How Can Doctors Photograph a Slice Through the Back of the Eye Without Cutting It?

What an OCT Scan Shows That an Ordinary Photo Cannot

A normal photograph of the back of the eye shows only the surface, like a photo of the top of a cake. Doctors often want to see the layers inside: is the retina swollen, is it thinner than it should be? A review in the Journal of Clinical Investigation says OCT can look into a living retina and the head of the optic nerve in real time, down to about 3 micrometers (thousandths of a millimeter) in detail, and that no other method can produce images this detailed from living tissue [2].

It is painless and needs no contact with the eye [1]. Before it, doctors had less direct ways to look inside, and some standard tests, such as measuring eye pressure or mapping the field of vision, could be unreliable for catching glaucoma, an eye disease that damages the optic nerve [1]. So the question is a good one: how do you get depth out of light?

The Echo Idea, and Why a Clock Cannot Time Light

Depth is easy to get from sound. Ultrasound sends in a pulse and listens for echoes. An echo from something far away takes longer to come back than an echo from something near, so the delay tells you the distance. The Lasker Foundation, which gave its 2023 clinical medical research award to the inventors of OCT, explains that the same idea would work with light, but the delays are so small that even electronic clocks are not fast enough to measure them [1].

To get a feel for it, here is a rough calculation of my own. Light travels about 300,000 kilometers in a second, which is about 30 centimeters in a billionth of a second. Across one millimeter of tissue and back, the delay is only a few trillionths of a second. (Light moves a little slower inside the eye than in empty space, so this is a ballpark figure.) No stopwatch, however fast, counts that. If a clock cannot measure the delay, it has to be found another way.

Overlapping Light Waves Reveal the Delay Without a Clock

The other way is interference. Light behaves like a wave, and when two waves meet they add up: where the peaks line up, the light gets stronger, and where a peak meets a trough, the two cancel. That strengthening and cancelling can be measured, even for very faint light [1].

In OCT, the light is split in two. One beam goes into the eye and bounces back from the different layers of the retina. The other goes to a reference mirror at a known distance. When the two come back together, they interfere. A review describes the method as low-coherence interferometry: the light used is made of many colors at once, so the two beams strengthen each other only when they have traveled almost exactly the same distance [1][2]. In other words, the reference mirror acts like a ruler. A strong overlap says that a layer is at the same distance as the mirror, and moving the reference or reading the overlapping light across many colors lets the machine check one depth after another. A simple way to picture it is two people walking from the same start: they arrive together only if their paths were the same length.

Why Looking at the Retina Was Hard Until 1991

The first step came from James Fujimoto, an MIT researcher who specialized in extremely short laser pulses. In 1986, his group used them to tell apart layers of skin and to measure the thickness of the cornea, the clear front of the eye, in anesthetized rabbits. The method was slow, the lasers were very expensive, and the setup was not sensitive enough for a surface that reflects little light, like the retina [1].

Two things changed that. David Huang, a student working with Fujimoto, took on the project, and Eric Swanson of MIT Lincoln Laboratory joined. Swanson had worked on laser links between satellites and on fiber-optic networks, which rely on interference to handle faint signals from far away. Using optical fiber made the light waves interfere efficiently, which raised the speed by orders of magnitude and made the equipment far more compact [1].

The 1991 paper in the journal Science by Huang, Swanson, Fujimoto and colleagues introduced the name optical coherence tomography. Its images, of a retina taken from a donor and of a coronary artery, showed detail of 12 to 17 micrometers, about the size of one cell [1][2].

From One Line of Echoes to a Whole Slice

One measurement gives a single line: a list of how strongly light bounced back from each depth. Huang noticed that the retina showed several layers in such a line. He then realized that if the beam were moved sideways a little at a time, and the lines were set side by side, the one-dimensional data would turn into a two-dimensional cross-section [1]. Imagine building a picture from thin vertical strips, each strip colored by one measurement.

Images of retinas in healthy living people followed in 1993, from the MIT group working with eye doctors at the New England Eye Center [1][2]. Later machines became much faster. A review says newer designs, including swept-source OCT, made scans much faster than the earlier ones [2].

How Doctors Use the Slices, and Why OCT Became Widespread

The first commercial eye OCT machine appeared in 1996. Use grew steadily, and then, in the early 2000s, a new kind of medicine arrived for an advanced form of age-related macular degeneration, a disease that damages the center of the retina in older people. The medicine is an anti-VEGF antibody treatment, given by injection. OCT helped doctors work out who would benefit, check how the retina responded, and decide when another injection was needed. That need drove OCT into everyday eye care [1].

Today the review calls OCT a standard of care in eye medicine, and notes that it has had its greatest effect on the care of retinal disease and glaucoma [2]. Engineers have also built it into thin probes that can enter blood vessels and into surgical microscopes [1].

Watch Waves Add Up and Cancel in a Bowl of Water

You can see a rough version of interference with water. Set a wide, shallow bowl on a towel and wait until the surface is still. Dip two fingertips in, a few centimeters apart, at the same moment. Watch where the two sets of ripples meet. Are there spots where the water bumps up higher, and spots that hardly move?

Light waves are far smaller and cannot be seen, so this is only a picture to help you imagine the idea, not the same thing. If you want to know more, the Lasker page [1] is a readable account by people who followed the story closely, and it includes the inventors’ own remarks.

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