Magnetic Resonance Imaging

As mentioned, MRI is the imaging method of choice for most patients with neuro-ophthalmic conditions. This technique relies on the magnetic properties of the spinning protons in soft tissue used for image generation. The magnetic field strength is measured in tesla (T); most commercially available machines range from 1–3 T in strength. Open MRI machines are of lower strength and provide poorer image quality. The superior image quality and shorter acquisition times provided by higher-strength machines are based on higher-quality signal-to-noise and contrast-to-noise ratios.

MRI is sensitive to changes in the water content of soft tissue, which depends on how the water is bound and how it moves within the tissue. The patient can be injected with gadolinium, a paramagnetic contrast agent with unpaired electrons in the outer shell; the gadolinium traverses a disrupted blood–brain barrier and alters the MRI signal characteristics. This alteration may be crucial in identifying infectious and inflammatory lesions as well as certain tumors with compositions that make them otherwise indistinguishable from normal cortical tissue. Gadolinium-based contrast agents may cause nephrogenic systemic fibrosis (NSF), a rare, multisystemic disease of the heart, lung, and liver characterized by soft-tissue collagen deposition that results in skin thickening and muscle contractures. Gadolinium-based contrast agents are a relative contraindication in patients with preexisting renal disease and patients who have recently had kidney or liver transplants. Most radiologists will not administer the contrast media if the calculated GFR is lower than 30 mL/min/1.73 m². The advantages and disadvantages of MRI are summarized in Table 2-1.

Table 2-1 Comparison of Magnetic Resonance Imaging and Computed Tomography

Magnetic resonance images are usually classified as T1- or T2-weighted (Fig 2-3). Each tissue type has a characteristic T1 and T2 relaxation time attributable to the amount of water in the tissue and how the water is bound in the tissue (Table 2-2). In the most common MRI sequence technique, known as spin echo (SE), a T1-weighted image is obtained by selecting the appropriate radiofrequency (RF) pulse timing, which is a relatively short time to repetition (TR; approximately 200–700 ms) and a short time to echo (TE; 20–35 ms). The information regarding TR and TE is traditionally displayed on the scan (the position of this information on the display varies according to the image software used). T1-weighted images are optimal for demonstrating anatomy. T1-weighted images also have a higher resolution than T2-weighted images, chiefly because of the increased intensity of the signal and shorter repetition time, which result in faster acquisition times and thereby minimize movement-related artifacts. However, T2-weighted images (long TR of 1500–3000 ms and TE of 75–250 ms) maximize differences in tissue water content and state and thus are more sensitive to inflammatory, ischemic, or neoplastic alterations in tissue (Figs 2-4, 2-5, 2-6). Proton density–weighted images are similar to T2-weighted images but have a long TR and a short TE, both of which allow a clear depiction of white versus gray matter; however, proton density–weighted images have now been replaced by fluid-attenuated inversion recovery (FLAIR) images.

Very intense tissue signals (eg, those from fat in T1-weighted images and those from cerebrospinal fluid [CSF] or vitreous in T2-weighted images) may obscure subtle signal abnormalities in neighboring tissues (see Table 2-2). Special RF pulse sequences are used to reduce such intense signals. Fat-suppression techniques, such as short tau inversion recovery (STIR), are used to obtain T1-weighted images without the confounding bright fat signal. This technique is particularly useful in studying the orbit (Figs 2-7, 2-8; see Fig 2-9D). FLAIR provides T2-weighted images without the high (bright)-CSF signal, making this imaging modality ideal for detecting periventricular white matter changes in a demyelinating process such as multiple sclerosis (Fig 2-9).

Figure 2-3 Normal axial MRI scans of the brain at the level of the orbits and midbrain, high-lighting the different signal characteristics depending on the water content and bound state of the tissue. A, T1-weighted image with contrast shows cerebrospinal fluid (CSF) and vitreous as hypointense (dark), orbital fat as hyperintense (bright), and gray matter as relatively hypointense in comparison to white matter. B, T2-weighted image shows that CSF, vitreous, and orbital fat are hyperintense and gray matter is hyperintense in comparison to white matter. C, Fluid-attenuated inversion recovery image (FLAIR) shows that orbital fat is hyperintense; however, vitreous and CSF appear hypointense, facilitating detection of abnormalities in periventricular tissue. Note that in the FLAIR image the gray matter is lighter than in the T1-weighted image.

(Courtesy of Rod Foroozan, MD.)

Table 2-2 MRI Signal Intensity (Brightness) by Tissue Type

Figure 2-4 A comparison between T1-weighted and T2-weighted MRI scans can yield information about the characteristics of a lesion and can be particularly helpful in dating hemorrhages. A 61-year-old patient presented with acute onset of severe headache. A hemorrhage is apparent in the parieto-occipital region in the 3 original scans: T1-weighted (A), proton density (B), and T2-weighted (C) images. The signal at the lesion periphery relates to the presence of oxyhemoglobin, whereas the core remains dark in all 3 images because of the presence of deoxyhemoglobin. When the MRI series was repeated 10 days later, the signal characteristics had changed as a result of the development of methemoglobin in the outer ring, which is bright on T1-weighted (D), proton density (E), and T2-weighted (F) images. The core remains dark.

(Courtesy of Steven A. Newman, MD.)

Figure 2-5 Various tumors may have specific sequence findings. This patient had a large frontal tumor invading the orbit. A, A T1-weighted sagittal sequence shows the tumor to be heterogeneous but mostly hypointense to gray matter. B, A gadolinium-enhanced, T1-weighted axial image demonstrates minimal rim enhancement. C, On a proton density image, the signal intensity becomes brighter; it is isointense with gray matter and brighter than white matter. D, On a T2-weighted image, the lesion becomes extremely bright. These findings are characteristic of an epidermoid.

(Courtesy of Steven A. Newman, MD.)

Figure 2-6 MRI scans of a patient with lateral medullary syndrome (also called Wallenberg syndrome). A, A T2-weighted axial MRI scan demonstrates a bright signal in the left lateral medulla (arrow). B, Diffusion-weighted imaging (DWI) shows a bright signal (arrow). C, The apparent diffusion coefficient (ADC) shows a dark signal (arrow) consistent with an acute infarction and not the phenomenon known as T2 shine-through.

(Courtesy of M. Tariq Bhatti, MD.)

Table 2-3 Edema: DWI and ADC

Figure 2-7 MRI scan of a patient with left optic nerve sheath meningioma. A, A T1-weighted axial orbital MRI scan shows the normal hyperintense orbital fat. B, Suppression of the orbital fat allows visualization of the optic nerve sheath enhancement (arrow) consistent with a meningioma. C, A T1-weighted coronal postcontrast image with orbital fat suppression shows enhancement of the optic nerve sheath (arrow).

(Courtesy of Valerie Biousse, MD.)

Figure 2-8 MRI scan from a 48-year-old woman who presented with slowly progressive decreased vision in the left eye. The T1-weighted axial postcontrast image with fat suppression shows a meningioma involving the middle cranial fossa of the skull base (arrow), including the optic canal. Abnormal enhancement in the orbital apex indicates that the meningioma extends along the optic nerve sheath (arrowhead).

(Courtesy of Eric Eggenberger, DO.)

Diffusion-weighted imaging (DWI) is sensitive to recent alterations in vascular perfusion and is thus ideal for identifying recent infarctions (Fig 2-10; see also Fig 2-6). An abnormal DWI signal occurs within minutes of the onset of cerebral ischemia (versus several hours with conventional MRI sequences) and persists for approximately 3 weeks, thus serving as a time marker for acute and subacute ischemic events. Apparent diffusion coefficient (ADC) is a derived image technique used for stroke and oncological imaging. ADC can help determine whether the DWI hyperintensity is real diffusion restriction or a T2 “shine-through” artifact (Table 2-3).

Figure 2-9 MRI scans of a 30-year-old woman with acute optic neuritis of the left eye. An axial FLAIR image (A) and an axial T2-weighted image (B) show bilateral, scattered, periventricular white-matter hyperintense lesions, which are more conspicuous on the FLAIR image. C, A T2-weighted sagittal image demonstrates the same periventricular white matter changes perpendicular to the corpus callosum (arrows), also known as Dawson fingers. D, A T1-weighted coronal postcontrast image with orbital fat suppression shows enhancement of the left optic nerve (arrow).

(Courtesy of M. Tariq Bhatti, MD.)

Figure 2-10 Axial MRI scans demonstrate the value of DWI in acute infarction. A, The FLAIR image is normal except for nonspecific tiny foci of hyperintense signal in the deep white matter. B, DWI reveals abnormal restricted diffusion (high-intensity signal) near the left middle cerebral artery. C, The low-intensity signal on the ADC map is consistent with an acute infarction.

(Courtesy of M. Tariq Bhatti, MD.)

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Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.