2, 3
1 FSAU NMITs "MNTK" Eye Microsurgery "them. acad. S. N. Fedorova "Ministry of Health of Russia, Moscow
2 FKU "TsVKG im. P.V. Mandryka "Ministry of Defense of Russia, Moscow, Russia
3 FSBEI IN RNIMU them. N.I. Pirogov, Ministry of Health of Russia, Moscow, Russia

Optical coherence tomography(OCT) was first used for visualization of the eyeball more than 20 years ago and still remains an indispensable diagnostic method in ophthalmology. With the help of OCT, it became possible to obtain non-invasive optical sections of tissues with a resolution higher than that of any other imaging method. The dynamic development of the method has led to an increase in its sensitivity, resolution, and scanning speed. Currently, OCT is actively used for diagnostics, monitoring and screening of diseases of the eyeball, as well as for performing scientific research. The combination of modern OCT technologies and photoacoustic, spectroscopic, polarization, Doppler and angiographic, elastographic methods made it possible to assess not only the morphology of tissues, but also their functional (physiological) and metabolic state. Operating microscopes with the function of intraoperative OCT have appeared. The presented devices can be used to visualize both the anterior and posterior segments of the eye. This review examines the development of the OCT method, presents data on modern OCT devices, depending on their technological characteristics and capabilities. Methods of functional OCT are described.

Please cite this paper as: Zakharova M.A., Kuroedov A.V. Optical coherence tomography: a technology that has become a reality // RMZh. Clinical ophthalmology. 2015. No. 4. P. 204–211.

For citation: Zakharova M.A., Kuroedov A.V. Optical coherence tomography: a technology that has become a reality // RMZh. Clinical ophthalmology. 2015. No. 4. S. 204-211

Optic coherent tomography - technology which became a reality

Zaharova M.A., Kuroedov A.V.

Mandryka Medicine and Clinical Center
The Russian National Research Medical University named after N.I. Pirogov, Moscow

Optical Coherence Tomography (OCT) was first applied for imaging of the eye more than two decades ago and still remains an irreplaceable method of diagnosis in ophthalmology. By OCT one can noninvasively obtain images of tissue with a resolution higher than by any other imaging method. Currently, the OCT is actively used for diagnosing, monitoring and screening of eye diseases as well as for scientific research. The combination of modern technology and optical coherence tomography with photoacoustic, spectroscopic, polarization, doppler and angiographic, elastographic methods made it possible to evaluate not only the morphology of the tissue, but also their physiological and metabolic functions. Recently microscopes with intraoperative function of the optical coherence tomography have appeared. These devices can be used for imaging of an anterior and posterior segment of the eye. In this review development of the method of optical coherence tomography is discussed, information on the current OCT devices depending on their technical characteristics and capabilities is provided.

Key words: optical coherence tomography (OCT), functional optical coherence tomography, intraoperative optical coherence tomography.

For citation: Zaharova M.A., Kuroedov A.V. Optic coherent tomography - technology which became a reality. // RMJ. Clinical ophthalomology. 2015. No. 4. P. 204–211.

The article is devoted to the use of optical coherence tomography in ophthalmology

Optical coherence tomography (OCT) is a diagnostic technique that allows high-resolution tomographic sections of internal biological systems to be obtained. The name of the method is first cited in a work by a team at the University of Massachusetts of Technology, published in Science in 1991. The authors presented tomographic images showing in vitro the peripapillary zone of the retina and the coronary artery. The first intravital studies of the retina and anterior segment of the eye using OCT were published in 1993 and 1994. respectively . The following year, a number of works were published devoted to the application of the method for the diagnosis and monitoring of diseases of the macular region (including macular edema with diabetes mellitus, macular holes, serous chorioretinopathy) and glaucoma. In 1994, the developed OCT technology was transferred to the foreign division of Carl Zeiss Inc. (Hamphrey Instruments, Dublin, USA), and already in 1996 the first serial OCT system intended for ophthalmic practice was created.
The principle of the OCT method is that a light wave is directed into the tissue, where it propagates and is reflected or scattered from the inner layers, which have various properties... The resulting tomographic images are, in fact, the dependence of the intensity of the signal scattered or reflected from structures inside tissues on the distance to them. The imaging process can be viewed as follows: a signal is sent to the tissue from a source, and the intensity of the returning signal is measured sequentially at regular intervals. Since the speed of propagation of the signal is known, the distance is determined by this indicator and the time of its passage. Thus, a one-dimensional tomogram (A-scan) is obtained. If you consistently move along one of the axis (vertical, horizontal, oblique) and repeat the previous measurements, you can get a two-dimensional tomogram. If you sequentially move along one more axis, you can get a set of such slices, or a volumetric tomogram. Low coherence interferometry is used in OCT systems. Interferometric methods can significantly increase the sensitivity, since they are used to measure the amplitude of the reflected signal, and not its intensity. The main quantitative characteristics of OCT devices are axial (depth, axial, along A-scans) and transverse (between A-scans) resolution, as well as the scanning speed (number of A-scans per 1 s).
The first OCT devices used a sequential (temporal) imaging method (time-domain optical coherence tomography, TD-OC) (Table 1). This method is based on the principle of interferometer operation proposed by A.A. Michelson (1852-1931). A beam of low coherence light from a superluminescent LED is divided into 2 beams, one of which is reflected by the object under study (eye), while the other passes along the reference (comparative) path inside the device and is reflected by a special mirror, the position of which is regulated by the researcher. When the length of the beam reflected from the tissue under study and the beam from the mirror are equal, the phenomenon of interference occurs, which is registered by the LED. Each measuring point corresponds to one A-scan. The obtained single A-scans are summed up, as a result of which a two-dimensional image is formed. The axial resolution of commercial first-generation instruments (TD-OCT) is 8-10 µm at a scan rate of 400 A-scans / s. Unfortunately, the presence of a movable mirror increases the examination time and decreases the resolution of the device. In addition, eye movements that inevitably occur with a given scan duration, or poor fixation during the study lead to the formation of artifacts that require digital processing and can hide important pathological features in tissues.
In 2001, a new technology was presented - ultra-high-resolution OCT (UHR-OCT), which made it possible to obtain images of the cornea and retina with an axial resolution of 2-3 microns. A femtosecond titanium-sapphire laser (Ti: Al2O3 laser) was used as a light source. Compared to the standard resolution of 8–10 µm, high-resolution OCT has begun to provide better visualization of the retinal layers in vivo. The new technology made it possible to differentiate the boundaries between the inner and outer layers of photoreceptors, as well as the outer boundary membrane. Despite the improvement in resolution, the use of UHR-OCT required expensive and specialized laser equipment, which did not allow its use in wide clinical practice.
With the introduction of spectral interferometers using the Fourier transform (Spectral domain, SD; Fouirier domain, FD), the technological process has acquired a number of advantages over the use of traditional time-domain OCT (Table 1). Although the technique has been around since 1995, it was not used for retinal imaging until almost the early 2000s. This is due to the advent of high-speed cameras (charge-coupled devices, CCDs) in 2003. The light source in the SD-OCT is a broadband superluminescent diode, which produces a low-coherence beam containing multiple wavelengths. As in traditional OCT, a light beam is divided into 2 beams in spectral OCT, one of which is reflected from the object under study (eye), and the second - from a fixed mirror. At the output of the interferometer, the light is spatially decomposed over the spectrum, and the entire spectrum is recorded by a high-speed CCD camera. Then, using the mathematical Fourier transform, the interference spectrum is processed and a linear A-scan is formed. Unlike traditional OCT, where a linear A-scan is obtained by sequentially measuring the reflecting properties of each individual point, in spectral OCT, a linear A-scan is formed by one-step measurement of the rays reflected from each individual point. The axial resolution of modern spectral OCT devices reaches 3–7 µm, and the scanning speed is more than 40 thousand A-scans / s. By far the main advantage of SD-OCT is its high scanning speed. First, it can significantly improve the quality of the resulting images by reducing artifacts arising from eye movements during examination. By the way, a standard linear profile (1024 A-scans) can be obtained on average in just 0.04 s. During this time, the eyeball makes only microsaccadian movements with an amplitude of several arc seconds, which do not affect the study process. Secondly, it became possible to reconstruct the image in 3D, which makes it possible to assess the profile of the structure under study and its topography. Acquisition of multiple images simultaneously with spectral OCT made it possible to diagnose small-sized pathological foci. So, with TD-OCT, the macula is displayed according to the data of 6 radial scans, as opposed to 128-200 scans of the same area when performing SD-OCT. Thanks to the high resolution, the retinal layers and the inner layers of the choroid can be clearly visualized. The outcome of the standard SD-OCT study is a protocol that presents the results obtained both graphically and in absolute values. The first commercial spectral optical coherence tomograph was developed in 2006, it was RTVue 100 (Optovue, USA).

Currently, some spectral tomographs have additional scanning protocols, which include: a pigment epithelium analysis module, a laser scanning angiograph, an Enhanced depth imagine (EDI-OCT) module, and a glaucoma module (Table 2).

A prerequisite for the development of the EDI-OCT module was the limitation of visualization of the choroid using spectral OCT due to the absorption of light by the retinal pigment epithelium and its scattering by the choroidal structures. A number of authors used a spectrometer with a wavelength of 1050 nm, with the help of which it was possible to qualitatively visualize and quantify the choroid itself. In 2008, a method of imaging the choroid was described, which was implemented by placing the SD-OCT device close enough to the eye, as a result of which it became possible to obtain a clear image of the choroid, the thickness of which could also be measured (Table 1). The principle of the method is the appearance of mirror artifacts from the Fourier transform. In this case, 2 symmetrical images are formed - positive and negative with respect to the zero delay line. It should be noted that the sensitivity of the method decreases with increasing distance from the eye tissue of interest to this conditional line. The intensity of the display of the retinal pigment epithelium layer characterizes the sensitivity of the method - the closer the layer is to the zero delay line, the greater its reflectivity. Most devices of this generation are designed to study the layers of the retina and the vitreoretinal interface, so the retina is located closer to the zero delay line than the choroid. During processing of scans, the bottom half of the image is usually deleted, only it is displayed top part... If you move the OCT scans so that they cross the zero delay line, then the choroid will be closer to it, this will allow it to be visualized more clearly. Currently, the module of increased image depth is available for Spectralis tomographs (Heidelberg Engineering, Germany) and Cirrus HD-OCT (Carl Zeiss Meditec, USA). EDI-OCT technology is used not only to study the choroid in various ocular pathologies, but also to visualize the ethmoid plate and assess its displacement depending on the stage of glaucoma.
Fourier-domain-OCT methods also include swept-source OCT (SS-OCT; deep range imaging, DRI-OCT). The SS-OCT uses frequency swept laser sources, that is, lasers in which the emission frequency is tuned at high speed within a certain spectral band. In this case, the change is recorded not in the frequency, but in the amplitude of the reflected signal during the frequency tuning cycle. The device uses 2 parallel photodetectors, thanks to which the scanning speed is 100 thousand A-scans / s (as opposed to 40 thousand A-scans in SD-OCT). SS-OCT technology has several advantages. The 1050 nm wavelength used in SS-OCT (SD-OCT wavelength is 840 nm) allows for clear visualization of deep structures such as choroid and lamina cribriform, while image quality is much less dependent on the distance of the tissue of interest to zero delay lines as in EDI-OCT. In addition, at this wavelength, less light is scattered as it passes through the cloudy lens, resulting in clearer images in patients with cataracts. The scanning window covers 12 mm of the posterior pole (for comparison: for SD-OCT - 6–9 mm), therefore, the optic nerve and macula can be simultaneously presented on the same scan. The results of the SS-OCT study are maps that can be presented as the total thickness of the retina or its individual layers (a layer of retinal nerve fibers, a layer of ganglion cells together with an inner pleximorphic layer, choroid). The swept-source OCT technology is actively used to study the pathology of the macular zone, choroid, sclera, vitreous body, as well as to assess the layer of nerve fibers and ethmoid plate in glaucoma. In 2012, the first commercial Swept-Source OCT was introduced, implemented in the Topcon Deep Range Imaging (DRI) OCT-1 Atlantis 3D SS-OCT (Topcon Medical Systems, Japan). Since 2015, a commercial DRI OCT Triton sample (Topcon, Japan) with a scanning speed of 100 thousand A-scans / s and a resolution of 2-3 microns has become available on the foreign market.
Traditionally, OCT has been used for pre- and postoperative diagnostics. With development technological process it became possible to use OCT technology integrated into a surgical microscope. Currently, several commercial devices with the function of performing intraoperative OCT are offered at once. Envisu SD-OIS (spectral-domain ophthalmic imaging system, SD-OIS, Bioptigen, USA) is a spectral optical coherence tomograph designed to visualize retinal tissue, it can also be used to obtain images of the cornea, sclera and conjunctiva. SD-OIS includes a handheld probe and microscope set-up, has an axial resolution of 5 μm and a scan rate of 27 kHz. Another company, OptoMedical Technologies GmbH (Germany), has also developed and presented an OCT camera that can be installed on an operating microscope. The camera can be used to visualize the anterior and posterior segments of the eye. The company points out that the device may be useful in surgical procedures such as corneal transplants, glaucoma surgery, cataract surgery, and vitreoretinal surgery. OPMI Lumera 700 / Rescan 700 (Carl Zeiss Meditec, USA), released in 2014, is the first commercially available microscope with an integrated optical coherence tomograph. The optical paths of the microscope are used to obtain real-time OCT images. The device can measure the thickness of the cornea and iris, the depth and angle of the anterior chamber during surgery. OCT is suitable for the observation and control of several stages in cataract surgery: limbal incisions, capsulorhexis and phacoemulsification. In addition, the system can detect viscoelastic residues and monitor the position of the lens during and at the end of the operation. During surgical intervention on the posterior segment, vitreoretinal adhesions, detachment of the posterior hyaloid membrane, the presence of foveolar changes (edema, rupture, neovascularization, hemorrhage) can be visualized. Currently, in addition to the existing ones, new installations are being developed.
OCT is, in fact, a method that makes it possible to assess at the histological level the morphology of tissues (shape, structure, size, spatial organization in general) and their constituent parts. Devices that include modern OCT technologies and methods such as photoacoustic tomography, spectroscopic tomography, polarization tomography, Doppler and angiography, elastography, optophysiology, make it possible to assess the functional (physiological) and metabolic state of the tissues under study. Therefore, depending on the capabilities that OCT can have, it is customary to classify it into morphological, functional and multimodal.
Photoacoustic tomography (PAT) exploits differences in tissue absorption of short laser pulses, their subsequent heating, and extremely rapid thermal expansion to produce ultrasonic waves that are detected by piezoelectric receivers. The predominance of hemoglobin as the main absorbent of this radiation means that contrast images of the vasculature can be obtained using photoacoustic tomography. At the same time, the method provides relatively little information about the morphology of the surrounding tissue. Thus, the combination of photoacoustic tomography and OCT makes it possible to assess the microvascular network and the microstructure of the surrounding tissues.
The ability of biological tissues to absorb or scatter light, depending on the wavelength, can be used to assess functional parameters - in particular, the saturation of hemoglobin with oxygen. This principle is implemented in spectroscopic OCT (Spectroscopic OCT, SP-OCT). Although the method is currently under development, and its use is limited to experimental models, it nevertheless seems promising in terms of studying blood oxygen saturation, precancerous lesions, intravascular plaques, and burns.
Polarization sensitive OCT (PS-OCT) measures the polarization state of light and is based on the fact that certain tissues can change the polarization state of the probing light beam. Various mechanisms of interaction between light and tissues can cause changes in the state of polarization, such as birefringence and depolarization, which have already been partially used in laser polarimetry. Birefringent tissues are the corneal stroma, sclera, eye muscles and tendons, trabecular meshwork, retinal nerve fiber layer and scar tissue. The effect of depolarization is observed in the study of melanin contained in the tissues of the retinal pigment epithelium (RPE), the pigment epithelium of the iris, nevi and choroidal melanomas, as well as in the form of pigment accumulations of the choroid. The first polarization low-coherence interferometer was implemented in 1992. In 2005, PS-OCT was demonstrated for in vivo imaging of the human retina. One of the advantages of the PS-OCT method is the possibility of a detailed assessment of RPE, especially in cases when on OCT, for example, in neovascular macular degeneration, the pigment epithelium is poorly distinguishable due to the strong distortion of the retinal layers and backscattering (Fig. 1). There is also a direct clinical purpose of this method. The fact is that visualization of atrophy of the RPE layer can explain why visual acuity does not improve in these patients during treatment after anatomical restoration of the retina. Polarizing OCT is also used to assess the condition of the nerve fiber layer in glaucoma. It should be noted that other structures depolarizing within the affected retina can be detected by PS-OCT. Initial studies in patients with diabetic macular edema showed that rigid exudates are depolarizing structures. Therefore, PS-OCT can be used to detect and quantify (size, quantity) hard exudates in this condition.
Optical coherence elastography (OCE) is used to determine the biomechanical properties of tissues. OCT elastography is analogous to ultrasound sonography and elastography, but with the advantages inherent in OCT, such as high resolution, non-invasiveness, real-time imaging, depth of tissue penetration. The method was first demonstrated in 1998 for the imaging of the mechanical properties in vivo of human skin. Experimental studies of donor corneas using this method demonstrated that OCT elastography can quantify the clinically significant mechanical properties of a given tissue.
The first spectral OCT with Doppler optical coherence tomography (D-OCT) for measuring ocular blood flow appeared in 2002. In 2007, total retinal blood flow was measured using circular B-scans around the optic nerve. However, the method has several limitations. For example, Doppler OCT makes it difficult to discern slow blood flow in small capillaries. In addition, most vessels pass nearly perpendicular to the scan beam, so Doppler signal detection is critically dependent on the angle of the incident light. An attempt to overcome the disadvantages of D-OCT is OCT angiography. To implement this method, high-contrast and ultra-high-speed OCT technology was required. An algorithm called split-spectrum amplitude decorrelation angiography (SS-ADA) became the key to the development and improvement of the technique. The SS-ADA algorithm implies an analysis using the division of the entire spectrum of an optical source into several parts, followed by a separate calculation of decorrelation for each frequency range of the spectrum. Simultaneously, anisotropic decorrelation analysis is performed and a series of full spectral width scans are performed, which provide high spatial resolution of the vasculature (Figs. 2, 3). This algorithm is used in the Avanti RTVue XR tomograph (Optovue, USA). OCT angiography is a non-invasive three-dimensional alternative to conventional angiography. The advantages of the method include non-invasiveness of the study, no need to use fluorescent dyes, and the ability to quantitatively measure ocular blood flow in vessels.

Optophysiology is a method of non-invasive study of physiological processes in tissues using OCT. OCT is sensitive to spatial changes in optical reflection or tissue scattering of light due to local changes in refractive index. Physiological processes occurring at the cellular level, such as membrane depolarization, cell swelling and metabolic changes, can lead to small but detectable changes in the local optical properties of biological tissue. The first evidence that OCT can be used to obtain and assess physiological responses to light stimulation of the retina was demonstrated in 2006. Subsequently, this technique was applied to study the human retina in vivo. Currently, a number of researchers continue to work in this direction.
OCT is one of the most successful and widely used imaging techniques in ophthalmology. Currently, devices for technology are on the list of products of more than 50 companies in the world. Over the past 20 years, the resolution has improved 10 times, and the scanning speed has increased hundreds of times. Continuous advances in OCT technology have made this method a valuable tool for the study of eye structures in practice. Over the past decade, the development of new technologies and additions to OCT makes it possible to make an accurate diagnosis, carry out dynamic observation and evaluate treatment results. This is an example of how new technologies can solve real-life medical problems. And, as is often the case with new technologies, further application experience and application development can provide an opportunity for a deeper understanding of the pathogenesis of eye pathology.

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There are a limited number of ways to visualize the exact structure and the smallest pathological processes in the structure of the organ of vision. The use of simple ophthalmoscopy is absolutely insufficient for a complete diagnosis. Relatively recently, since the end of the last century, optical coherence tomography (OCT) has been used to accurately study the state of eye structures.

OCT of the eye is a non-invasive, safe method of examining all structures of the organ of vision in order to obtain accurate data on the smallest damage. In terms of resolution, no high-precision diagnostic equipment can be compared with coherence tomography. The procedure allows you to detect damage to the eye structures with a size of 4 microns.

The essence of the method is the ability of an infrared light beam to reflect unequally from various structural features of the eye. The technique is close at the same time to two diagnostic manipulations: ultrasound and computed tomography. But in comparison with them, it wins significantly, since the images are clear, the resolution is large, there is no radiation exposure.

What can you explore

Optical coherence tomography of the eye allows you to evaluate all parts of the organ of vision. However, the most informative manipulation is when analyzing the features of the following eye structures:

• cornea;
• retina;
• optic nerve;
• front and rear cameras.

A particular type of research is optical coherence tomography of the retina. The procedure allows you to identify structural abnormalities in this eye area with minimal damage. For the examination of the macular zone - the area of ​​greatest visual acuity, OCT of the retina has no full-fledged analogues.

Indications for manipulation

Most diseases of the organ of vision, as well as symptoms of eye damage, are indications for coherence tomography.

The conditions in which the procedure is carried out are as follows:

• retinal tears;
• dystrophic changes in the macula of the eye;
• glaucoma;
• optic nerve atrophy;
• tumors of the organ of vision, for example, choroidal nevus;
• acute vascular diseases of the retina - thrombosis, ruptured aneurysms;
• congenital or acquired anomalies of the internal structures of the eye;
• myopia.

In addition to the diseases themselves, there are symptoms that are suspicious of retinal damage. They also serve as indications for research:

• a sharp decrease in vision;
• fog or "flies" in front of the eye;
• increased eye pressure;
• sharp pain in the eye;
• sudden blindness;
• exophthalmos.

In addition to clinical indications, there are social ones. Since the procedure is completely safe, it is recommended to be carried out by the following categories of citizens:

• women over 50;
• men over 60;
• all those suffering from diabetes mellitus;
• in the presence of hypertension;
• after any ophthalmic interventions;
• in the presence of severe vascular accidents in history.

How is the study going

The procedure is carried out in a special room equipped with an OCT tomograph. This is a device with an optical scanner, from the lens of which infrared light beams are directed to the organ of vision. The result of the scan is recorded on the connected monitor in the form of a layer-by-layer tomographic image. The device converts the signals into special tables, according to which the structure of the retina is assessed.

Preparation for the examination is not required. Can be done at any time. The patient, being in a seated position, focuses his gaze at a special point indicated by the doctor. It then remains still and focused for 2 minutes. This is enough for a full scan. The device processes the results, the doctor assesses the state of the eye structures and within half an hour a conclusion is issued on the pathological processes in the organ of vision.

Eye tomography using an OCT scanner is performed only in specialized ophthalmology clinics... Even in large metropolitan areas, there is not a lot of medical centers offering a service. The cost varies depending on the scope of the study. Fully OCT of the eye is estimated at about 2 thousand rubles, only the retina - 800 rubles. If you need to diagnose both organs of vision, the cost doubles.

Since the examination is safe, there are few contraindications. They can be represented as follows:

• any condition when the patient is unable to fix his gaze;
• mental illness, accompanied by a lack of productive contact with the patient;
• lack of consciousness;
• the presence of a contact medium in the organ of vision.

The last contraindication is relative, since after washing out the diagnostic medium, which may be after various ophthalmological examinations, for example, gonioscopy, the manipulation is performed. But in practice, the two procedures are not combined in one day.

Relative contraindications are also associated with the opacity of the eye media. Diagnostics can be performed, but the images are not as good quality. Since there is no radiation, there is also no effect of the magnet, the presence of pacemakers and other implanted devices is not a reason for refusing the examination.

Diseases for which the procedure is prescribed

The list of diseases that can be detected by OCT of the eye looks like this:

• glaucoma;
• retinal vascular thrombosis;
• diabetic retinopathy;
• benign or malignant tumors;
• retinal rupture;
• hypertensive retinopathy;
• helminthic invasion of the organ of vision.

Thus, optical coherence tomography of the eye is absolutely safe method diagnostics. It can be used in a wide range of patients, including those for whom other high-precision research methods are contraindicated. The procedure has some contraindications, it is performed only in ophthalmological clinics.

Taking into account the harmlessness of the examination, it is advisable to perform OCT for all people over 50 to detect small structural defects of the retina. this will allow diagnosing diseases in the early stages and preserving high-quality vision for a longer time.

The possibilities of modern ophthalmology have been significantly expanded in comparison with the methods of diagnosing and treating diseases of the organs of vision some fifty years ago. Today, complex, high-tech devices and techniques are used to make an accurate diagnosis, to detect the slightest changes in the structures of the eye. Optical coherence tomography (OCT) performed with a special scanner is one such method. What is it, to whom and when to conduct such a survey, how to properly prepare for it, are there any contraindications and are complications possible - the answers to all these questions are below.

Benefits and Features

Optical coherence tomography of the retina and other elements of the eye is an innovative ophthalmological study, in which superficial and deep structures of the organs of vision are visualized in high quality resolution. This method is relatively new, uninformed patients treat it with prejudice. And it is completely in vain, since today OCT is considered the best that exists in diagnostic ophthalmology.

OCT takes only a few seconds, and the results will be prepared within an hour after the examination - you can come to the clinic at lunchtime, perform OCT, get a diagnosis immediately and start treatment on the same day

The main advantages of OCT include:

• the ability to examine both eyes at the same time;
• the speed of the procedure and the efficiency of obtaining accurate results for the diagnosis;
• in one session, the doctor gets a clear idea of ​​the state of the macula, optic nerve, retina, cornea, arteries and capillaries of the eye at the microscopic level;
• tissues of the elements of the eye can be thoroughly examined without a biopsy;
• the resolution of OCT is many times higher than the indicators of conventional computed tomography or ultrasound - tissue damage not exceeding 4 microns in size, pathological changes at the earliest stages are detected;
• no need to inject intravenously contrasting dyes;
• the procedure is non-invasive, therefore it has almost no contraindications, does not require special preparation and recovery period.

When conducting coherent tomography, the patient does not receive any radiation exposure, which is also a great advantage, taking into account the harmful effects of external factors, and without this, every modern person is exposed.

What is the essence of the procedure

If light waves are passed through the human body, they will be reflected from different organs in different ways. The delay time of light waves and the time of their passage through the elements of the eye, the intensity of reflection is measured using special devices during tomography. Then they are transferred to the screen, after which the decryption and analysis of the data obtained are carried out.

Retinal oc is an absolutely safe and painless method, since the devices do not come into contact with the organs of vision, nothing is injected subcutaneously or inside the eye structures. But at the same time, it provides much higher information content than standard CT or MRI.

This is how the image on a computer monitor, obtained by scanning with OCT, looks like; to decipher it, special knowledge and skills of a specialist will be required

It is in the method of decoding the received reflection that the main feature of OCT lies. The fact is that the waves of light move at a very high speed, which does not allow directly measuring the required indicators. For these purposes, a special device is used - a Meikelson interferometer. It splits the light wave into two beams, then one beam is passed through the eye structures that need to be examined. And the other goes to the mirror surface.

If an examination of the retina and macular zone of the eye is required, a low-coherence infrared beam of 830 nm is used. If you need to do OCT of the anterior chamber of the eye, you will need a wavelength of 1310 nm.

Both beams are connected and enter the photodetector. There they are transformed into an interference image, which is then analyzed by a computer program and displayed on a monitor in the form of a pseudo-image. What will it show? Areas with a high degree of reflection will be colored in warmer hues, while those that reflect light waves weakly will appear almost black in the picture. Nerve fibers and pigment epithelium are displayed "warm" in the picture. Nuclear and plexiform retinal layers have an average degree of reflectivity. And the vitreous body looks black, since it is almost transparent and transmits light waves well, almost not reflecting them.

To obtain a full-fledged, informative picture, it is necessary to pass light waves through the eyeball in two directions: transverse and longitudinal. Distortions of the resulting image can occur if the cornea is edematous, opacities of the vitreous body, hemorrhages, and foreign particles occur.

One procedure lasting less than a minute is enough to obtain the most complete information about the state of the eye structures without invasive intervention, to identify developing pathologies, their forms and stages

What can be done with optical tomography:

• Determine the thickness of the eye structures.
• Set the size of the optic nerve head.
• To identify and evaluate changes in the structure of the retina and nerve fibers.
• Assess the condition of the elements of the anterior region of the eyeball.

Thus, during OCT, the ophthalmologist gets the opportunity to study all the components of the eye in one session. But the most informative and accurate is the study of the retina. Today, optical coherence tomography is the most optimal and informative way to assess the state of the macular zone of the organs of vision.

Indications for

Optical tomography, in principle, can be prescribed to every patient who consults an ophthalmologist with any complaints. But in some cases this procedure is indispensable, it replaces CT and MRI and is even ahead of them in terms of information content. The indications for OCT are the following symptoms and patient complaints:

• "Flies", cobwebs, lightning and flashes before the eyes.
• Blurred vision.
• An unexpected and dramatic decrease in vision in one or both eyes.
• Severe pain in the organs of vision.
• A significant increase in intraocular pressure with glaucoma or for other reasons.
• Exophthalmos - bulging of the eyeball from the orbit spontaneously or after injury.

Glaucoma, increased intraocular pressure, changes in the optic nerve head, suspected retinal detachment, as well as preparation for surgical interventions on the eyes - all these are indications for optical coherence tomography

If vision correction using a laser is to be performed, such a study is carried out before and after surgery in order to accurately determine the angle of the anterior chamber of the eye and assess the degree of drainage intraocular fluid(if glaucoma is diagnosed). OCT is also necessary for keratoplasty, implantation of intrastromal rings or intraocular lenses.

What can be identified and detected with coherence tomography:

• changes in intraocular pressure;
• congenital or acquired degenerative changes in retinal tissue;
• malignant and benign neoplasms in the structures of the eye;
• symptoms and severity of diabetic retinopathy;
• various pathologies of the optic nerve head;
• polyiferative vitreoretinopathy;
• epiretinal membrane;
• blood clots of the coronary arteries or central vein eyes and other vascular changes;
• tears or detachment of the macula;
• macular edema, accompanied by the formation of cysts;
• corneal ulcers;
• deeply penetrating keratitis;
• progressive myopia.

Thanks to such a diagnostic study, it is possible to identify even minor changes and abnormalities of the organs of vision, correctly diagnose, determine the degree of lesions and the optimal method of treatment. OCT actually helps to maintain or restore the patient's visual function. And since the procedure is completely safe and painless, it is often performed in preventive purposes for diseases that can be complicated by eye pathologies - with diabetes mellitus, hypertension, cerebrovascular accidents, after trauma or surgery.

When OCT is not allowed

The presence of a pacemaker and other implants, conditions in which the patient is unable to focus his gaze, is unconscious, or is unable to control his emotions and movements, most diagnostic tests are not carried out. In the case of coherent tomography, everything is different. A procedure of this kind can be carried out with confusion of consciousness and an unstable psychoemotional state of the patient.

Unlike MRI and CT, which, although informative, have a number of contraindications, OCT can be used to examine children without any fear - the child will not be afraid of the procedure and will not receive any complications

The main and, in fact, the only obstacle to performing OCT is the simultaneous conduct of other diagnostic studies. On the day for which OCT is prescribed, it is impossible to use any other diagnostic methods of examining the organs of vision. If the patient has already undergone other procedures, then OCT is transferred to another day.

Also, high myopia or severe opacity of the cornea and other elements of the eyeball can become an obstacle to obtaining a clear, informative image. In this case, the light waves will be poorly reflected and give a distorted image.

OCT technique

It must be said right away that optical coherence tomography is usually not performed in district polyclinics, since ophthalmological offices do not have the necessary equipment. OCT can only be done in specialized private medical institutions... In large cities, it will not be difficult to find a trustworthy ophthalmology office with an OCT scanner. It is advisable to agree on the procedure in advance, the cost of coherence tomography for one eye starts from 800 rubles.

No preparation for OCT is required, only a functioning OCT scanner and the patient himself are needed. The examinee will be asked to sit on a chair and focus on the indicated mark. If the eye, the structure of which is to be examined, is unable to focus, then the gaze is fixed as much as possible by the other, healthy eye. It takes no more than two minutes to be in a stationary state - this is enough to pass beams of infrared radiation through the eyeball.

During this period, several images are taken in different planes, after which the medical officer selects the clearest and highest quality images. Their computer system checks against an existing database compiled from examinations of other patients. The base is represented by various tables and diagrams. The less matches are found, the higher the likelihood that the structures of the patient's eye are pathologically altered. Since all analytical actions and transformations of the obtained data are performed by computer programs in an automatic mode, it will take no more than half an hour to obtain the results.

The OCT scanner produces perfectly accurate measurements, processes them quickly and efficiently. But in order to make a correct diagnosis, it is still necessary to correctly decipher the results obtained. And this requires high professionalism and deep knowledge in the field of histology of the retina and choroid of an ophthalmologist. For this reason, the interpretation of the research results and the diagnosis are carried out by several specialists.

Abstract: The majority of ophthalmic diseases are extremely difficult to recognize and diagnose at early stages, all the more so to establish the real degree of damage to the eye structures. For suspicious symptoms, ophthalmoscopy is routinely prescribed, but this method is not enough to get the most accurate picture of the condition of the eyes. More complete information is provided by computed tomography and magnetic resonance imaging, but these diagnostic measures have a number of contraindications. Optical coherence tomography is completely safe and harmless, it can be performed even in cases where other methods of examining the organs of vision are contraindicated. Today it is the only non-invasive way to get the most complete information about the condition of the eyes. The only difficulty that may arise is that not all ophthalmological offices have the equipment necessary for the procedure.

Almost all eye diseases, depending on the severity of the course, can have a negative impact on the quality of vision. In this regard, the most important factor determining the success of treatment is timely diagnosis... The main reason, partial or complete loss of vision in ophthalmic diseases such as glaucoma or various retinal lesions, is the absence or weak manifestation of symptoms.

Thanks to the capabilities of modern medicine, the detection of such a pathology on early stage, allows you to avoid possible complications and stop the progression of the disease. However, the need for early diagnosis implies a conditional examination healthy people who are not ready to undergo grueling or traumatic procedures.

The advent of optical coherence tomography (OCT) not only helped to resolve the issue of choosing a universal diagnostic technique, but also changed the opinion of ophthalmologists about some eye diseases... What is the principle of OCT operation based on, what is it and what are its diagnostic capabilities? The answer to these and other questions can be found in the article.

Operating principle

Optical coherence tomography is a diagnostic ray method used mainly in ophthalmology, which allows you to obtain a structural image of eye tissue at the cellular level, in cross-section, and with high resolution. The mechanism for obtaining information in OCT combines the principles of two main diagnostic techniques - ultrasound and X-ray CT.

If data processing is carried out according to principles similar to computed tomography, which records the difference in the intensity of X-ray radiation passing through the body, then when performing OCT, the amount of infrared radiation reflected from tissues is recorded. This approach has some similarities with ultrasound, where the transit time of an ultrasonic wave from the source to the inspected object and back to the recording device is measured.

A beam of infrared radiation used in diagnostics, having a wavelength of 820 to 1310 nm, is focused on the object of study, and then the magnitude and intensity of the reflected light signal is measured. Depending on the optical characteristics of various tissues, part of the beam is scattered, and part is reflected, allowing you to get an idea of ​​the structure of the examined area at different depths.

The resulting interference pattern, with the help of computer processing, takes the form of an image on which, in accordance with the provided scale, the zones characterized by high reflectivity are colored in the colors of the red spectrum (warm), and low - in the range from blue to black (cold) ... The layer of pigment epithelium of the iris and nerve fibers is characterized by the highest reflectivity, the plexiform layer of the retina has an average reflectivity, and the vitreous body is absolutely transparent to infrared rays, therefore it is colored black on the tomogram.

Important! The short infrared wavelength used in OCT does not allow the study of deeply located organs, as well as tissues with a significant thickness. In the latter case, it is possible to obtain information only about the surface layer of the investigated object, for example, mucous membrane.

Pain syndrome - an indication for optical coherence tomography

Views

All types of optical coherence tomography are based on the registration of an interference pattern created by two beams emitted from one source. Due to the fact that the speed of a light wave is so great that it cannot be fixed and measured, they use the property of coherent light waves to create an interference effect.

For this, the beam emitted by the superluminescent diode is split into 2 parts, the first being directed to the study area, and the second to the mirror. A prerequisite for achieving the interference effect is an equal distance from the photodetector to the object and from the photodetector to the mirror. Changes in the intensity of radiation, allow you to characterize the structure of each specific point.

There are 2 types of OCT used to study the orbit of the eye, the quality of the results of which varies significantly:

• Time-dоmаin OST (Michelson's method);
• Srestral OCT (spectral OCT).

Time-dоmain OST is the most common, until recently, scanning method, the resolution of which is about 9 microns. To obtain 1 two-dimensional scan of a certain point, the doctor had to manually move the movable mirror, located on the support arm, until an equal distance between all objects was achieved. The scanning time and the quality of the results obtained depended on the accuracy and speed of movement.

Spectral OCT. In contrast to Time-dоmain OST, a broadband diode was used as an emitter in spectral OCT, which made it possible to obtain several light waves of different lengths at once. In addition, it was equipped with a high-speed CCD camera and spectrometer, which simultaneously recorded all the components of the reflected wave. Thus, to obtain multiple scans, it was not required to manually move the mechanical parts of the instrument.

The main problem of obtaining the highest quality information is the high sensitivity of the equipment to minor movements of the eyeball, which cause certain errors. Since one study on the Time-dоmain OST takes 1.28 seconds, during this time, the eye manages to complete 10-15 micro-movements (movements called "micro-saccades"), which makes it difficult to read the results.

Spectral tomographs allow you to get twice the amount of information in 0.04 seconds. During this time, the eye does not have time to move, respectively, the final result does not contain distorting artifacts. The main advantage of OCT is the ability to obtain a three-dimensional image of the object under study (cornea, optic nerve head, retinal fragment).

An imaging principle widely used in ophthalmology

Indications

Indications for optical coherence tomography of the posterior segment of the eye are diagnostics and monitoring of treatment results for the following pathologies:

• degenerative changes in the retina;
• glaucoma;
• macular tears;
• macular edema;
• atrophy and pathology of the optic nerve head;
• retinal disinsertion;
• diabetic retinopathy.

Pathologies of the anterior segment of the eye requiring OCT:

• keratitis and ulcerative damage to the cornea;
• grade functional state drainage devices for glaucoma;
• assessment of the thickness of the cornea before carrying out laser correction vision by the LASIK method, lens replacement and installation of intraocular lenses (IOL), keratoplasty.

Preparation and implementation

Optical coherence tomography of the eye does not require preparation. However, in most cases, when examining the structures of the posterior segment, drugs are used to expand the pupil. At the beginning of the examination, the patient is asked to look through the lens of the fundus camera at an object blinking there, and fix his gaze on it. If the patient does not see the object due to low visual acuity, then he should look straight ahead without blinking.

Then, the camera is moved towards the eye until a clear image of the retina appears on the computer monitor. The distance between the eye and the camera, which allows obtaining the optimal image quality, should be equal to 9 mm. At the moment of achieving optimal visibility, the camera is fixed with a button and the image is adjusted to achieve maximum clarity. The control of the scanning process is carried out using the knobs and buttons located on the control panel of the tomograph.

The next stage of the procedure is to align the image and remove artifacts and noise from the scan. After receiving the final results, all quantitative indicators are compared with the indicators of healthy people of a similar age group, as well as with the patient's indicators obtained as a result of previous examinations.

Important! OCT is not performed after ophthalmoscopy or gonioscopy, since the use of a lubricating fluid necessary for the above procedures will not allow a high-quality image to be obtained.

Scanning takes no more than a quarter of an hour

Interpretation of results

Interpretation of the results of computed tomography of the eye is based on the analysis of the obtained images. First of all, attention is paid to the following factors:

• the presence of changes in the outer contour of tissues;
• the relative position of their various layers;
• the degree of light reflection (the presence of foreign inclusions that enhance reflection, the appearance of foci or surfaces with reduced or increased transparency).

With the help of quantitative analysis, it is possible to reveal the degree of decrease or increase in the thickness of the studied structure or its layers, to estimate the size and changes of the entire examined surface.

Corneal examination

When examining the cornea, the most important thing is to accurately determine the area of ​​existing structural changes and fix their quantitative characteristics. Subsequently, it will be possible to objectively assess the presence of positive dynamics from the applied therapy. OCT of the cornea is the most accurate method for determining its thickness without direct contact with the surface, which is especially important in case of damage.

Examination of the iris

Due to the fact that the iris consists of three layers with different reflectivity, it is almost impossible to visualize all layers with equal clarity. The most intense signals come from the pigment epithelium, the posterior layer of the iris, and the weakest, from the anterior boundary layer. With the help of OCT, it is possible to diagnose with high accuracy a number of pathological conditions that do not have any clinical manifestations at the time of examination:

• Frank-Kamenetsky syndrome;
• pigment dispersion syndrome;
• essential mesodermal dystrophy;
• pseudoexfoliative syndrome.

Retinal examination

Optical coherence tomography of the retina allows you to differentiate its layers, depending on the light reflectance of each. The layer of nerve fibers has the highest reflectivity, the layer of the plexiform and nuclear layers is the middle, and the layer of photoreceptors is absolutely transparent to radiation. On the tomogram, the outer edge of the retina is limited, painted in red, by a layer of choriocapillaries and RPE (retinal pigment epithelium).

Photoreceptors are displayed as a darkened strip located immediately in front of the layers of the choriocapillaries and RPE. The nerve fibers located on the inner surface of the retina are colored bright red. Strong contrast between colors allows accurate measurements of the thickness of each layer of the retina.

Retinal tomography reveals macular ruptures at all stages of development - from pre-rupture, which is characterized by detachment of nerve fibers while maintaining the integrity of the remaining layers, to complete (lamellar) rupture, determined by the appearance of defects in the inner layers while maintaining the integrity of the photoreceptor layer.

Important! The degree of preservation of the RPE layer, the degree of tissue degeneration around the rupture, are factors that determine the degree of preservation of visual functions.

Retinal tomography will show even a macular rupture

Examination of the optic nerve. Nerve fibers, which are the main building blocks of the optic nerve, have a high reflectivity and are clearly defined among all the structural elements of the fundus. Particularly informative is the three-dimensional image of the optic nerve head, which can be obtained by performing a series of tomograms in various projections.

All parameters that determine the thickness of the layer of nerve fibers are automatically calculated by the computer and supplied in the form of quantitative values ​​for each projection (temporal, superior, inferior, nasal). Such measurements make it possible to determine both the presence of local lesions and diffuse changes optic nerve. Evaluation of the reflectivity of the optic nerve head (optic nerve disc) and comparison of the obtained results with the previous ones allows one to assess the dynamics of improvement or progression of the disease during hydration and degeneration of the optic disc.

Spectral optical coherence tomography provides the doctor with extremely extensive diagnostic capabilities. However, each new diagnostic method requires the development of different criteria for assessing the main groups of diseases. The multidirectionality of the results obtained during OCT in the elderly and children significantly increases the requirements for the qualifications of an ophthalmologist, which becomes a determining factor when choosing a clinic where to do an examination.

Today, many specialized clinics have new models of OK tomographs, which are used by specialists who have completed additional education courses and received accreditation. The international center "Yasny Vzor" made a significant contribution to improving the qualifications of doctors, which provides an opportunity for ophthalmologists and optometrists to improve their knowledge without interrupting their work, as well as obtain accreditation.

Optical coherence tomography is a relatively new method for studying ocular structures.

It requires high-tech equipment, and allows you to obtain comprehensive information about the state of the retina and anterior structures of the eye without traumatic intervention. The infrared beam of light does not cause damage, does not cause any inconvenience either during the diagnosis, after it.

The very idea of ​​diagnostics using infrared radiation was proposed only in 1995 by an ophthalmologist from the United States, Carmen Puliafito. The first apparatus for optical coherence tomography appeared 2 years later. Today, this relatively young method of eye examination is widely used.

Tomograph device for OCT

This is a high-tech apparatus, which consists of a device for the production of low-coherence ultraviolet rays, reflective mirrors, a Michelson interferometer and computer equipment.

The beams generated by the device are divided into two beams, one passes through the tissues of the eye, and the other through special mirrors. The speed of transmission of light rays is recorded and analyzed (during ultrasound, radio waves are analyzed), but not direct (their speed is too high), but reflected.


The structures of the eye (skin, mucous membranes, lens, vitreous humor, veins, etc.) reflect light rays in different ways, and this difference is recorded by the interferometer. The equipment converts numerical measurements into an image that is displayed on the monitor. Beams with high level reflections are drawn in a "warm" spectrum (red shades), the lower the reflection level, the colder the color (down to dark blue and black). So, the vitreous body in the image will be black (it almost does not reflect light), and the nerve fibers (like the epithelium) have high degree reflections and will turn out to be red.

It follows that the study will be difficult with opacity of optical media, corneal edema, and hemorrhage.

Scanning is carried out in two planes along, as well as across, a lot of plane cuts are made. This allows you to simulate an accurate three-dimensional image of the eye. Resolution level from 1 to 15 microns. To study the bottom of the retina, a beam with a wavelength of 830 nm is used, for the study of the anterior section - 1310 nm.

The level of technical equipment today allows examining the anterior region and the posterior pole of the eye. To obtain high-quality diagnostic results, transparency of optical media and tear film normally (artificial tears are often used), the pupil should be dilated (special mydriatic preparations are used).

The obtained and deciphered result will be presented in the form of maps, pictures and protocols.

Many ophthalmologists call OCT a non-invasive biopsy, which is, in fact, true.

When is coherence tomography prescribed?

I prescribe this examination for a number of diseases of the anterior part of the eye. Among them will be:

• various forms of glaucoma (investigate and evaluate the work of drainage systems),
• corneal ulcers
• complex keratitis.

Coherent tomography is prescribed to study the anterior parts of the eye before and after:

• laser vision correction, keratoplasty,
• implantation of a phakic intraocular optical lens (IOL), or intrastromal corneal rings.

Examine the posterior part of the eye when detecting:

• age-related, degenerative changes in the retina;
• macular ruptures or macular cystic edema.

• if you suspect a retinal detachment,
• in the case of the presence of an epiretinal membrane (cellophane macula),
• with anomalies of the optic disc, ruptures, atrophies,
• with thrombosis of the central retinal vein,
• in case of suspicion of polyiferative vitreoretinopathy or if it is detected.

Often, coherent tomography is prescribed for patients with diabetic retinopathy (they are examined without mydriatics), as well as in a number of other ophthalmic diseases that require biopsy.

Examination procedure on a coherent tomograph

The diagnosis itself is absolutely painless, it takes 2-3 minutes in time, it is carried out in a comfortable environment for the patient. The patient is placed in front of the lens of the fundus camera (the head is fixed) and looks at the blinking dot. If vision is reduced and the point is not visible, then you just need to sit still and look at one point in front of you.

The operator will preliminarily enter the patient data into the computer. Then a scan is performed within 1–2 minutes. The patient is required not to move or blink.

After that, the received data is processed. The results obtained are compared with those available in the database of healthy people, digital data are converted into maps, drawings that are easy to understand. All results will be presented to the examinee in the form of maps, tables and protocols.

Coherence tomography results

The interpretation of the results is carried out by a qualified specialist and will contain the following aspects:

• morphological features of tissues: external contours, relationship and ratio of various layers, structures and departments, connective tissues;
• indicators of light reflection: their changes, increase or decrease, pathology;
• quantitative analysis: cellular, tissue thinning or thickening, the volume of structures and tissues (here a map of the diagnosed surface is drawn up).

When examining the cornea, it is imperative to accurately indicate the location of the lesions, their size and quality, and the thickness of the cornea itself. OCT allows you to very accurately determine the required parameters. Here, non-contact methodology is of great importance.

Diagnostics of the iris makes it possible to determine the size of the boundary layer, stroma and pigment epithelium. Although the signals from the light and more pigmented iris differ, in any case, they make it possible to identify at the early (often preclinical) stages such diseases as mesodermal dystrophy, Frank-Kamenetsky syndrome, and others.

Coherent tomography of the retina will give a normal macular profile with a depression in the center. The layers should be uniform in thickness, without foci of destruction. The nerve fibers and pigment epithelium will have warm (red-yellow) shades, the plexiform and nuclear layers have medium reflectivity, they will turn out to be blue and green, the photoreceptor layer will be black (it has low reflectivity), the outer layer is bright red. Measurements of dimensions should be like this: in the area of ​​the fossa macular slightly more than 162 microns, at its edge - 235 microns.

Examination of the optic nerve makes it possible to assess the thickness of the layer of nerve fibers (about 2 mm), their angle of inclination relative to the optic nerve head and retina.

Identification of pathologies on a coherent tomograph

During coherence tomography, many pathologies of both the anterior parts of the eye and the retina are revealed. Studies of the retina and macula will be especially valuable, since the study performed allows you to determine the pathology as accurately as with a biopsy. But OCT is not an invasive technique and does not violate tissue integrity. So, among the most frequently detected diseases will be:

• Retinal defects, idiopathic breaks ... They are often found in the elderly, occur for no apparent reason. The study establishes the focus, size at all stages of the disease, as well as degenerative processes around the focus, the presence of interritinal cysts.
• Age-related macular degeneration. OCT allows detecting these diseases (typical for the elderly), as well as assessing the effectiveness of the therapy.
• Diabetic edema attributed to the most severe forms of diabetic retinopathy, it is difficult to treat. Coherent tomography allows you to determine the affected area, the severity and degeneration of tissues, the degree of damage to the vitreomacular space.
• Stagnant disc ... The degree of light reflection determines the hydration and degeneration of tissues. The presence of a stagnant disc will indicate high intracranial pressure.
• Congenital defects of the optic nerve fossa ... Among them, delamination is the most common.
• Retinitis pigmentosa ... Defining this progressive hereditary disorder is often difficult. The method is very informative for babies, when other methods are powerless over the anxiety of the baby.