Spectral Domain vs. Swept Source OCT: What’s the Difference?

Have you ever had an eye exam where your doctor asked you to rest your chin on a machine and stare at a light? That machine was likely an OCT scanner. It takes detailed pictures of the inside of your eye without touching it.

But not all OCT machines are the same. The two main types are called spectral-domain OCT (SD-OCT) and swept-source OCT (SS-OCT). They both do the same basic job, but they work a little differently.
 

How They Work

Think of SD-OCT like using a camera flash that captures everything at once. It sends out a broad beam of light and reads what bounces back. It is fast and gives very clear images.

SS-OCT works differently. It sweeps through different wavelengths of light one at a time. Because it uses a longer wavelength, it can see deeper into the eye. This makes it helpful for examining structures buried deep within, such as the layer beneath the retina or deep parts of the optic nerve.
 

What Studies Show

Both machines work well, but research has found they each have strengths.

For age-related macular degeneration (AMD), the two are very similar. One study examined eyes with AMD and found that SD-OCT and SS-OCT yielded nearly identical results when measuring areas of damage.

Either machine could reliably track the disease’s progression. SS-OCT did catch a few more tiny spots of damage because its scan pattern is denser, but the difference was not big enough to matter in a real-world setting. For most patients, both machines give doctors the information they need.

For optic disc drusen (ODD), the story is different. Optic disc drusen are tiny calcium deposits buried deep in the optic nerve. They can be hard to spot, especially when they are small. One study compared SD-OCT with enhanced depth imaging (EDI) to SS-OCT in eyes with deep ODD.

The results showed that SD-OCT with EDI actually worked better. It gave clearer images of the deep drusen and the structures around them. Researchers think this is because SD-OCT can average multiple scans of the same spot, reducing background noise and making the image sharper. SS-OCT scans were faster but had more motion artifacts and could not average scans the same way.
 

So Which One Is Better?

It depends on what your doctor is looking for.

• SS-OCT is better at seeing deep into the eye. Its longer wavelength penetrates deeper tissues, such as the choroid. It is also very fast, which helps reduce blurring from eye movements.
• SD-OCT with EDI produces sharper, clearer images of deep structures. The ability to average multiple scans together gives it an edge when doctors need to see very small details, like tiny calcium deposits in the optic nerve.

What This Means for You

For common conditions like AMD, either machine will give your doctor reliable information. The choice often comes down to what equipment your clinic has.

If you have a condition involving deep structures, like suspected optic disc drusen, your doctor may specifically recommend an SD-OCT scan with EDI. Studies show it gives the clearest view in those cases. If you need fast imaging of deeper layers, SS-OCT may be the better choice.

Both machines are amazing tools. They let doctors see inside your eye in ways that were not possible just a few decades ago. Whether you get scanned with SD or SS, you are getting a detailed look at your eye health.

For more on the difference between spectral domain and swept source OCT, visit Texas State Optical. Our office is in Lumberton, Texas. Call (409) 751-3937 to book an appointment today.

https://www.sciencedirect.com/science/article/pii/S2666914524001295

https://pmc.ncbi.nlm.nih.gov/articles/PMC9600200/