Optical coherence tomography (OCT) is a noninvasive, non-contact imaging technique which produces cross-sectional images with millimeter penetration (approximately 2–3 mm in tissue) and micrometer scale axial and transverse resolution of not only the retina and optic nerve but also the anterior segment of the eye. The technique was first demonstrated in 1991 with ∼30 µm axial resolution. With advancement in technology, today we have the conventional OCT with an axial resolution of ≤ 10 µm and the 3D-OCT with a much higher resolution of 5 µm.
The working principle of OCT is similar to ultrasound, which uses echoes to locate structures within the body. The speed of light being almost a million times faster than sound allows measurement of structures with a resolution of ≤ 10 microns as compared to 100 microns scale for ultrasound.
OCT works through the magic of low-coherence interferometry. Broad bandwidth near infrared light (820 nm) generated by using a superluminescent diode projected on the retina. The light is then broken into two arms—a sample arm (containing the item of interest) and a reference arm (usually from a mirror). The combination of reflected light from the sample arm and light from the reference arm gives rise to an interference pattern. By scanning the mirror in the reference arm, a reflectivity profile of the sample can be obtained (this is time domain OCT). This reflectivity profile, called an A-scan contains information about the spatial dimensions and location of structures within the item of interest. A cross-sectional tomograph (B-scan) may be achieved by laterally combining a series of these axial depth scans (A-scan).
OCT can clearly image the cornea, sclera, iris, and lens in the anterior segment. Using infrared light having a wavelength of 13.10 nm (Figure 1.1).
OCT is the only technique which resolves the substructure of the retina in living eyes. It has therefore developed into an indispensable diagnostic tool for managing patients with vitreoretinal diseases. Figure 1.2 illustrates the capability of OCT to detect sub-surface microscopic tissue layers in vivo.
New three-dimensional optical coherence tomography (3D-OCT) instruments developed by companies such as Topcon, Inc. are cutting-edge ophthalmic imaging solutions that merge technology, speed, quality, and clinical versatility. These devices are up to 50 times faster and 25% more accurate than conventional OCT.
Using higher speeds, 3D-OCT acquires more data in less time easing the scanning process for both patients and technicians. 3D-OCT provides unparalleled views of retinal and subretinal structures with the best resolution available in any commercial device. Capturing, viewing, and analyzing images are easier with the software improvements. Integration with a non-mydriatic retinal camera as in the Topcon 3D-OCT allows for a direct comparison between the raster scans on the retinal image to the corresponding OCT tomographic layers.
3D-OCT based on spectral domain technology (SD-OCT) replaces the moving parts found in conventional OCT instruments with a stationary spectrometer. By using mathematical calculations, an SD-OCT instrument can capture scans very quickly-before the patient shifts his or her gaze. The faster speed, higher resolution and improved signal-to-noise ratio of SD-OCT systems may ultimately improve the detection of the true outer retinal boundary and retinal layers, and enable the detection and quantification of clinically relevant disease features, such as subretinal fluid.5
One of the distinguishing features of the 3D-OCT device is that it can capture 256 B-scans in rapid succession, resulting in a dense three-dimensional cube of OCT information. This 3-D data can then be viewed en face as a ‘C-scan’, or in an infinite number of orientations and directions to visualize specific regions of the fundus.
Despite numerous, rapid technological advances in the last decade, conventional time domain OCT devices have many hardware and software limitations. Because these instruments rely on mechanical movements of internal components to make thickness measurements, their image acquisition speed is limited. This restricts the number of scans that can be acquired before a patient looks away from the fixation point. For instance, current time domain OCT instruments construct a high-resolution retinal thickness map using 6 radially oriented B-scans captured in 8–10 seconds. In this mode, however, the instrument measures less than 5% of the total macular area and must approximate more than 95% of the output data. Therefore, the device may incorrectly estimate retinal thickness, and may miss small lesions that fall between the radial-line scans (Figures 1.3A to C).
Figure 1.3: (A) The white lines on this fundus image indicate the locations of the six radial line B-scans performed by Stratus OCT. (B) Red lines show points measured by Stratus OCT, and non-red points are estimated. (C) Example of a focal lesion that could be missed by the radial-line protocol
In contrast, 3D-OCT captures a dense, uniform grid of data that is unlikely to miss small, focal lesions (Figure 1.4). Furthermore, the consistency of the information allows blood vessels on the scans to be aligned between patient visits, and comparison of scans to other diagnostic imaging modalities, such as fluorescein angiograms.
Figure 1.4: Examples of data from the Topcon 3D-OCT instrument showing: Dense grid of B-scans (from the area with the soft drusen)(Courtesy: Yijun Huang, Topcon Medical Systems, Inc.)
Role of OCT Imaging
OCT is useful in the diagnoses and staging of diseases which help in their management, assessing the response of disease to treatment and in monitoring the progress of the disease.
| Comparison of three different types of OCT |
Feature | Conventional | OCT UHR-OCT* | 3D-OCT |
|---|---|---|---|
Measurement principle Measurement capabilities | Time domain A-scan B-scan | Time domain A-scan B-scan | Spectral domain A-scan B-scan C-scan 3D-scan |
Light source Wavelength Bandwidth Axial resolution Transverse resolution Scanning speed Dilatation Macular scan | Superluminescent diode laser 820 nm 25 nm 10 μm 20 μm 1.3 sec Required 6 radial scans | Femtosecond sapphire laser 815 nm 125 nm 3 μm 15–20 μm 4sec Required 6 radial scans | Superluminescent diode laser 840 nm 50 nm 5 μm <20 μm 0.05 sec Not required Raster scan |
Various types of Spectral/Fourier Domain OCT are commercially available:
- TOPCON – 3D OCT-1000
- OTI – OCT/SLO
- OPTOVUE – RTVue-100
- OPTOPOL – SOCT COPERNICUS.
SUGGESTED READING
- Sunitha R, Andrew MR, Jonathan ER, Siavash Y, Volker W, David SB, et al. Real-time optical coherence tomography of the anterior segment at 1310. Arch Ophthalmol 2001;119:1179–85.
- Tony H Ko, Andre J Witkin, James G Fujimoto, Annie Chan, Adam H Rogers, Caroline R Baumal, et al. Ultrahigh-resolution optical coherence tomography of surgically closed macular holes. Arch Ophthalmol 2006; 124(6): 827–36.