CHAPTER 2
Neuroimaging in Neuro-Ophthalmology
Ophthalmologists have a variety of neuroimaging modalities at their disposal in the diagnosis and management of ocular, orbital, and intracranial diseases. Three broad clinical questions should be addressed before selecting the appropriate neuroimaging technique:
1.What is the clinical presentation?
2.Where is the lesion resulting in the clinical presentation?
3.What are the potential etiologies of the lesion?
The choice of imaging modality and the timing of the study should be based on the
differential diagnosis
type of lesion suspected (eg, vascular) patient characteristics
age
weight
comorbidities
issues with claustrophobia
possible allergies
presence of metallic foreign bodies or medical devices (eg, cardiac pacemaker) availability of imaging modalities to match the clinical urgency of the condition
In general, most neuro-ophthalmic diseases are best assessed with magnetic resonance imaging (MRI) of the brain and orbit using contrast and fat-saturation techniques. If a vascular abnormality is suspected, magnetic resonance angiography (MRA) or magnetic resonance venography (MRV) can be added.
The importance of open communication between the ophthalmologist and radiologist cannot be stressed enough, not only in selecting the best imaging modality for the patient but also when interpreting the study results. Several types of prescriptive or interpretive errors may occur when ordering and interpreting an MRI:
Prescriptive errors include
failure to apply a dedicated study
inappropriate use of a dedicated study
omission of intravenous contrast material
omission of specialized sequences Interpretive errors include
failure to detect a lesion due to misleading clinical information
rejection of a clinical diagnosis because an expected imaging abnormality is absent the assumption that a striking imaging abnormality accounts for the clinical abnormality failure to consider the lack of clinical specificity of an imaging abnormality
This chapter discusses the 2 most common neuroimaging techniques used in neuro-ophthalmology clinical practice: computed tomography (CT; Fig 2-1) and MRI (Fig 2-2). The use of plain (x-ray) films in the evaluation of neuro-ophthalmic diseases has largely become obsolete, except to evaluate for a metallic foreign body, and is not discussed. A glossary of neuroimaging terminology appears at the end of the chapter.
Figure 2-1 Computed tomography (CT) scans. Axial (A) and coronal (B) orbital views of a healthy subject. (Courtesy of Rod
Foroozan, MD.)
Figure 2-2 Brain and orbital MRI scans showing anatomical visual and orbital structures from the chiasm to the anterior orbit. Note the left eyeball is abnormal but not pertinent to the objective of this figure. Axial T1-weighted (A), coronal T1weighted (B–D), coronal T2-weighted with fat saturation (E), and coronal T1-weighted (F) images. ACF = anterior cranial fossa; Ant segment = anterior segment; ICA = internal carotid artery; IO = inferior oblique muscle; IR = inferior rectus muscle; LR = lateral rectus muscle; Lev P = levator palpebrae superioris muscle; MCF = middle cranial fossa; MR = medial rectus muscle; olf fossa = olfactory fossa; SO = superior oblique muscle; sph sinus = sphenoid sinus; sph wing = sphenoid wing; SR = superior rectus muscle; temp lobe = temporal lobe; vit = vitreous. (Courtesy of M. Tariq Bhatti, MD.)
Jäger HR. Loss of vision: imaging the visual pathways. Eur Radiol. 2005;15(3):501–510. Epub 2005 Jan 26.
Mafee MF, Karimi A, Shah JD, Rapoport M, Ansari SA. Anatomy and pathology of the eye: role of MR imaging and CT. Magn Reson Imaging Clin N Am. 2006;14(2):249–270.
Vaphiades MS. Imaging the neurovisual system. Ophthalmol Clin North Am. 2004;17(3):465–480, viii.
Wolintz RJ, Trobe JD, Cornblath WT, Gebarski SS, Mark AS, Kolsky MP. Common errors in the use of magnetic resonance imaging for neuro-ophthalmic diagnosis. Surv Ophthalmol. 2000;45(2):107–114.
Computed Tomography
Computed tomography (CT) uses computer (digital) processing to generate a 2-dimensional or 3- dimensional image from a series of 2-dimensional x-ray (sectional) images. By manipulating (“windowing”) the data, various structures can be visualized. The orbit is particularly suited to x- ray–based imaging because fat provides excellent contrast to the globe, lacrimal gland, optic nerve, and extraocular muscles. Bone and other calcium-containing processes can be visualized easily because of their marked x-ray attenuation. Soft-tissue details can be further enhanced by the injection of iodinated contrast material, which crosses a disturbed blood–brain barrier to accumulate within a local lesion and reveals inflammatory and neoplastic processes.
Tremendous advancements in CT technology and techniques have occurred since the introduction of CT into clinical practice in the early 1980s. Compared with the single-step imaging technique of first-generation CT scanners, most modern CT scanners image the patient with simultaneous sectioning (ie, spiral or helical) and have multiplanar reconstruction capabilities using a volumerendering method known as maximum intensity projection. The advantages of CT include relatively low cost, rapid image acquisition, wide availability, and excellent spatial resolution. The speed of CT and its ability to identify acute blood or bone abnormalities accurately make this technique especially useful in trauma, which may involve a confused or combative patient, as well as when identification of hemorrhage or bony abnormalities is crucial. The advantages and disadvantages of CT are summarized in Table 2-1.
Table 2-1