While plain radiographs and arteriograms may not be ready for the ophthalmologic history books just yet, they are fast becoming relics among neuro-ophthalmologists, thanks to advances in imaging technology.
Here’s an update on leading trends in the field, from improvements in software to completely new technology.
More Powerful Magnetic Fields
While 1.5-tesla MRI machines still offer very good spatial resolution, 3-tesla scanners are becoming more widely available and are moving out of the research centers and into wider clinical use. Even though most clinical MRI systems still use 1.5-tesla magnetic fields (equal to about 30,000 times the earth’s magnetic field), the more sophisticated 3-tesla scanners (tipping the scales at more than seven tons) provide even greater image resolution and do it more rapidly than the older MRI machines.
“The stronger the magnet, the better the resolution capability and the greater the ability to look at anatomic structures,” said Andrew G. Lee, MD, associate professor of ophthalmology, neurology and neurosurgery at the University of Iowa. “So with MRI, bigger is better. As our experience with the 3-tesla magnet increases, I expect it will probably replace the smaller magnets because its pictures are much better.”
Stronger magnets also translate into faster scanning times, which is appealing to patients who become claustrophobic during a prolonged MRI study. And in patients for whom breath-holding is an issue, a faster scan has obvious appeal.
However, if highly claustrophobic patients opt for an open MRI, they’ll sacrifice some image precision in doing so. Not only are magnets smaller with open MRI scans—often only 0.2- or 0.5-tesla magnets—but the greater distance between the magnet and the area being studied reduces spatial resolution.
Strides in Software
As valuable as greater magnet strength can be, other factors are at play in the value of an imaging system. In fact, the more widely available 1.5-tesla may be just as useful for most ophthalmic purposes, particularly with updates in computer software that allow improvements in the way images are retrieved with MRI scans.
Diffusion-weighted imaging (DWI), for instance, can be used with a 1.5-tesla magnet; this makes the procedure much more sensitive for detecting acute strokes than non-DWI imaging.
“A patient who has a sudden onset of peripheral vision loss in each eye may have normal findings with a conventional MRI, when in fact his status isn’t normal at all,” said Lanning B. Kline, MD, professor and chairman of ophthalmology at the University of Alabama, Birmingham. “In patients like these, a diffusion-weighted MRI scan can help detect problems such as stroke, thus offering the clinician a huge advantage in making the diagnosis.”
While conventional MRIs cannot detect signs of injury or trauma for many hours or days after tissue changes have occurred, DWI can identify them just minutes after the event. “It’s a way of detecting alterations in the water content within brain cells that become disturbed within minutes of a cerebrovascular accident,” said Dr. Kline. “By contrast, the more conventional T1- and T2-weighted images that are obtained with conventional MRI can take up to 48 hours to detect a stroke.”
Dr. Lee sees a trend toward the use of more functional imaging—using scans like positron emission tomography (PET), magnetic resonance spectroscopy and functional MRI—that can image what the brain is doing rather than what it looks like. Functional MRIs, for example, use readily available equipment (the MRI scanner) and are useful for detecting hypermetabolic states associated with tumors, as well as differentiating tumors from areas of inflammation and in detecting regions of ischemia.
At times, PET scans can reveal abnormalities that structural imaging may be incapable of finding. PET is often used for evaluating cortical visual loss, stroke and migraine, and for mapping the visual cortex. “If a patient has a homonymous hemianopsia but his structural scan is normal, we’d consider performing a functional scan, such as a PET or a functional MRI scan,” said Dr. Lee.
The PET scan’s spatial resolution isn’t as good as that of MRI or CT scans, but its functional nature compensates for its shortcomings in other areas. “PET scans are helpful in finding occult inflammations or occult tumors,” said Larry P. Frohman, MD, director of neuro-ophthalmology and associate professor of ophthalmology and neurosciences at UMDNJ-New Jersey Medical School and president of the North American Neuro-Ophthalmology Society.
In eight case studies reported in Ophthalmology, visual symptoms were the presenting manifestation of Alzheimer’s disease, with symptoms such as homonymous visual field loss or presumed cortical visual impairment. Structural neuroimaging (MRI or CT scans) can be normal in these Alzheimer’s patients, although functional imaging with PET scans may show hypoperfusion in the occipital lobes.1
From Old to New CT
Amid all the high-tech advances in neuro-ophthalmology, CTs haven’t been forgotten. For example, Dr. Frohman noted that they remain useful in imaging orbital pathology, including tumors and trauma.
“CT scans are still valuable in some cases,” Dr. Lee agreed. “If a patient has an acute hemorrhage, a CT scan is a good choice. Or if you’re looking for calcification, or if you need a faster scan in an emergency situation, a CT scan should be considered.”
A new technology, CT angiography (CTA), has emerged in the last two years. It uses high-speed spiral CT scans of medium- to large-sized arteries, with images obtained after injection of an IV bolus of iodinated contrast.
Unlike MRI, CTA can be used in patients with pacemakers, as well as those who become claustrophobic during an MRI scan.
The CTA technology can image blood vessels behind the eye in exquisite detail, as well as nearby vessels in the neck and brain. It is capable of detecting very small aneurysms (as little as 1.7 millimeters).
“CT angiography is a major advance, and is becoming the best way to image the vasculature of the brain in a noninvasive way,” said Dr. Kline. “It is the procedure of choice for imaging arteriovenous malformations and intracranial aneurysms. The images are excellent, and the system allows us to visualize the area of abnormality not only in relation to the other vascular structures in the brain, but also other major components like bone and nearby neural structures.”
However, Dr. Frohman cautioned that the quality of CTAs is influenced by the experience of those performing the procedure.
Advances in neuroimaging should continue to accelerate, with testing of 7-tesla MRI systems already in progress. GE Medical Systems, for example, delivered its first 7-tesla MRI system to the NIH in December 2002, and research on this ultra-high-field scanner is under way at a number of medical schools.
1 Lee, A. G. et al. Ophthalmology 2004;111: 376–380.
How to Get a Good Scan
All scans are not created equal. To help get the information you’re seeking from a neuroimaging procedure, it’s important to communicate clearly with the radiologist about what’s needed, including the type of scan and what region should be imaged.
The radiologist should help the ophthalmologist tailor the study to the proper area and determine whether contrast agents are going to be helpful. The radiologist also needs relevant clinical information, as well as a patient history, from the ophthalmologist to properly select the appropriate custom sequences.
According to Dr. Frohman, the importance of communication between the ophthalmologist and the radiologist cannot be overemphasized. “If you ask me, ‘What’s more important: to have a 3-tesla machine [rather than 1.5] or to have a neuroradiologist clearly understand what the imaging study should look for and monitor the study and the technician?’—the latter is much more important than the magnetic strength.
As for physician oversight, “It's important for the ordering physician to speak to the radiologist who annotates the case and sets up the protocol for the sequences of the scans,” he added. “Once the right scan is done, if one has a question about the reading, the film can be shown to someone else, but if the right images are not first acquired, one is stuck.”
Who Needs an MRI?
There are numerous ophthalmic indications for an MRI, but “every patient with optic neuritis without a pre-existing diagnosis of multiple sclerosis should have one,” said Dr. Frohman. In fact, optic neuritis is frequently an initial manifestation of MS. According to a recent report of the Optic Neuritis Study Group, the strongest predictor of MS in people with acute optic neuritis is the presence of white matter lesions on a baseline MRI scan of the brain. The existence of one such lesion (with a diameter of at least 3 mm) correlated with more than a doubling of the 10-year risk of MS.1
1 Arch Ophthalmol 2003;121: 944–949.