Figure 2-10 Axial MRI scan demonstrates the value of diffusion-weighted imaging in acute infarction. A, Fluid-attenuated inversion recovery image is normal except for nonspecific tiny foci of hyperintense signal in the deep white matter. B, Diffusion-weighted image reveals abnormal restricted diffusion (high-intensity signal) in the territory of the left middle cerebral artery. C, Low-intensity signal on the apparent diffusion coefficient map is consistent with an acute infarction.

(Courtesy of M. Tariq Bhatti, MD.)

Table 2-4

Mukherji SK, Chenevert TL, Castillo M. Diffusion-weighted magnetic resonance imaging. J Neuroophthalmol. 2002;22(2):118– 122.

Symms M, Jäger HR, Schmierer K, Yousry TA. A review of structural magnetic resonance neuroimaging. J Neurol Neurosurg Psychiatry. 2004;75(9):1235–1244.

Vascular Imaging

Several techniques are commonly used to image blood vessels; they are important because of the frequency with which ischemic processes affect the nervous system.

Catheter or Contrast Angiography

The gold standard for intracerebral vascular imaging remains catheter, or contrast, angiography (Fig 2-11). With this technique, a catheter is placed intra-arterially and iodinated radiodense contrast dye is injected. Digital subtraction angiography (DSA) is a technique that reduces artifacts by subtracting densities created by the overlying bony skull. The contrast dye outlines the column of flowing blood within the injected vessel and demonstrates stenosis, aneurysms, vascular malformations, flow dynamics, and vessel wall irregularities such as dissections or vasculitis. The procedure has an overall morbidity of approximately 2.5%, primarily related to ischemia from

emboli or vasospasm, dye-related reactions, or complications at the arterial puncture site (eg, hematoma). The use of digital subtraction technology has enhanced the ability to visualize vascular structures with smaller amounts of contrast dye.

Figure 2-11 Contrast angiogram, lateral view, demonstrates aneurysm of the posterior communicating artery (arrow). ACA = anterior cerebral artery; ICA = internal carotid artery; MCA = middle cerebral artery. (Courtesy of Rod Foroozan, MD.)

Magnetic Resonance Angiography and Magnetic Resonance Venography

Because an MRI signal requires detection of a brief period of excitation and decay, moving tissue often passes out of the plane of assessment before the return signal can be detected. This is the basis of the black “flow void” that characterizes MRI scans of vascular channels with flow. However, protons that are excited in one slice and then move to another may be specifically imaged using the 3- dimensional assessment technique that underlies MRA and MRV. Several techniques, such as 2- and 3-dimensional time-of-flight angiography, phase contrast angiography, and multiple overlapping thinslab acquisition (MOTSA), have been used to obtain images. MRA signals may also be obtained from gadolinium-enhanced images. This technique is particularly useful for imaging the proximal large vessels of the chest and neck. MRA with gadolinium contrast has a very short acquisition time,

making patient movement–related artifacts less of an issue. MRA provides excellent noninvasive information about largeand medium-size vessels (Fig 2-12). Because MRA depends on flow physiology, however, it tends to overestimate vascular stenosis, and the reduced image resolution limits the ability to visualize smaller vessels or vasculitis. MRV may be helpful in excluding thrombosis within the dural venous sinuses (Fig 2-13), a condition that may cause papilledema (see Chapter 14).

Figure 2-12 Magnetic resonance angiography (MRA) provides excellent noninvasive information about largeand medium-size vessels. A, MRA source image showing an abnormality at the junction of the posterior communicating artery and internal carotid artery (arrow). Volume rendered 3-dimensional image (B) and maximal intensity projection image (C) clearly show a posterior communicating artery aneurysm (arrows). (Courtesy of M. Tariq Bhatti, MD.)

Figure 2-13 Venous sinus thrombosis imaging. Magnetic resonance venography image (A) and cerebral angiogram (B) in the venous phase showing absence of flow within the left transverse sinus, sigmoid sinus, and internal jugular vein.

(Reprinted with permission from Foroozan R, Kline LB. Papilledema. In: Kline LB, Foroozan R, eds. Optic Nerve Disorders. 2nd ed. Ophthalmology Monograph 10. New York: Oxford University Press, in cooperation with the American Academy of Ophthalmology; 2007:55.)

Computed Tomography Angiography and Computed Tomography Venography

Computed tomography angiography (CTA) uses a high-speed spiral scanner that provides excellent vessel resolution, with 3-dimensional capability that is complementary to MRA. The technique requires use of iodinated contrast dye and employs ionizing radiation; the procedure takes approximately 15 minutes. Sensitivities in the detection of aneurysms >3 mm in diameter or stenosis >70% are approximately 95%. Some medical centers prefer the use of CTA to MRA in the detection of cerebral aneurysms, including aneurysms causing ocular motor cranial nerve palsies (see Chapter 8, Fig 8-9). New multidetector CTA techniques can accurately detect aneurysms >3–4 mm in diameter in almost all patients with cranial nerve (CN) III palsies.

Computed tomography venography (CTV) is an excellent imaging modality for visualizing the cerebral venous system and is comparable to MRV.

Mathew MR, Teasdale E, McFadzean RM. Multidetector computed tomographic angiography in isolated third nerve palsy. Ophthalmology. 2008;115(8):1411–1415.

Osborn AG, Blaser SI, Salzman KL, et al. Diagnostic Imaging: Brain. 1st ed. Salt Lake City: Amirsys; 2004.

Metabolic and Functional Imaging Modalities

Magnetic resonance spectroscopy (MRS) can provide information about the integrity and metabolism of neural tissue. This technique generally depends on the presence of hydrogen and phosphorus in tissue and produces 5 principal spectra: N-acetylaspartate (NAA), choline, creatinine, lactate, and lipid. These MRS spectra have the following significance:

NAA is associated with neuronal integrity; a decrease in NAA is associated with neuronal loss. Choline is a component of cell membranes.

Creatinine is relatively stable in the brain and can serve as an internal control. Lactate, normally barely visible, is a marker of anaerobic metabolism.

Lipid is not significant.

Changes in the pattern of the spectra peaks are associated with different disease processes; for example, a nonnecrotic brain neoplasm typically produces an elevated choline peak and a reduced NAA peak. Use of MRS can help characterize abnormalities detected initially on MRI.

Functional MRI (fMRI) depends on changes in regional blood flow in the brain in accord with metabolic demand. With very fast MRI analysis, local differences in blood oxygenation (blood oxygenation level–dependent [BOLD] responses) can be evaluated. Based on this technique, areas of higher metabolic activity can be identified during specific tasks, such as reading, speaking, or moving a finger. In neuro-ophthalmology, fMRI is a reliable technique allowing accurate visualization of the cortical areas involved in ocular motility, and of cortical retinotopy within the visual cortex. Besides being less expensive than positron emission tomography (discussed next), fMRI has the additional advantages of faster imaging speed, higher spatial resolution, practical repeatability, and being less