus makes it more prone for partial volume artifact2.3 Susceptibility artifact Signal changes produced by disorders affecting the inferior rectus muscle, such as myositis [Fig. 4a] can occasionally be seen on fat saturated sequences, in individuals with metallic dental implants [Fig. 4b]. Disturbance of the magnetic field along the floor of the orbit by dental implants or air in the maxillary sinus interferes with fat saturation and creates a false appearance of edematous changes [Fig. 4c]. The artefact is recognizable as it extends beyond the muscle boundaries and is worst inferiorly. A similar artefact can be seen when the patient uses eye makeup (mascara) containing metallic iron particles.Figure 4: (a) Inferior rectus myositis, (b) Susceptibility artefact due to dental implants (*) causes hyperintensity of all tissues along the orbital floor, (c) Magnetic field disturbance by the metallic dental implant extends into the orbital floor (dotted line)2.4 Chemical shift artifact The appearance of tumor deposits along the optic nerve [Fig. 5a] can be seen even in normal individuals [Fig. 5b] as the so-called “Chemical shift artifact.” This artifact is typically seen at the interface between fluid and fat containing structures (such as optic nerve and retroorbital fat) as a band of dark signal intensity on one side (along the frequency encoding direction). This is caused by “mis-mapping” (shift) of the signal from retroorbital fat, in relation to the perioptic fluid [Fig. 5c]. Fat suppressed sequences [Fig. 5d] are immune to this artifact.Figure 5: (a) Leukemic deposits on left optic nerve, (b) Curvilinear bands of bright and dark signal along superior and inferior aspects of optic nerves in a normal individual, (c) Illustration of chemical shift artifact, (d) Fat saturated scan in the same individual shows normal optic nervesThis artifact is also seen at the margin of fat containing structures surrounded by fluid (e.g., silicone oil in the globe or intracranial lipomas) and fluid containing structures surrounded by fat (e.g., cysts or hemangiomas within the retroorbital fat). Presence of this artifact gives vital clues in the diagnosis of these conditions. 2.5 Financial support and sponsorship Nil. 2.6 Conflicts of interest There are no conflicts of interest.