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).
Table 2-2 MRI Signal Intensity (Brightness) by Tissue Type
Table 2-3 Edema: DWI and ADC
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).
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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.