51– 54 With SD-OCT, it is challenging to image deep structures due to relatively poor wavelength penetrance and decreasing sensitivity and resolution with increasing depth. 51 Swept-source OCT can evaluate RNFL and macular thickness, but can also more clearly image deeper ocular structures such as the choroid and lamina cribrosa (LC) in patients. 50 Swept-source OCT uses a longer wavelength (generally 1050 nm) compared with SD-OCT (840 nm). Swept-source OCT (SS-OCT), a newer generation of OCT, has recently been commercially introduced. This greatly decreased the need for interpolation compared with TD-OCT. Previous TD-OCT B-scans had an axial resolution of approximately 10 μm, whereas the introduction of typical commercially available SD-OCT instruments improved resolution to approximately 5 μm axially with broad bandwidths at near infrared wavelengths. 17, 34– 43 One of OCT's main strengths is its unparalleled high axial image resolutions.
24– 33 The introduction of spectral-domain OCT (SD-OCT), which instead acquired all information within a single axial scan simultaneously through the tissue by evaluating the frequency spectrum of the interference between the stationary reference mirror and reflected light, increased reproducibility and accuracy in quantifying glaucomatous damage by further improving scan density and resolution and reducing imaging artifacts and scan acquisition time. However, this technology was limited by slow scan acquisition times and two-dimensional imaging. Previously available OCT instruments used a technique referred to as time-domain OCT (TD-OCT), which encoded the location reflections in the time information and related the location of the reflection to the position of the moving reference mirror, could obtain images of the fundus, discriminate glaucomatous eyes from normal, and detect change over time. Hardware advances in commercial systems improved resolution and increased scanning speeds. Optical coherence tomography technology has advanced since it was first applied to the eye and continues to rapidly evolve.