Popular Mechanics | May 11, 2012 11:00 AM
Stargazing Science Used to See Inside the Human Eye
A technique originally developed to see distant stars now allows researchers to diagnose eye diseases such as age-related macular degeneration months sooner than possible with other current methods.
The enchanting twinkling of the stars is disheartening to astronomers studying distant galaxies. Distortions in the atmosphere such as turbulence and wind shear create those twinkling effect we see while stargazing, but when you look through a telescope, there's no twinkle—just a big blur.
Human eyes also have their own "twinkling" problem. When ophthalmologists and vision scientists try to look deep into our eyes, distortions within the cornea and lens impair light as it travels through the eyeball, frustrating their efforts. Lately, a growing group of vision scientists have turned to a solution pioneered by astronomers—adaptive optics, originally developed by the military and used by astronomers such as Scot Olivier at California's Lawrence Livermore National Laboratory to produce clear images of faraway stars. Applied to looking into the human eye, the technique allows researchers to view minute details never before seen and to diagnose blinding disease like macular degeneration months before current methods allow.
A Twinkle in the Eye
In astronomy, adaptive optics uses a device known as a wave-front sensor to detect the degree to which light has been distorted as it approaches the telescope. To make this work out correctly, telescopes such as Hawaii's Keck Observatory use a guide star, which astronomers create by aiming a laser into the upper atmosphere. When tuned to the right frequency, the laser excites sodium atoms leftover from meteorites that have condensed in a layer atop the mesosphere, about 60 miles up. Excited sodium atoms then send light back toward the telescope, which can be measured by observatory's wave-front sensor. This information is then continuously fed to a computer that precisely bends an intricate mirror to adjust for and cancel out these distortions.
It was back in the 1990s that University of Rochester vision scientist David Williams had his eureka moment: Why not apply the techniques of adaptive optics to imaging the human eye to be able to produce clearer images of the retina? With the human eye, Williams realized, scientists could shine a laser—that's about one million times weaker—onto the back of the retina, which functions as a weak mirror. That light spot is then used to measure and adjust for the imperfections in the focusing optics of the eye, Olivier says.
Williams got to work on the problem of using adaptive optics on the eye and, along with collaborators, made a system that allowed him to produce the first images of the three types of cone photoreceptors—responsible for color vision—in a study in Nature in 1999. Building on this work, Olivier joined forces with UC Davis vision researcher John Werner and Indiana University scientist Donald Miller to create more advanced imaging prototypes.
Adaptive optics is tremendous for minute details, but it produces only 2D images. So the group combined it with another technique called optical coherence topography, used to measure the thickness of tissues like the retina. The result: the ability to produce 3D images of cells within the retina, such as cone photoreceptors, which isn't possible with any other technology, Olivier says. Funded by the National Eye Institute and the National Institutes of Health, the group has made three prototype imaging devices, two of which are used to diagnose disease among eye patients in California and Bloomington, Indiana.
The system is much smaller than those used for telescopes—it fits into a 1.5-foot cube and contains just 140 tiny actuators compared to 4000. But while the prototypes are still primarily research tools, they're teaching scientists things they didn't know before, which could soon be used to detect disease—and used to help develop medications to fight blindness.
The technology has already allowed Werner to detect eye diseases that didn't show up with any other technique, such as a case of epiretinal membrane wherein cells coalesce on the surface of the retina to distort vision. Spotting this and other irregularities such as macular holes changed the course of treatment in several patients, likely saving their vision. Olivier said that in the future it could be used to detect macular degeneration years ahead of current methods. Williams and his group have also found that changes happen in the retinal epithelium—the layer beneath the retina that supplies it with blood and nutrients—early on in the course of macular degeneration.
Adaptive optics optical coherence topography (AO-OCT) has shown that in glaucoma, the cone's outer segment, where light is absorbed, becomes shorter. The number of cones is also reduced, which came as a surprise because glaucoma is thought to arise from damage to the optic nerve, often caused by increased pressure within the eye. "Nobody thought this would happen, because the cones are upstream of the damage," Werner said. By revealing other changes that accompany the optic nerve's decline, AO-OCT opens the door to alternative treatments. Perhaps if a drug could be developed to prevent cone cells from dying, the decline could be slowed or halted.
Werner's team is also using a variation of the technique that employs fluorescence to record electrical activity of retinal neurons. The human eye has more than 12 different kinds of "wires" for sending info up to the brain, he says, and nobody understands why we need all these different kinds.
And Miller's doctoral student Ravi Jonnal used a variation of the technique to measure the rate at which cone photoreceptors grow—something never previously measured. To do it, Jonnal bounced one half of a split laser beam off a healthy volunteer's retina. The other half was reflected back by a mirror at a set distance. By recombining the beams, Jonnal's group then worked out the minute differences between the two, caused by the interference of structures within the eye, to determine the position of the cone cells. He found that they grow about 150 nanometers per hour, about 100 times slower than human hair grows.
From Stars to Retinas
For their next projects, Olivier and colleagues will borrow more from the adaptive optics of astronomers. They are working to improve the system to be able to produce images of ganglion cells, nerves that transmit signals to the brain, which are damaged in glaucoma. "It's a contrast issue—it's tough to see a transparent cell in front of a transparent tissue." They also are working toward being able to see rod cells, which are important for vision but appear as much as 100 million times dimmer. This involves using techniques from astronomy that can pick out dim stars next to brighter neighbors, or find potentially life-friendly planets around alien suns.
"It's great to see the same tricks developed in astronomy being used to solve problems in vision science," Olivier says.