Swept-Source OCT vs. Spectral-Domain OCT: What Patients Should Know
How OCT Technology Works
Both types of OCT use the same underlying principle to create images of your eye, but they differ in how they generate and detect light. Knowing the basics of how each device works helps explain why one may be preferred over the other for certain conditions.
Both SD-OCT and SS-OCT use a technique called low-coherence interferometry to image the retina. In practical terms, the device sends a safe beam of near-infrared light into the eye and measures how that light reflects back from the different retinal layers. The system then builds a precise, cross-sectional picture of the retina's structure and thickness. The process is completely noninvasive and takes only seconds.
SD-OCT uses a broadband light source, typically called a superluminescent diode, that emits light centered at a wavelength of 840 nanometers. All wavelengths within that broad beam illuminate the retina at the same time, and the reflected light is captured by a spectrometer, a device that separates and analyzes each wavelength simultaneously. This approach allows SD-OCT devices to capture tens of thousands of depth profiles per second, which made it the standard for retinal imaging when it was introduced and remains widely used today.
SS-OCT uses a tunable laser that sweeps rapidly through a range of wavelengths one at a time rather than emitting them all at once. The reflected light is detected by a single photodiode instead of a spectrometer. The laser operates at a longer wavelength, typically around 1050 nanometers, and the overall design allows for significantly faster scanning speeds than SD-OCT. This combination of a longer wavelength and greater speed gives SS-OCT several meaningful clinical advantages.
Key Differences and Their Clinical Impact
While both technologies produce high-quality retinal images, their technical differences translate into real distinctions in what each can show and how well each performs in certain patients. These differences guide which device a provider selects for a given clinical question.
One of the most important differences between the two technologies is how deeply they can image into the eye. The 840-nanometer wavelength used by SD-OCT images the retinal layers well but is substantially scattered by the retinal pigment epithelium (RPE), the cell layer at the back of the retina. This scattering limits how clearly SD-OCT can show the choroid, the vascular layer beneath the RPE. The longer 1050-nanometer wavelength used by SS-OCT penetrates more deeply into the tissue, providing much clearer views of the choroid and even the sclera, the white outer wall of the eye. This makes SS-OCT especially valuable when conditions involving those deeper structures need to be assessed.
The faster scanning speed of SS-OCT means more data points can be collected in the same amount of time. This allows for denser scan patterns, which reduce the chance of missing small lesions that might fall between scan lines on a less dense image. Faster scanning also enables wider field imaging, capturing a larger portion of the retina in a single session. For patients, this may also mean a shorter time spent holding their gaze steady during the scan.
The longer wavelength of SS-OCT is less affected by media opacities, which are conditions that cloud the normally clear path of light through the eye. Cataracts and vitreous opacities (cloudiness inside the gel-like substance that fills the eye) can degrade SD-OCT image quality, but SS-OCT is more resistant to these effects. For older patients, who frequently have some degree of lens clouding, this can be a meaningful practical advantage when clear imaging is needed.
SD-OCT can experience a reduction in signal quality for structures that are farther from the device's reference point, a limitation known as sensitivity roll-off. This means the deepest structures in an SD-OCT image may appear less sharp or noisier. SS-OCT maintains more consistent image quality across the full depth of the scan, which is one reason it provides superior visualization of the choroid and other deeper structures.
When Each Technology Is Most Useful
Choosing between SD-OCT and SS-OCT is a clinical decision based on your specific condition and the information your provider needs. Both are excellent tools, and many retinal practices use both, selecting the most appropriate one for each patient.
Spectral-domain OCT remains the standard of care for the majority of retinal conditions and provides all the detail needed for accurate diagnosis and ongoing monitoring. It is commonly used for conditions including the following.
- Diabetic macular edema (swelling in the center of the retina caused by diabetes)
- Wet age-related macular degeneration, to monitor response to treatment
- Epiretinal membranes and macular holes, which are disorders at the surface of the retina
- Retinal nerve fiber layer thickness measurement in glaucoma management
- Tracking structural changes in inherited retinal diseases
For these applications, SD-OCT provides the resolution and clinical detail needed to guide accurate diagnosis and treatment decisions.
Swept-source OCT is particularly advantageous when the choroid or deeper structures need to be evaluated. Conditions where SS-OCT may provide additional diagnostic information include the following.
- Central serous chorioretinopathy, a condition where fluid builds up beneath the retina
- Polypoidal choroidal vasculopathy, an abnormal blood vessel condition beneath the RPE
- Choroidal tumors and lesions
- High myopia (severe nearsightedness), which is associated with choroidal thinning
- Evaluation of the sclera and the structures behind and around it
SS-OCT is also preferred for widefield imaging of the peripheral retina and for patients with cataracts or other conditions that reduce image clarity with SD-OCT.
OCT angiography (OCTA) is an advanced technique available on both SD-OCT and SS-OCT platforms. It maps blood flow in the retinal and choroidal vessels without the need for a dye injection. When comparing their angiographic capabilities, the advantages of SS-OCT are most evident in the deeper vascular layers. The choriocapillaris (the dense capillary network beneath the retina) and the larger choroidal vessels are visualized more clearly with SS-OCTA because of the deeper tissue penetration. For imaging the retinal blood vessels closer to the surface, both technologies perform comparably and provide clinically useful information.
What to Expect During Your OCT Scan
From a patient's perspective, having an OCT scan feels virtually the same regardless of which type of device is used. Both are fast, comfortable, and require no contact with your eye. Knowing what to expect can help you feel prepared and relaxed during imaging.
During an OCT scan, you will sit at the imaging device with your chin resting on a support and your forehead against a stabilizing bar. You will be asked to look at a small fixation target while the scan is captured. The imaging light is gentle and not painful, and the full scan takes only seconds. Your pupils may or may not need to be dilated beforehand, depending on the device and the clinical situation. Dilation generally improves image quality, and your provider will let you know if it is needed. There is no contact with the surface of your eye, and there are no known risks associated with either type of OCT imaging.
The type of OCT used for your visit depends on which equipment is available at your provider's office and what clinical questions need to be answered. If your provider is evaluating a condition primarily affecting the retinal layers, SD-OCT may be the right choice. If deeper choroidal imaging is needed, SS-OCT may be selected. In either case, the goal is the same: to give your provider the clearest possible picture of your eye health so that the right treatment decisions can be made for you.
Availability and the Future of OCT
The availability of each technology varies depending on the type of practice and the equipment they have invested in. Both technologies are continuing to advance, and access to SS-OCT is growing as the technology becomes more established in clinical settings.
SD-OCT is the more established technology and is found in the vast majority of ophthalmology and optometry practices, from general eye clinics to specialized retinal centers. SS-OCT has historically been more common at academic medical centers and specialized retinal practices, but it is becoming increasingly available as more commercial platforms enter the market and equipment costs decrease. Patients who may benefit from SS-OCT imaging but whose regular provider does not have the device may be referred to a center that offers it.
Both SD-OCT and SS-OCT continue to improve. Newer SD-OCT systems offer faster scanning speeds and more sophisticated software tools, narrowing some of the performance gap with SS-OCT. At the same time, SS-OCT platforms are evolving with even faster lasers, wider imaging fields, and integration with artificial intelligence tools for automated image analysis. The two technologies are increasingly viewed as complementary, with many practices using both in their daily workflow to serve different clinical needs.
Frequently Asked Questions
These questions address common points of confusion about OCT imaging, including how the choice of device affects your care and what to do if you have specific concerns.
For most retinal conditions, both technologies provide the image quality needed for an accurate diagnosis. The difference becomes clinically significant when the choroid or deeper structures are involved. In those cases, swept-source OCT may reveal details that spectral-domain OCT cannot show as clearly, which could change or refine a diagnosis. If you have been told you have a condition involving the choroid, it is worth asking your provider whether SS-OCT is available or whether a referral to a center that offers it would be appropriate.
In most situations, the choice of OCT device is a clinical decision best made by your eye care provider. Your provider knows your condition, the imaging options available, and what information is needed to guide your care. If you have a condition known to involve the choroid, such as central serous chorioretinopathy or a pachychoroid spectrum disease, you can ask whether swept-source imaging is available and whether it would add useful information for your specific case. For the majority of retinal conditions, SD-OCT provides everything needed for excellent care.
Both types feel the same to the patient and are generally comfortable. The scan itself is painless, takes only seconds, and involves no contact with the eye. If your pupils are dilated beforehand, you may experience temporary light sensitivity and blurred near vision for a few hours afterward. It is a good idea to arrange a ride home if you know dilation will be part of your visit, since reading and driving may be temporarily affected.
Rather than one replacing the other, both technologies are increasingly being used side by side in clinical practice. SD-OCT has a strong, well-established track record for retinal layer imaging and remains the right tool for a wide range of conditions. SS-OCT adds value in specific situations, particularly for deeper tissue imaging. As both technologies continue to improve, the trend is toward using them together as complementary tools rather than one becoming obsolete.
If your condition is one where deeper choroidal imaging would be clinically useful, your provider can refer you to a center that has SS-OCT available. Many specialized retinal practices have both types of devices and can perform the most appropriate scan based on your needs. For routine monitoring of common retinal conditions, SD-OCT at your regular provider's office is likely to be entirely sufficient. Open communication with your provider about your condition and your imaging options is the best approach.
Advanced Retinal Imaging at Rhode Island Eye Institute
At Rhode Island Eye Institute, our team of fellowship-trained retinal specialists uses advanced OCT imaging technology to provide precise, thorough evaluations for patients across Rhode Island and southeastern Massachusetts. We are committed to selecting the right diagnostic tools for each patient's unique condition, so that every care decision is grounded in the clearest possible picture of your eye health. If you have questions about retinal imaging or would like to schedule a comprehensive evaluation, we welcome you to reach out to our team.