Description

1. MRI uses a large magnetic field to excite protons of water molecules. The energy given off as the protons re-equilibrate to their normal state is detected by specialized receivers (coils), and that information is reconstructed into a computer image.
2. MRI obtains multiplanar images without loss of resolution.
3. The basic principles of MRI are listed in Table 14.3.1. Figures 14.3.1 to 14.3.3 provide specific examples.
4. Contrast studies can be ordered using gadolinium, a well-tolerated, non–iodine-based paramagnetic agent.
5. Diffusion weighted imaging (DWI) is a technique that measures the Brownian motion of water within tissue. Lesions that are tightly packed by cells with a high nuclear to cytoplasmic ratio will minimize Brownian motion and “restrict diffusion.” DWI is often coupled with an apparent diffusion coefficient (ADC) image. The use of DWI/ADC in the orbit is limited because of proximity of the CNS and limitations of resolution, but it can be useful in certain disorders such as lymphoma.

14-3.3 Axial T2-weighted image.

The vitreous is hyperintense (bright) relative to the orbital fat. The lesion is also bright but in some cases may be isointense with the surrounding fat.

14-3.2 Axial T1 image with fat suppression and gadolinium.

Note how both the vitreous and fat are dark, but the extraocular muscles become bright. The orbital mass is now clearly visible. This technique should be performed in all orbital magnetic resonance imagings.

14-3.1 Axial T1-weighted image without fat suppression or gadolinium.

The vitreous is dark (hypointense) relative to the bright signal from fat. A well-circumscribed mass is clearly visible in the right orbit, also hypointense. Most orbital lesions are dark in T1 prior to gadolinium injection. The notable exceptions are listed in Table 14.3.2.

NOTE:

In general, conventional “closed” MRI scanners provide better resolution and fat suppression than open scanners, unless the open scanner contains a stronger magnet. As a rule, always try to use conventional “closed” scanners for orbital studies.

Uses in Ophthalmology

1. Excellent for defining the extent of orbital/central nervous system masses. Signal-specific properties of certain pathology may be helpful in diagnosis (see Table 14.3.2).
2. Poor bone definition (e.g., fractures).
3. Excellent for diagnosing intracranial, cavernous sinus, and orbital apex lesions, many of which affect neuro-ophthalmic pathways.
4. Gadolinium is essential in defining lesion extent in suspected neurogenic tumors (meningioma, glioma).
5. All patients with clinical signs or symptoms of optic neuritis from suspected demyelinating disease should undergo brain MRI with gadolinium. Fluid-attenuated inversion recovery images are especially useful. See Figure 14.3.4 and 10.14, OPTIC NEURITIS.
6. For orbital studies, fat suppression (also called fat saturation) should always be used in conjunction with intravenous gadolinium to enhance the visualization of the underlying pathology (e.g., optic neuritis, fat-containing lesions). Note that gadolinium without fat suppression may cause pathology to “disappear” into surrounding orbital fat.
7. DWI can help differentiate the various phases of cerebral infarction (e.g., hyperacute, acute, subacute, and chronic). DWI may also help to distinguish tightly packed, highly cellular tumors (e.g., lymphoma) from inflammation.

14-3.4 Magnetic resonance imaging with fluid-attenuated inversion recovery sequence of demyelinating lesions in multiple sclerosis.

Guidelines for Ordering the Study

1. For the vast majority of orbital studies, a head coil is indicated to provide bilateral orbital views extending to the optic chiasm.
2. Intravenous gadolinium is a useful adjunct for evaluating ocular, orbital, and perineural masses. In patients who have kidney failure, sepsis, or recent major surgery, risk of developing nephrogenic systemic fibrosis (NSF) may be a contraindication. NSF is a rare but devastating complication of gadolinium that occurs weeks to months after administration. NSF is characterized by a scleroderma-like fibrosis of the skin, especially over extremities and trunk, which may involve the viscera. There is no known effective therapy or prophylaxis. Evaluate renal function in patients in whom renal insufficiency is suspected and discuss options with your radiology and nephrology departments. Recently, gadolinium deposition with specific areas of the brain has been noted in patients undergoing serial imaging. The clinical significance of this finding is unknown and is under review by the Food and Drug Administration.
3. Contraindications to MRI include severe claustrophobia, marked obesity, some cardiac pacemakers, some cardiac valves, suspected magnetic intraocular/intraorbital foreign bodies, spinal stimulators, vagal nerve stimulators, stapes implants, and specific breast and penile implants. Titanium plates and newer aneurysm clips are MRI safe, as are gold and platinum weights placed in the eyelids. When in doubt, ask the radiologist to look up the specific device in an MRI safety catalog. Any patient with a poorly documented implanted device should NOT be scanned with MRI until the issue is clarified. On occasion, specific types of pacemakers must be turned off and reset for an MRI.