Optical Coherence Tomography (OCT) is a non-invasive imaging method used to capture detailed, cross-sectional pictures of the eye. Unlike ultrasound, OCT uses light waves to measure echoes from different structures within the eye. The resulting images provide a microscopic view of tissue layers, which is invaluable for diagnosing and managing eye conditions. Swept Source OCT (SS-OCT) represents a significant advancement, utilizing a newer light source and detection method to produce faster, more detailed scans. This generation of OCT provides deeper insight into ocular anatomy than was previously possible.
How the Swept Source Mechanism Works
The difference between SS-OCT and earlier spectral-domain systems lies in the light source and how reflected light is processed. Older systems use a broadband light source and separate the reflected light into wavelengths simultaneously using a spectrometer.
Swept Source technology, in contrast, employs a rapidly tunable laser that “sweeps” across a narrow range of wavelengths over time. Instead of a spectrometer, the system uses a single, high-speed photodetector to measure the light returning from the eye sequentially as the laser sweeps. This sequential detection process allows for improved speed of data acquisition.
The SS-OCT light source also operates at a longer wavelength, typically around 1050 nanometers, compared to the 850 nanometers used by many spectral-domain systems. This longer wavelength penetrates denser tissues more effectively. By sweeping the light source and detecting the signal sequentially, the system generates a complete depth profile (A-scan) with each sweep, which is then compiled into a detailed cross-sectional image.
Performance Gains Over Standard Imaging
The sweeping laser mechanism provides several performance advantages that enhance scan quality and utility.
Increased Scanning Speed
One major gain is the dramatically increased scanning speed, with many SS-OCT systems capable of capturing 100,000 or more depth profiles per second. This speed significantly reduces the impact of microscopic eye movements that can blur or distort images, resulting in clearer, more reliable data. Faster scanning also decreases the total time a patient needs for the procedure, improving comfort and clinic efficiency.
Deeper Tissue Penetration
The longer wavelength of the swept source light is less scattered by the overlying structures of the retina. This allows for high-resolution visualization of the choroid, the vascular layer beneath the retina. Deeper penetration also helps capture clear images in eyes with media opacities, such as those with dense cataracts.
Wider Field of View
SS-OCT is capable of acquiring a wider field of view in a single scan. The combination of high speed and deep penetration enables the device to capture large-area scans, sometimes up to 12 by 9 millimeters. Capturing a larger area provides a more comprehensive overview for diagnosis and monitoring.
Primary Uses in Eye Care
The enhanced speed and penetration of SS-OCT make it an invaluable tool for diagnosing and managing a wide spectrum of eye conditions.
Posterior Segment Imaging
In the posterior segment, including the retina and choroid, SS-OCT monitors progressive diseases. For age-related macular degeneration (AMD), it detects subtle fluid accumulation and the presence of drusen (fatty deposits beneath the retina). It is also highly effective for patients with diabetic retinopathy, allowing for precise measurement of retinal swelling, known as macular edema. Detailed visualization of the choroid and its vascular network helps guide treatment decisions and monitor the patient’s response to therapy.
Anterior Segment and Glaucoma
Beyond the retina, SS-OCT images the anterior segment, which includes the cornea, iris, and drainage angle. The fast acquisition speed is beneficial for obtaining three-dimensional images of the angle structures, which is an important component of glaucoma diagnosis. Precise measurements of the retinal nerve fiber layer thickness around the optic nerve are also obtained, tracking the progression of glaucomatous damage over time.
What to Expect During the Scan
Undergoing a Swept Source OCT scan is a quick, non-contact, and painless process. The patient is seated comfortably in front of the machine, similar to having a photograph taken of the eye. The technician will ask the patient to rest their chin and forehead on supports to ensure the head remains steady during the brief imaging sequence.
The patient looks at a fixed target while the machine directs a low-intensity beam of light into the eye. The invisible nature of the light source and the high speed of the technology mean the entire scan is completed in seconds per eye, minimizing movement that could compromise image quality.
In many cases, clear images are obtained without the need for pupil-dilating eye drops. However, dilation may still be required depending on the specific structures the doctor needs to examine. Once the scan is complete, the detailed images are immediately available for the eye care professional to review.