102 Neuroimaging in Ophthalmology

Table 4-2. Guidelines for Ordering Imaging Studies in Ophthalmology

1.Decide whether CT or MRI is indicated. The MRI scan is superior to CT for most neuroophthalmic indications, but CT is superior to MRI for calcification, bone, acute hemorrhage, if an emergent scan is needed, or if the patient cannot undergo MRI.

2.Decide if contrast is needed. In most cases, contrast material should be ordered for both CT and MRI studies. Contrast may not be necessary in acute hemorrhage, thyroid eye disease, or trauma cases. Caution is necessary for both iodinated contrast and gadolinium contrast in patients with renal failure, and contrast may be contraindicated in these settings.

3.Topographically localize the lesion clinically (“where is the lesion”), define the differential diagnosis (“what is the lesion”), establish the urgency of the imaging request, and then order the best study tailored to the lesion location (e.g. head, orbit, or neck).

4.Order specific imaging sequences (e.g., fat suppression for orbital postcontrast study, fluid attenuation inversion recovery for white matter lesions, gradient recall echo for hemorrhage, diffusion-weighted imaging for stroke or posterior reversible encephalopathy syndrome) depending on clinical indication.

5.Order special imaging for specific vascular indications (e.g., MRA or CTA, MRV, CA). See Table 3–1 for catheter angiography, MRA, and CTA recommendations for third nerve palsy.

6.Call the radiologist if there is any doubt about localization, image study of choice, contrast selection, indications, or the final report.

7.If the imaging shows either no abnormality or an abnormality that does not match the clinical localization, then call the radiologist or, better yet, review the films directly with him or her. Ask the radiologist if the area of interest has been adequately imaged, if artifact might be obscuring the lesion, or if additional studies might show the lesion.

8.If the clinical picture suggests a specific lesion or localization and initial imaging is “normal,” consider repeating the imaging with thinner slices and higher magnification of the area of interest, especially if the clinical signs and symptoms are progressive.

9.Recognize that the lack of an imaging abnormality does not exclude pathology.

Modified from Reference 54.

CA, conventional angiography; CT, computed tomography; CTA, computed tomography angiography; MRA, magnetic resonance angiography; MRI, magnetic resonance imaging; MRV, magnetic resonance venography.

4-3 EXAMINATION OF IMAGES

After review of the identifying and acquisition information, attention is turned to the images themselves. It is important to orient oneself to the plane of scanning. This may be done by examining the scout film, which shows the slices as sectioned by the computer. Axial and coronal images can be confused if one is not familiar with the area of the brain being imaged. Many software programs allow the user to measure the size of lesions with a virtual ruler and to check the location of a suspected lesion seen on multiple imaging planes. Reviewing the information on the scans requires experience. It is necessary to examine bone, soft tissue, blood vessels, and cerebrospinal fluid–containing structures to assess normality. Ideally, the prescribing clinician would review the imaging study directly with the interpreting radiologist to obtain the ideal combination of clinicoradiologic correlation.