Magnetic Resonance Imaging
MRI is the imaging method of choice for evaluating many disease processes. The technique depends on the physical properties of soft tissue for image generation. Atoms containing an odd number of nucleons (protons and neutrons) have a small magnetic moment (ie, a spinning magnetic field). The most common element in the body with such properties is hydrogen. When exposed to the powerful magnetic field of an MRI scanner, the magnetic moments of the hydrogen nuclei align in the field. Radiofrequency (RF) pulses then transiently perturb the hydrogen nuclei, knocking their magnetic moments out of alignment. After a sequence of such pulses, their rate of relaxation (ie, return to alignment with the scanner’s magnetic field) generates a tissue-specific signal corresponding to the magnetic properties of the tissue. By varying the RF pulses and the timing of signal recording, different types of images are produced. Modern MRI devices record a 3-dimensional data set that can construct views in axial, coronal, sagittal, or even oblique planes without patient repositioning.
MRI is sensitive to changes in water content of soft tissue, depending on how the water is bound and how it moves within tissue. Injection of gadolinium, a paramagnetic contrast agent that traverses a disrupted blood–brain barrier, 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. Although initially believed to be free of toxicity, gadolinium has been reported to cause a systemic fibrosis syndrome that often involves the skin (ie, nephrogenic fibrosing dermopathy) in patients with preexisting renal disease. See Table 2-1 for a list of advantages and disadvantages of MRI.
Magnetic resonance images are usually classified as T1or T2-weighted (Fig 2-3). Each tissue type has a characteristic T1 and T2 relaxation time attributable to the amount of water, and how water
is bound, in the tissue. In the most common MRI sequence technique, known as spin-echo, a T1weighted image is obtained by selecting appropriate RF pulse timing: a relatively short time to repetition (TR; approximately 200–700 ms) and a short time to echo (TE; 20–35 ms) (Table 2-2). T1weighted images are optimal for demonstrating anatomy. Resolution is also higher than in T2weighted images, chiefly because of the increased intensity of the signal and thus the faster acquisition times (minimizing motion artifact). 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 the most sensitive to inflammatory, ischemic, or neoplastic alterations in tissue (Fig 2-4, 2-5, 2-6).
Figure 2-3 Normal axial MRI of the brain at the level of the orbits and midbrain highlighting the different signal characteristics depending on the water content and bound state of 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 compared with white matter. B, T2-weighted image shows that CSF, vitreous, and orbital fat are hyperintense and gray matter is hyperintense compared with white matter. C, Fluid-attenuated inversion recovery image shows that orbital fat is hyperintense; however, vitreous and CSF appear hypointense, which facilitates detection of abnormalities in periventricular tissue. (Courtesy of Rod Foroozan, MD.)
Table 2-2
Figure 2-4 Comparison of T1-weighted and T2-weighted images 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 scans originally taken: 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, Sagittal T1-weighted sequence shows the tumor to be heterogeneous but mostly hypointense to gray matter. B, Axial gadolinium-enhanced T1-weighted image demonstrates minimal rim enhancement. C, On the proton density image, the signal intensity becomes brighter, being isointense with gray matter and brighter than white matter. D, On T2-weighted images, the lesion becomes extremely bright. These findings are characteristic of an epidermoid. (Courtesy of Steven A.
Newman, MD.)
Figure 2-6 MRI scan of patient with the lateral medullary syndrome (also called Wallenberg syndrome). A, Axial T2weighted MRI scan demonstrates a bright signal in the left lateral medulla (arrow). B, The diffusion-weighted image shows a bright signal (arrow). C, The apparent diffusion coefficient 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.)
Very intense tissue signals (eg, from fat in T1-weighted images and cerebrospinal fluid [CSF] or vitreous in T2-weighted images) may obscure subtle signal abnormalities in neighboring tissues
(Table 2-3). 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 relatively T1-weighted images without the confounding bright fat signal. This technique is particularly useful in studying the orbit (Fig 2-7, 2-8 see Fig 2-9D). Fluid-attenuated inversion recovery (FLAIR) provides T2weighted images without the high (bright)-CSF signal, making it ideal for viewing the periventricular white matter changes in a demyelinating process such as multiple sclerosis (Fig 2-9). Diffusionweighted 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 (as compared with several hours using conventional MRI sequences) and persists for approximately 3 weeks, thus serving as a time marker for acute and subacute ischemic events (Table 2-4).
Table 2-3
Figure 2-7 MRI scan of patient with meningioma. A, Axial T1-weighted orbital MRI scan with contrast 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. (Part A courtesy of Rod Foroozan, MD; part B reprinted with permission from Foroozan R, Hinckley L. Compression of the anterior visual pathways. 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; 2007:109.)
Figure 2-8 MRI scan from a 48-year-old woman who presented with slowly progressive decreased vision OS. The axial T1-weighted fat-suppressed postcontrast MRI scan shows a meningioma involving the middle cranial fossa of the skull base (arrow), including the optic canal. Abnormal enhancement in the orbital apex indicates extension of the meningioma along the optic nerve sheath (arrowhead).
Figure 2-9 MRI scan of a 30-year-old woman with acute optic neuritis of the left eye. Axial fluid-attenuated inversion recovery (FLAIR) image (A) and axial T2-weighted image (B) show bilateral, scattered, periventricular white matter hyperintense lesions, which are more conspicuous on the FLAIR image. C, Sagittal T2-weighted image demonstrates the same periventricular white matter changes perpendicular to the corpus callosum (ie, Dawson fingers). D, Coronal postcontrast, T1-weighted image with orbital fat suppression shows enhancement of the left optic nerve (arrow). (Courtesy
of M. Tariq Bhatti, MD.)