The Last 30 Years & Optical Coherence Tomography

The Last 30 Years & Optical Coherence Tomography

October 21, 2020

An optical signal acquisition method called optical coherence tomography (OCT) is now employed in clinical diagnostics to assess organs. OCT can produce three-dimensional images with a resolution of a few micrometers inside biological tissues. In fact, OCT has emerged as a crucial imaging tool for identifying and monitoring macular diseases.

It has frequently been used in conjunction with fluorescein angiography, particularly for the diagnosis and treatment of a variety of retinal illnesses, such as macular edema and age-related macular degeneration.

Additionally, advancements made it possible to use optical coherence tomography to assess the corneal topography, anterior chamber angle, optic nerve cupping, and nerve fiber layer integrity.

When optical coherence tomography was initially introduced in 1991, it built on earlier research on ocular interferometry, which effectively demonstrated that imaging the eye could be done by measuring reflected light. Imaging of the retina through to the choroid is possible by using laser light in the infrared band.

What is feasible has been increased thanks to faster ways of scanning from the time domain to the Fourier domain, swept-source OCT, enhanced depth imaging OCT, and OCT angiography, among other innovations.

The development of treatments that call for meticulous evaluation of ocular structures—in particular, evaluation of macular fluid on OCT—has increased demand. This evaluation informs the use of anti-VEGF injections and treatment regimens.

To commemorate the discovery of OCT 25 years ago, this study will offer a succinct history of the technology while also offering an update on current OCT capabilities and how they might support clinical practice.


Similar to how ultrasound imaging works, optical coherence tomography detects images by employing an interferometer since light waves travel at too high a speed to be recorded directly.

The interferometer for time-domain OCT (TD-OCT) has a reference arm with a mirror and a sample arm that detects the reflected light from the retina. A mirror on the reference arm can be mechanically adjusted to change the time delay and measure interference.

Because the light from both arms has traveled a similar distance, when backscattered light from the sample arm and reference light from the reference arm are measured together, an interference pattern is produced. As a result, a reflectivity profile, or A-scan, is produced. A reviewable B-scan is then produced by combining numerous A-scan photos.

Although this imaging modality had limitations in terms of picture resolution and acquisition speed, it marked a significant advancement in ophthalmological imaging at the time it was established.

Over a decade after the invention of OCT, Fourier domain OCT (FD-OCT) with a subset known as spectral-domain OCT represented a significant technological advance (SD-OCT). An interferometer and a stationary reference arm use Fourier transformation to detect light echo interference patterns, which is how SD-OCT operates.

A broadband source is used, and a spectrometer disperses the spectral interference pattern while simultaneously collecting it on an array detector. Compared to TD-OCT, this technology has enabled imaging that is more faster and has higher resolution.

OCT was initially developed and commercialized by Zeiss in 1997 with the OCT1. With the introduction of the Stratus (OCT3) in 2002 and the Zeiss Visante in 2006, imaging of the anterior segment became widely used and simple to use.

Zeiss's newest imaging technologies include the Cirrus 5000 OCTA (OCT angiography). By monitoring blood flow and extracting that information to display a picture of the retinal circulation, this technique uses OCT to image the retinal circulation. This makes it possible to see the choroidal neovascularization and some choroid features as well as the superficial and deep capillary plexae.

The most recent Zeiss IOL master 700 biometry system uses swept-source OCT to photograph the entire eye, increasing measurement accuracy because it can assess fixation, find anomalies in lens position, and penetrate a wider range of cataract.

The OCT system may also have additional imaging and functional testing modalities such as color or multi-color imaging, autofluorescence, fundus fluorescein angiography (FFA), indocyanine green (ICG), and microperimetry. Many companies are currently involved, including Topcon, Heidelberg, Nidek, Optovue, Canon, and Optos. Some have eye-tracking, which improves repeatability and resolution, while others employ focusing, which makes acquisition easier.

Medical Retina

Since the Stratus OCT was introduced, it has been widely utilized for macular assessments in clinics, screenings, and "virtual clinics" because it is more reliable than clinical biomicroscopy at detecting retinal and subretinal fluid. The retinal architecture might be resolved down to around 10 microns using the Stratus (a TD-OCT).

Finer detail of the retinal architecture as well as the posterior vitreous and choroid may now be seen thanks to the development of SD-OCT devices like the Spectralis (Heidelberg) or Cirrus (Zeiss) OCT and, more recently, swept-source technology, which is found in the Topcon Atlantis.

With the numerical assessment of the central foveal subfield, the four inner and outer subfields, as well as the macular volume, these devices have made it possible to quantify macular edema. For many retinal disorders that result in macular edema or retinal thickness, this has emerged as the gold standard of measurement.

The algorithmic measurement of the central macular thickness varies between the stratus (TD-OCT) and SD-OCTs as well as between various SD-OCTs. Therefore, depending on the machine used, various numerical figures are obtained.

OCT only reveals the lesion's shape, not its activity, but frequently these two concepts are the same. OCT is the cornerstone for assessing the effectiveness of treatment because the majority of current retinal therapies aim to reduce edema. FFA will demonstrate the presence of a leak, complementing OCT results.

When there is no leak on FFA but "fluid" is seen on OCT, as is the situation with central serous retinopathy (CSR), there may be a delay between the leak ceasing and the subretinal fluid clearing up. Current recommendations for macular degeneration include starting FFA before starting anti-VEGF therapy.

However, a choroidal neovascular membrane can often be observed on an OCT, therefore less and less FFAs are performed, especially if this might cause a delay in treatment.

As choroid imaging becomes more reliable, it can be used to diagnose CSR, which is marked by a thicker choroid, and to track the effectiveness of treatment in some inflammatory disorders.

This is now achievable because to the technology known as enhanced depth imaging OCT (EDI-OCT), which was initially introduced in 2008 by Spaide et al. This was accomplished by moving the OCT device closer to the subject's eye, which produced an inverted image that was then reoriented using the appropriate software.

As opposed to traditional OCT, this caused the peak sensitivity to shift from the posterior vitreous to the inner sclera. For EDI-OCT, this can now be done automatically thanks to software correction. The use of swept-source OCT and longer wavelengths, among other advancements, can offer a greater depth of resolution from the posterior vitreous to the choroid.

It has been demonstrated that choroidal neovascular lesions can be found using the commercially available OCT angiography (OCTA), particularly if type 2 lesions are present in front of the retinal pigment epithelium (RPE). It creates a map of blood flow by comparing the backscatter or echo of light waves from successive B-scans taken in the same location.

There are three typical ways to perform OCTA using swept-source OCT or SD-OCT. They entail measuring the OCT signal's amplitude/intensity, phase, or complex properties (involving both phase and amplitude).

Speckle variance, intensity-based optical microangiography (OMAG), intensity-based doppler variance, cross-correlation mapping, and split-spectrum amplitude-decorrelation angiography are a few amplitude/intensity OCTA technique examples (SSADA).

In order to build a map of blood flow, these algorithms measure variance, decorrelation, cross-correlation, or absolute differences of OCT signal amplitude/intensity from sequential B-scans. Motion artifact is a less significant issue with these techniques.

Through software correction, motion artifacts in comparative phase and complex based OCTAs like phase variance, doppler variance, or OMAG must be removed. Although OCTA doesn't reveal leak, it might offer sufficient information in a non-invasive manner to reduce the need for FFA and ICG, which could quicken the treatment process.

With the development of medications that more successfully diminish the extent of neovascularization, such as those shown with anti-PDGF therapies that are currently in phase 3 trials, the measurement of the size of the neovascular network may become increasingly significant.


In addition to clearly aiding in the detection of medical retinal disorders like age-related macular degeneration (AMD), OCT imaging has also made it possible to diagnose a number of vitreoretinal conditions such macular holes and epiretinal membranes.

By imaging the vitreomacular interface, OCT imaging can supplement vitreoretinal surgery and may be more and more helpful in clinical settings. For instance, Steel D et al. found that OCT data taken during therapy can be used to predict the likelihood of macular hole closure using ocriplasmin.

The Topcon sweeping source systems and Heidelberg systems enable higher-resolution wide-field OCT imaging, allowing for the visualization of the posterior vitreous, macular, and optic disc details in a single image. Intraoperative OCT, such as that provided by Zeiss and Optovue systems, may further assist vitreoretinal surgery.


OCT is increasingly employed in the diagnosis and monitoring of glaucoma because it may be used to evaluate the thickness of the retinal nerve fiber layer (RNFL) both at the optic disc and at the macular. This is getting better as a result of stronger normative databases and higher resolution technologies working together. Ganglion cell loss coincides with RNFL thinning at the discs, which suggests glaucomatous alterations to the optic disc.

Since different OCT devices, such the Cirrus and Spectralis OCT, employ various RNFL algorithms, it is not possible to compare the results of their measurements. More precise measurements of the RNFL are now possible thanks to the advent of Fourier domain OCTs. There are still some issues in determining the disc edge to assess the degree of cupping and where to measure the nerve fiber layer.

With the advancement of computational measures of the RNFL, this has been partially rectified. In certain methods, two rings around the disc are measured to increase the precision of calculating the thickness of the nerve fiber layer. Glaucoma can also be identified and monitored using macular RNFL measures above and below the fovea.


In neuro-ophthalmology, SD-OCT is helpful for evaluating lesions in the optic nerve and visual pathway. Through measurements of the RNFL, SD-OCT can assess the optic nerve head for optic disc edema, disc asymmetry, and optic nerve head drusen.

Swept-source imaging may be more effective for locating optic disc drusen because of its capacity for deeper penetration. OCT can be used to accurately assess the degree of optic nerve edema and, in some situations, to differentiate between papilloedema and an optical disc that is "packed."

Due to its repeatability, it can be used to track the enlargement of the optic disc in cases of benign intracranial hypertension. Macular issues have been found to be related to several optic nerve diseases, either in the form of true macular edema or a cystic appearance of the macula brought on by retrograde nerve fiber degeneration, which can be validated by OCT.

Anterior segment

The anterior region and cornea could be examined using OCT for the first time, as demonstrated by Izatt et al. There are several systems available, with some dedicated systems (1310nm, for example, Zeiss Visante, Heidelberg SL-OCT) supplying longer wavelength light sources and retinal scanner conversions giving lower wavelength light sources (830nm e.g. Zeiss Cirrus, Heidelberg Spectralis).

Deeper penetration made possible by longer wavelength imaging allows for a clearer view of deeper structures, such as the scleral spur at the iridocorneal angle. A better axial resolution is possible with shorter wavelength imaging.

Prior section OCT can offer a precise evaluation of the tear film. Measurements of conjunctival lesions like pingueculas and pterygiums may be useful for tracking post-operative changes. The anterior chamber angle can be evaluated using measurements of the angle, iris, and anterior chamber.

In order to diagnose illnesses such an iris cyst or melanoma that may be the cause of an anterior displacement of the iris, an anterior segment ultrasound may still be required to view behind a pigmented iris. OCT has demonstrated some useful applications in assessing the success of filtration blebs created during trabeculectomy surgery.

OCT has been looked into as a helpful tool for detecting corneal pathology and performing corneal refractive surgery. It enables the accurate measurement of corneal pathology together with associated measurements of corneal thickness.

Planning operations beforehand and analyzing outcomes afterward may be made simpler. OCT has been demonstrated to be helpful in monitoring the maintenance of certain diseases, including as keratoconus and corneal ectasias, with collagen cross-linking when used in conjunction with confocal microscopy. OCT can be used to measure and grade post-LASIK ectasias in refractive surgery. Intraoperative OCT may improve the evaluation of the location of different keratoplasty surgeries.

Future of OCT

Software updates that improve the 3D representation of vascular networks and ocular structures using various laser wavelengths and polarized light are currently being developed.

As part of a current effort, Google is examining a database of one million OCTs to develop automation for understanding OCT results and predicting patient outcomes.

With prototypes being created, newer laser technologies are projected to enable considerably smaller adaptive systems. Binocular OCT, for instance, is being developed and will be able to measure visual acuity, pupil reflexes, and eye movement in addition to imaging the ocular structures.