We are, as Carl Sagan famously said, made of star stuff. And now, your doctor may use a technology designed for studying the stars to examine the inner workings of your eyes. It could one day save you from blindness – here's how it works.
Astronomers use a technology called adaptive optics to correct blurring in images captured by telescopes. The system works by tilting, rotating, and flexing telescope mirrors to correct distortions in the light from distant stars. Now, a miniature version can help ophthalmologists correct blurring in images of the eye, revealing microscopic details that doctors couldn’t see before in live patients.
Adaptive optics have been around since the mid-2000s but the hardware has been too expensive for the average clinic. Now, a team of researchers says they’ve developed computer software that can correct the blurring without extra hardware. University of Illinois doctor and engineer Stephen Boppart and his colleagues published a paper on their new technique in the journal Nature Photonics.
Seeing the Retina
Slit-lamp exam. Image credit: Mike Blyth via Wikimedia Commons
It’s a familiar part of most eye exams: the ophthalmologist shines a bright light into your eyes while peering at them through an instrument called an ophthalmoscope. The lamp and the ophthalmoscope’s magnifying power give your doctor a decent look inside your eye, all the way back to your retina, the layer of cells at the back of your eye. Cells in the retina translate light into nerve signals and then send those signals to your brain.
Your eye doctor may also use a more advanced technique called optical coherence tomography (OCT), which uses low-powered lasers to image your retinas. It’s one of those parts of the eye exam that requires you to put your face in a chin rest that looks like it would be at home in a medieval torture chamber, but it’s painless and usually pretty quick. Plus, there are lasers, which make anything cooler.
OCT is the best technology available for retinal exams at the moment, and it can provide a pretty detailed view of your retina, with resolution ranging from 5 to 10 micrometers. It’s a good way to look for signs of problems like macular degeneration, for instance. It gives your doctor a cross-section view of your retina, which look like this.
OCT cross-section of a retina. Image Credit: medOCT-group, Department of Medical Physics, University of Vienna.
But even with OCT, it’s hard for doctors to see some of the smallest structures in your eye, which might provide early clues about diseases that could affect your vision. Even the slightest movement during scanning can blur an OCT image, and the human eye is in constant motion. Try to hold your head still and focus your gaze on something stationary, like a painting on the wall. You may think your eyes are perfectly still, but they’re still making tiny movements called saccades, to scan the scene for new details and keep it in focus.
So even using OCT, ophthalmologists can’t focus on individual cells in your retina. “The eye has always been a bit of a challenge to image,” Boppart said in a press release. “It’s a very complicated organ. There are many microscopic structures that are hard to see.”
Why Doctors Need a Closer Look
That’s unfortunate, because many of the diseases that can threaten your vision start out as microscopic problems in your retina. For example, macular degeneration is one of the major causes of vision loss, especially in older adults, and it starts with the death of light-sensing cells called rods and cones around a spot near the centre of the retina, called the macula.
If doctors can look at individual cells and see them dying, they might catch the disease earlier in its development, giving patients a better chance of treatment. That’s not possible with OCT alone, though, so doctors have to wait for other signs of the disease.
“They can’t directly look at the photoreceptors and watch them die off during macular degeneration, for example. They just have to guess what’s going on,” said University of Illinois graduate student Fredrick South, a co-author of the newly published research, in a press release. He added, “It could be possible to use adaptive optics in these real-world applications, both for diagnosis of disease and tracking of treatment.”
The macula (dark spot in the centre) and optic nerve (bright green spot). Image credit: Hey Paul via Flickr
The retina can also tell doctors about the health of your entire nervous system. Nerves form the whole top layer of the retina, which means that your doctor can actually look into your eyes to see part of your nervous system. For example, doctors can look at the nerves in the retina for early signs of multiple sclerosis, which breaks down the myelin lining around the nerves. Of course, that requires a microscopic view of nerves and cells in the retina, which isn’t really possible with OCT alone.
That’s why Boppart and his colleagues turned to adaptive optics. But why would eye doctors want to use a technology that has been the purview of astronomers?
Look up at the sky on a clear night, and you’ll see stars twinkling against the darkness. Of course, stars only appear to twinkle because Earth’s atmosphere blurs and distorts their light. Earth’s atmosphere is in constant, turbulent motion, and that’s enough to bend light waves passing through the layers of air.
“This turbulence causes the stars to twinkle in a way that delights poets but frustrates astronomers,” wrote the European Southern Observatory, “since it blurs the finest details of the cosmos.”
To correct for that blurring, modern astronomical telescopes use adaptive optics: mirrors that can tilt, rotate, and flex, under the control of computers that calculate corrections for the atmosphere’s blurring effect. Because that blurring effect changes constantly, those corrections require a constant stream of calculations and mirror adjustments.
To get them right, the computers need a bright point of reference in the sky. A bright star or planet will do the trick, if there’s one near the section of sky astronomers want to observe, but some observatories also fire high-powered laser beams up into Earth’s upper atmosphere, creating artificial stars for the computers to reference. As a side benefit, observatories firing high-powered lasers into the sky look pretty amazing.
Artist’s impression of adaptive optics reference lasers. Image Credit: European Southern Observatory.
It’s not a new idea. The US military worked on adaptive optics systems during the Cold War, hoping it would provide a more precise way to track Soviet satellites. Science fiction author Poul Anderson even mentioned the technology in his 1970 novel Tau Zero. Adaptive optics didn’t really take off for real-world astronomers until the 1990s, however, when computers finally caught up enough to perform the quick, complex calculations required to make the corrections in real time.
And it works beautifully. Adaptive optics lets Earthbound telescopes capture images almost as clear and sharp as those sent back to Earth by space telescopes, for a fraction of the cost of sending an observatory into orbit. You can tell the difference.
Correction with adaptive optics. Image credit: European Southern Observatory
Mirrors and lenses help correct distorted rays of light reflecting from your retinas too, creating much clearer images than would be possible without adaptive optics. It’s been pretty successful, especially in research: for the first time, doctors could actually see cones — the cells that enable colour vision — in a live patient.
Boppart and his team claim that their new computational adaptive optics will make the technology more accessible to clinics, because it will be less expensive than hardware-based adaptive optics. Their method will require some hardware updates, especially for older OCT equipment, to make sure it’s compatible with the software, and it’s not clear yet how expensive those updates — or the software itself — would be.
So the new technique looks promising, but we may have to wait...and see. [Nature Photonics]
Top image: Retina. Image credit: Ku C. Yong et al via Biomed Central.