Optical Coherence Tomography (OCT) is an imaging technique that uses light interference to non-invasively obtain cross-sectional images of the retina and choroid. Unlike X-ray CT, which uses sound waves or radiation, OCT uses near-infrared light.
Introduced by Huang et al. in 1991, it rapidly spread in ophthalmology. Today, it is a standard examination for retinal diseases, glaucoma, and anterior segment diseases.
There are three main generations of OCT. Their characteristics are shown below.
TD-OCT
Wavelength: 810 nm
Speed: 400 A-scans/sec
Axial resolution: approximately 10 μm
First-generation method that obtains tomographic images by changing the optical path length with a movable reference mirror. It has now been largely replaced by SD-OCT.
SD-OCT
Wavelength: 840 nm
Speed: 20,000–70,000 A-scans/second
Axial resolution: 5–7 μm
Second-generation method that acquires depth information all at once using a spectrometer and Fourier transform. Currently the clinical standard. Suitable for detailed evaluation of the macula and optic nerve head.
SS-OCT
Wavelength: 1050 nm
Speed: 100,000–400,000 A-scans/second
Axial resolution: approximately 5 μm
Third-generation method using a swept-source laser and dual-balanced detector. The longer wavelength provides excellent visualization of deep structures such as the choroid. EDI (enhanced depth imaging) is not required.
OCT is a non-invasive, non-contact test that is completely painless. Although eye drops for pupil dilation may be necessary, it only involves shining light and does not touch the cornea or retina. The test usually takes only a few minutes.
OCT is used for diagnosis and follow-up of various diseases of the retina, macula, and choroid. The main diseases and representative OCT findings are shown below.
An overview of representative OCT findings for each disease is provided.
| Disease | Representative OCT Findings |
|---|---|
| Macular hole | Full-thickness retinal defect ± VMT |
| Epiretinal membrane | Hyperreflective layer on the inner surface |
| VMT | Partial adhesion of the posterior vitreous |
| Diabetic macular edema | Retinal thickening and cystoid edema |
| Pigment epithelial detachment | RPE elevation |
| CNVM | Subretinal hyperreflective material |
OCT is an essential tool for quantitative measurement of retinal thickness and monitoring of diabetic macular edema4). Key findings are shown below.
OCT enables quantitative assessment of macular edema and detection of vitreoretinal interface changes5). Evaluating the presence of cystoid macular edema, subretinal fluid, and VMT helps determine treatment strategy and follow-up.
OCT has excellent diagnostic accuracy for macular and posterior pole diseases, but is not suitable for detecting peripheral retinal lesions (e.g., lattice degeneration, retinal tears). Image quality and diagnostic reliability decrease in the presence of dense cataract or vitreous opacity. Wide-field fundus photography and indirect ophthalmoscopy are used for peripheral lesions.
Various artifacts can contaminate OCT images. Identifying artifacts is essential for accurate interpretation.
Due to imaging conditions
Mirror artifact: Caused by incorrect scan range settings, resulting in inverted and duplicated display of the actual image.
Vignetting: Signal attenuation at the periphery, depending on the incident angle of the illumination light.
Out-of-range error: Structures outside the set depth range are displayed as wraparound.
Patient Factors
Blink artifact: Blinking during imaging causes horizontal gaps.
Eye movement: Poor fixation leads to image misalignment or distortion.
Position shift: Caused by head movement during scanning.
Software Factors
Segmentation error: The automatic layer segmentation algorithm misidentifies retinal layers. This occurs frequently in lesions or severe cataracts.
Manual correction or rescanning is used to address this.
Reflectivity patterns on OCT images reflect the type and severity of disease. Representative patterns are shown below.
| Pattern | Findings | Representative Disease |
|---|---|---|
| Diffuse high reflectivity | Swelling of inner retinal layers | CRAO |
| HRF | Punctate hyperreflective foci <30 μm | Diabetic macular edema, retinal vein occlusion |
| DRIL | Disorganization of inner retinal layers | Diabetic macular edema |
| CME | Round to oval hyporeflective cavities | Diabetic macular edema, retinal vein occlusion |
In the management of diabetic retinopathy, regular measurement of macular thickness by OCT is an important indicator for initiating and retreatment decisions of anti-VEGF therapy4).
Poor fixation, blinking, and eye movements are major causes of artifacts and degrade image quality. Severe cataract, vitreous opacity, and poor pupil dilation (miosis) also reduce signal strength. Segmentation errors frequently occur at lesion sites, so it is important to visually confirm the validity of automated measurements.
OCT is based on the principle of the Michelson interferometer. Near-infrared light is split into a measurement beam and a reference beam, which are directed to the sample (fundus) and a reference mirror, respectively. The interference pattern (interferogram) generated when the reflected beams from both are recombined is used to calculate the reflection intensity at each depth. The profile of reflection intensities along depth is an A-scan, and A-scans arranged laterally form a B-scan (cross-sectional image).
The main difference lies in the wavelength used and the ability to visualize deep structures. SD-OCT uses the 840 nm band, while SS-OCT uses the 1050 nm band. Since 1050 nm light is less scattered by melanin pigment and penetrates the RPE more easily, SS-OCT is superior for observing the choroid and sclera. In addition, the imaging speed of SS-OCT exceeds that of SD-OCT, making wide-angle scanning easier. On the other hand, the axial resolution of both is about 5–7 μm, with no significant difference.
OCTA is attracting attention for its ability to non-invasively visualize retinal vascular structures, and is being applied to detect fine avascular areas and neovascularization that were difficult to identify with FA. Research is underway to improve screening accuracy for diabetic retinopathy and to monitor the therapeutic effects of anti-VEGF therapy.
The high-speed, wide-angle scanning capability of SS-OCT is making it possible to perform tomographic imaging of broad areas including the peripheral retina. Efforts are underway to evaluate lesions in the macula and periphery in a single scan. In addition, detailed assessment of choroidal thickness and sclera using the characteristics of the 1050 nm wavelength is contributing to the elucidation of pathologies in myopia and choroidal diseases.
Evaluation of ocular complications of systemic diseases through OCT findings is also a growing area of research.
Dou et al. (2024) reported a case of a giant RPE tear in a patient with membranous nephropathy and provided a literature review on the association between renal and ocular diseases1). OCT confirmed that the RPE tear occurred as a sudden flattening of a pigment epithelial detachment, suggesting that OCT is useful for monitoring ocular complications in patients with systemic diseases.
