Few subjects can intimidate even a seasoned optometrist like neuroimaging. When to order? What type of scan to order? How to utilize the results?
Knowing the answers to these fundamental questions and having a basic understanding of imaging will aid the optometrist in knowing how to best manage the patient and when and where to refer the patient if needed.
Types of imaging
Computerized tomography (CT) scan. A conventional CT takes multiple X-rays, often moving the patient between scans, and these images are pieced together with computer software to produce a 3-dimensional image. Most CT devices utilize a spiral or helical approach for increased resolution and less radiation.1
This technology is best for bone fractures from trauma and, increasingly, vascular disorders with CT angiography.2 CT scanners are often used to locate larger orbital and pituitary tumors, although a magnetic resonance image (MRI) is the instrument of choice for small tumors such as meningiomas.3,4
Related: Clinical imaging of macular holes
A CT scan is typically done with intravenous iodine contrast dye, provided the patient has no known sensitivity to iodine, to enhance the images. Advantages of the CT include being readily available and relatively inexpensive. A CT may also be used if a patient has a contraindication to an MRI, such as a pacemaker or other known or suspected metal in the body.4
Magnetic resonance imaging (MRI). A magnetic field causes hydrogen protons in water and fat to spin and release photon energy. The MRI is the instrument of choice for soft tissue concerns such as plaques associated with multiple sclerosis.5 MRIs are superior to CT scans for subtle tumors such as gliomas and menigiomas.4,6 MRIs may also be used as a screening device for vascular disorders, although a magnetic resonance angiogram (MRA) or CT angiogram is typically the definitive test.2
With an MRI, a contrast dye such as gadolinium is often administered intravenously to aid the images seen. Before ordering imaging with contrast, it is prudent to evaluate the patient’s kidney function by ordering a blood urea nitrogen (BUN) and creatinine level test.
When ordering an MRI for optic neuritis and suspected multiple sclerosis, fluid attenuated inversion recovery (FLAIR) is added to suppress the return from cerebrospinal fluid that might mask demyelinating plaques.
What to order
Before ordering a test, communicate with your radiologist and staff. They can help you determine the best test for what potential condition or diagnose you’re examining.
As an example, Tommy Hinton, MD, a radiologist in northwestern Arkansas, says, “The more clinical information we as radiologists can get from the referring doctor, the better off we are. Unfortunately, many of the imaging requests we receive have very little clinical history. If you can state a brief patient history and what you are trying to rule out, it would help us immensely to tailor our protocol accordingly.”
While interdisciplinary communication is always key, collaborating with radiology has the additional benefit of taking some of the burden off the referring optometrist to order the precisely correct scan, contrast dye, etc. Instead, a good patient history and a clear indication of what you’re looking for or wishing to rule out will help ensure the patient receives the proper radiological care.
Related: Using glaucoma diagnostic imaging
Typically, trauma such as a blow-out orbital fracture or a possible space-occupying lesion will require a CT scan. A potential example of the latter would be a patient with bitemporal visual field loss and a suspected pituitary adenoma or a patient with suspected idiopathic intracranial hypertension in which an actual tumor must be excluded from possible diagnoses. Other instances may include recent onset proptosis or an atypical ischemic optic neuropathy in which a lesion must be ruled out.
A scan of the brain and orbits to discover demyelinating disease is the most common use of an MRI for the optometrist, but the MRI may also be used if a subtle tumor is theorized. A suspected aneurysm, possibly associated with a worsening, painful third cranial nerve palsy or a blown pupil, may require a conventional catheter angiogram, an MRA or, increasingly, a spiral CT angiogram.2,7