It is 16 years since David Huang and colleagues in the James Fujimoto laboratory at the Massachusetts Institute of Technology developed optical coherence tomography (OCT).1 Following the introduction of Zeiss Stratus Optical Coherence Tomography (third-generation OCT or OCT 3) by Carl Zeiss Meditec Inc., it became a ‘gold standard’ for ocular examination of macular diseases. The technology used during examination enables the presentation of the retinal structure analogous to histological sections. This allows visualisation of the retinal and optic nerve pathology. Nowadays, no retina specialist can imagine his or her work without OCT. Various clinical disorders have been presented in OCT. As new developments in ophthalmic therapy emerged, it became clear that perfect visualisation of the retina and measurements of retina thickness allow monitoring of therapy with triamcinolone or anti-vascular endothelial growth factor (VEGF) drugs.
OCT is based on the measurement of back-reflected infrared light. This is performed indirectly because of the high speed at which light spreads. The technique is called low-coherence interferometry. In this technique, a back-reflected beam of light is interfered with by a reference beam with a known echo time delay. The optical interference measures the echo time delay of light from the ocular structures and gives us axial information similar to A-scans in ultrasonography. The sum of many A-scans provides the opportunity to produce a cross-sectional image similar to a B-scan. Even though this technology was so revolutionary to ophthalmology it has still two important disadvantages, the first with resolution and the second with imaging speed. Although the resolution of Stratus OCT is the best-known commercially available diagnostic instrument in medicine, the resolution is still too low to detect early structural changes in ocular tissue. This can be improved by using a more advanced light source such as femtosecond titanium:sapphire lasers.2–4 This technology allows axial image resolution more than three times higher than that of Stratus OCT to be achieved. However, the application of this technology is limited because this light source is large and expensive. Low-coherence interferometry does not allow increased imaging speed, nor would a longer examination be more precise because it increases the amount of artefacts created by eye movements. Current commercially available systems allow 512–1,024 axial scans per image up to 1.9 seconds.5 With these aspects in mind, the search for improvement needed new technology.
It would appear that the recently developed spectral OCT (SOCT) – relating to the Fourier Domain OCT – might solve the resolution and speed problems. Wojtkowski et al.6 were the first to report retinal imaging using SOCT. Wojtkowski et al./7 presented a combination of Fourier Domain OCT and ultra-high-resolution OCT. SOCT can measure all echoes of light from different delays simultaneously. This increases the speed of imaging by 50–100 times, thus providing much more information. The sensitivity of the examination is increased. In recent publications we observe growing interest in Fourier Domain SOCT and rapid development in this field. The use of femtosecond titanium:sapphire technology will probably not be so important in the future since rapid improvements in super-luminescent diode technology have been demonstrated. They also allow the achievement of ultra-high axial image resolutions of 3.1μm.8