OCT of the retina - what is it. Optical coherence tomography of the retina Optical coherence tomography contraindications

Antipyretics for children are prescribed by a pediatrician. But there are emergency situations for fever when the child needs to be given medicine immediately. Then the parents take responsibility and use antipyretic drugs. What is allowed to give to infants? How can you bring down the temperature in older children? What medicines are the safest?

The possibilities of modern ophthalmology are 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 using a special scanner, is one such method. What is it, who needs to conduct such an examination and when, how to properly prepare for it, whether there are contraindications and whether complications are possible - the answers to all these questions are below.

Advantages 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 resolution. This method is relatively new, uninformed patients are prejudiced against it. And completely in vain, since today OCT is considered the best that exists in diagnostic ophthalmology.

It only takes a few seconds to perform an OCT and results are ready within an hour after the examination – you can drop by the clinic during your lunch break, perform an OCT, get an immediate diagnosis and start treatment on the same day

Key benefits 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 making a diagnosis;
in one session, the doctor gets a clear picture 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 studied without a biopsy;
resolution capabilities of OCT are many times higher than those of conventional computed tomography or ultrasound - tissue damage no larger than 4 microns is detected, pathological changes at the earliest stages;
no need to administer intravenous contrast stains;
the procedure is non-invasive, therefore it has almost no contraindications, does not require special training and a recovery period.

With coherence tomography, the patient does not receive any radiation exposure, which is also a great advantage, considering what harmful effects external factors and without it every modern man is exposed.

What is the essence of the procedure

If light waves are passed through the human body, they will be reflected from various bodies differently. 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 decoding and analysis of the received data are carried out.

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

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

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

If an examination of the retina and macular area 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 wave length of 1310 nm.

Both beams are combined and enter the photodetector. There they are transformed into an interference picture, which is then analyzed by a computer program and displayed on a monitor as a pseudo-image. What will it show? Areas with a high degree of reflection will be painted in warmer shades, and those that reflect light waves weakly appear almost black in the picture. "Warm" in the picture are nerve fibers and pigment epithelium. Nuclear and plexiform layers of the retina have an average degree of reflectivity. And the vitreous body looks black, since it is almost transparent and transmits light waves well, almost without reflecting them.

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

One procedure lasting less than a minute is enough to obtain the most complete information about the state of 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.
Determine the size of the optic nerve head.
Detect and evaluate changes in the structure of the retina and nerve fibers.
Assess the condition of the elements of the anterior part of the eyeball.

Thus, when performing OCT, an 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. To date, 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 carrying out

Optical tomography in principle, it can be prescribed to every patient who has applied to an ophthalmologist with any complaints. But in some cases, this procedure cannot be dispensed with; it replaces CT and MRI and even outstrips them in terms of information content. Indications for OCT are the following symptoms and complaints of patients:

"Flies", cobwebs, lightning and flashes before the eyes.
Blurred visual image.
Sudden and sudden decrease in vision in one or both eyes.
Strong pain in the organs of vision.
A significant increase in intraocular pressure in glaucoma or for other reasons.
Exophthalmos - protrusion of the eyeball from the orbit spontaneously or after injury.

Glaucoma, increased intraocular pressure, changes in the optic nerve head, suspected retinal detachment, and preparation for eye surgery are all indications for optical coherence tomography.

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

What can be determined and detected using coherence tomography:

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

Thanks to such a diagnostic study, even minor changes and anomalies of the organs of vision can be detected, a correct diagnosis can be made, the degree of damage can be determined and the optimal method of treatment can be determined. OCT actually helps to preserve or restore the patient's visual function. And since the procedure is completely safe and painless, it is often performed as a preventive measure for diseases that can be complicated by eye pathologies - with diabetes, hypertension, disorders of cerebral circulation, after injuries or surgical intervention.

When not to do OCT

The presence of a pacemaker and other implants, conditions in which the patient cannot focus his eyes, is unconscious or unable to control his emotions and movements, most diagnostic tests not carried out. In the case of coherence tomography, everything is different. A procedure of this kind can be carried out with confusion and an unstable psycho-emotional 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 the OCT is scheduled, apply any other diagnostic methods visual examination is not possible. If the patient has already undergone other procedures, then OCT is transferred to another day.

Also, a high degree of myopia or severe clouding 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 clinics, since ophthalmological rooms do not have 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, all you need is a functioning OCT scanner and the patient himself. The subject will be asked to sit on a chair and focus on a specified mark. If the eye whose structure is to be examined is not able to focus, then the gaze is fixed as far as possible by another, healthy eye. It takes no more than two minutes to be stationary - this is enough to pass beams of infrared radiation through the eyeball.

During this period, several pictures are taken in different planes, after which the medical officer selects the most clear and high-quality ones. Their computer system checks against the existing database compiled on the basis of examinations of other patients. The database is represented by various tables and diagrams. The fewer matches are found, the higher the probability that the structures of the eye of the examined patient are pathologically changed. Since all analytical actions and transformations of the obtained data are performed by computer programs in 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 decoding of research results and the diagnosis are carried out by several specialists.

Summary: most ophthalmic diseases are extremely difficult to recognize and diagnose in the early stages, especially to establish the real degree of damage to eye structures. With suspicious symptoms, ophthalmoscopy is usually prescribed, but this method is not enough to get the most accurate picture of the condition of the eyes. More complete information is given CT scan 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. To date, this is the only non-invasive way to obtain the most complete information about the condition of the eyes. The only difficulty that may arise is that not all ophthalmological rooms have the equipment necessary for the procedure.

Method optical coherence tomography(optical coherence tomography, abbreviated as OST (eng.) or OCT (rus.)) is a modern high-precision non-invasive study of various structures of the eye. OCT is a non-contact method that allows the specialist to visualize the tissues of the eye with a very high resolution (1 - 15 microns), the accuracy of which is comparable to microscopic examination.

The theoretical foundations of the OCT method were developed in 1995 by the American ophthalmologist C. Pulafito, and already in 1996-1997 Carl Zeiss Meditec introduced the first device for optical coherence tomography into clinical practice. Today, OCT devices are used to diagnose various diseases of the fundus and anterior segment of the eye.

Indications for OST

The method of optical coherence tomography allows:

visualize morphological changes in the retina and the layer of nerve fibers, as well as assess their thickness;
assess the condition of the optic nerve head;
examine the structures of the anterior segment of the eye and their relative spatial arrangement.

The method can be used in ophthalmology for the diagnosis of many pathologies of the posterior part of the eye, such as:

degenerative changes in the retina (congenital and acquired, AMD)
cystoid macular edema and macular hole
epiretinal membrane
changes in the optic disc (anomalies, edema, atrophy)
diabetic retinopathy
thrombosis of the central retinal vein
proliferative vitreoretinopathy.

With regard to pathologies of the anterior part of the eye, OST can be applied:

to assess the angle of the anterior chamber of the eye and the functioning of the drainage systems in patients with glaucoma
in case of deep keratitis and corneal ulcers
during examination of the cornea during preparation and after laser vision correction and keratoplasty
for control in patients with phakic IOLs or intrastromal rings.

Video of our specialist

How is the study going

The patient is offered to fix the gaze with the examined eye on a special mark, after which the doctor performs a series of scans and selects the most informative image, which allows assessing the condition of the organ of vision. Diagnosis is completely painless and takes a minimum of time.

2, 3
1 FGAU NMIC "IRTC "Eye Microsurgery" named after A.I. acad. S. N. Fedorova» of the Ministry of Health of Russia, Moscow
2 FKU "TsVKG im. P.V. Mandryka” of the Ministry of Defense of Russia, Moscow, Russia
3 FGBOU VO RNIMU them. N.I. Pirogov of the Ministry of Health of Russia, Moscow, Russia

Optical coherence tomography (OCT) was first used to visualize the eyeball more than 20 years ago and still remains an indispensable diagnostic method in ophthalmology. With OCT, it has become possible to non-invasively obtain optical tissue sections with higher resolution than any other imaging modality. The dynamic development of the method has led to an increase in its sensitivity, resolution, and scanning speed. Currently, OCT is actively used for the diagnosis, monitoring and screening of diseases of the eyeball, as well as for scientific research. Combination modern technologies OCT 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 discusses the development of the OCT method, presents data on modern OCT devices depending on their technological characteristics and capabilities. The methods of functional OCT are described.

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

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

Optical 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 higher resolution 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. Optical 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 method that allows obtaining tomographic sections of internal biological systems with high resolution. The name of the method is first given in a work by a team from the Massachusetts Institute of Technology, published in Science in 1991. The authors presented tomographic images demonstrating in vitro the peripapillary zone of the retina and the coronary artery. The first in vivo 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 papers were published on the use of the method for the diagnosis and monitoring of diseases of the macular region (including macular edema in 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 designed for ophthalmic practice was created.
The principle of the OCT method is that a light wave is directed into the tissues, where it propagates and reflects or scatters from the inner layers, which have different properties. The resulting tomographic images are, in fact, the dependence of the intensity of the signal scattered or reflected from the structures inside the 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 successively measured at certain time intervals. Since the speed of signal propagation 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 sequentially shift along one of the axes (vertical, horizontal, oblique) and repeat the previous measurements, you can get a two-dimensional tomogram. If you sequentially shift along one more axis, then you can get a set of such sections, or a volumetric tomogram. OCT systems use weak coherence interferometry. Interferometric methods can significantly increase the sensitivity, since they 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 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 operation of the interferometer, proposed by A.A. Michelson (1852–1931). The low coherence light beam from the 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 adjusted by the researcher. When the length of the beam reflected from the tissue under study and the beam from the mirror are equal, an interference phenomenon occurs, which is recorded by the LED. Each measurement point corresponds to one A-scan. The resulting single A-scans are summed, resulting in a two-dimensional image. The axial resolution of first generation commercial 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 reduces the resolution of the instrument. In addition, eye movements that inevitably occur during 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 introduced - Ultrahigh-resolution OCT (UHR-OCT), which made it possible to obtain images of the cornea and retina with an axial resolution of 2–3 µm. 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 limiting 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 gained a number of advantages over the use of traditional time-based OCT (Table 1). Although the technique has been known since 1995, it was not used for retinal imaging until almost the early 2000s. This is due to the appearance in 2003 of high-speed cameras (charge-coupled device, CCD). 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, in spectral OCT the light beam is divided into 2 beams, 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 into a 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. In contrast to traditional OCT, where a linear A-scan is obtained by sequentially measuring the reflective properties of each individual point, in spectral OCT a linear A-scan is formed by simultaneously measuring 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 40,000 A-scans/s. Undoubtedly, the main advantage of SD-OCT is its high scanning speed. First, it can significantly improve the quality of the resulting images by reducing the artifacts that occur during eye movements during the study. 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 performs only microsaccade movements with an amplitude of several arc seconds, which do not affect the research process. Secondly, 3D reconstruction of the image has become possible, which makes it possible to evaluate the profile of the structure under study and its topography. Obtaining multiple images simultaneously with spectral OCT made it possible to diagnose small pathological foci. So, with TD-OCT, the macula is displayed according to 6 radial scans, as opposed to 128–200 scans of the same area when performing SD-OCT. Thanks to the high resolution, the layers of the retina and the inner layers of the choroid can be clearly visualized. The result of a standard SD-OCT study is a protocol that presents the results both graphically and in absolute terms. 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 Enhanced Image Depth Module (EDI-OCT) was the limitation of choroid imaging with spectral OCT by light absorption by the retinal pigment epithelium and scattering by choroidal structures. A number of authors used a spectrometer with a wavelength of 1050 nm, with which it was possible to qualitatively visualize and quantify the choroid itself. In 2008, a method for 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 lies in the appearance of mirror artifacts from the Fourier transform. In this case, 2 symmetrical images are formed - positive and negative relative 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 the processing of scans, the lower half of the image is usually removed, only its upper part is displayed. If you move the OCT scans so that they cross the zero delay line, then the choroid will be closer to it, which will allow you to visualize it more clearly. Currently, the enhanced image depth module is available from Spectralis (Heidelberg Engineering, Germany) and Cirrus HD-OCT (Carl Zeiss Meditec, USA) tomographs. EDI-OCT technology is used not only to study the choroid in various eye pathologies, but also to visualize the cribriform plate and assess its displacement depending on the stage of glaucoma.
Fourier-domain-OCT methods also include OCT with a tunable source (swept-source OCT, SS-OCT; deep range imaging, DRI-OCT). SS-OCT uses frequency-swept laser sources, i.e. lasers in which the emission frequency is tuned at a high rate within a certain spectral band. In this case, a change is recorded not in 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 a number of advantages. The 1050 nm wavelength used in SS-OCT (versus 840 nm in SD-OCT) enables clear visualization of deep structures such as the choroid and lamina cribrosa with much less image quality dependent on the distance of tissue of interest from zero delay lines, as in EDI-OCT. In addition, at a given wavelength, light is less scattered as it passes through a cloudy lens, resulting in clearer images in cataract patients. The scan window covers 12 mm of the posterior pole (compared to 6–9 mm for SD-OCT), so the optic nerve and macula can be seen simultaneously 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 (retinal nerve fiber layer, ganglion cell layer together with the 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 the cribriform 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 instrument (Topcon Medical Systems, Japan). Since 2015, a commercial sample of DRI OCT Triton (Topcon, Japan) with a scanning speed of 100,000 A-scans/s and a resolution of 2–3 µm has become available on the foreign market.
Traditionally, OCT has been used for pre- and postoperative diagnosis. With the development of the technological process, it became possible to use the OCT technology integrated into the 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 portable probe and microscope setup, has an axial resolution of 5 µm and a scan rate of 27 kHz. Another company, OptoMedical Technologies GmbH (Germany), 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 indicates that this device may be useful in performing surgical procedures such as corneal transplantation, 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 for real-time OCT imaging. Using the device, you can measure the thickness of the cornea and iris, the depth and angle of the anterior chamber during surgery. OCT is suitable for observation and control of several stages in cataract surgery: limbal incisions, capsulorhexis and phacoemulsification. In addition, the system can detect viscoelastic residue and monitor lens position during and at the end of surgery. During surgery in the posterior segment, vitreoretinal adhesions, detachment of the posterior hyaloid membrane, and the presence of foveolar changes (edema, rupture, neovascularization, hemorrhage) can be visualized. Currently, new installations are being developed in addition to the existing ones.
OCT is, in fact, a method that allows assessing at the histological level the morphology of tissues (shape, structure, size, spatial organization in general) and their components. Devices that include modern OCT technologies and methods such as photoacoustic tomography, spectroscopic tomography, polarization tomography, dopplerography and angiography, elastography, optophysiology, make it possible to assess the functional (physiological) and metabolic state of the tissues under study. Therefore, depending on the possibilities that OCT may have, it is usually classified into morphological, functional and multimodal.
Photoacoustic tomography (PAT) uses differences in the absorption of short laser pulses by tissues, 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 photoacoustic tomography can provide contrast images of the vasculature. 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, oxygen saturation of hemoglobin. 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 appears promising in terms of investigating 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 some tissues can change the polarization state of the probe 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, ocular 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 (REP), the pigment epithelium of the iris, nevi and melanomas of the choroid, as well as in the form of pigment accumulations of the choroid. The first polarizing 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 PES, especially in cases where the pigment epithelium is poorly visible on OCT, for example, in neovascular macular degeneration, due to 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 RPE layer atrophy may explain why visual acuity does not improve in these patients during treatment after anatomical retinal repair. Polarization OCT is also used to evaluate the condition of the nerve fiber layer in glaucoma. It should be noted that other depolarizing structures within the affected retina can be detected using PS-OCT. Initial studies in patients with diabetic macular edema showed that hard exudates are depolarizing structures. Therefore, PS-OCT can be used to detect and quantify (size, number) hard exudates in this condition.
Optical coherence elastography (OCE) is used to determine the biomechanical properties of tissues. OCT elastography is similar to ultrasound sonography and elastography, but with the advantages of OCT, such as high resolution, non-invasiveness, real-time imaging, depth of tissue penetration. The method was first demonstrated in 1998 for in vivo imaging of the mechanical properties of human skin. Experimental studies of donor corneas using this method have demonstrated that OCT elastography can quantify the clinically relevant mechanical properties of this tissue.
The first Doppler optical coherence tomography (D-OCT) to measure 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 a number of limitations. For example, slow blood flow in small capillaries is difficult to discern with Doppler OCT. In addition, most vessels run nearly perpendicular to the scan beam, so Doppler shift signal detection is critically dependent on the angle of incident light. An attempt to overcome the shortcomings of D-OCT is OCT angiography. To implement this method, a high-contrast and superfast OCT technology was needed. The algorithm called split-spectrum amplitude decorrelation angiography (SS-ADA) became the key to the development and improvement of the technique. The SS-ADA algorithm involves analysis using the division of the full spectrum of an optical source into several parts, followed by a separate calculation of the decorrelation for each frequency range of the spectrum. Simultaneously, an anisotropic decorrelation analysis is performed and a number of full spectral width scans are performed, which provide high spatial resolution of the vasculature (Fig. 2, 3) . This algorithm is used in the Avanti RTVue XR tomograph (Optovue, USA). OCT angiography is a non-invasive 3D alternative to conventional angiography. The advantages of the method include the non-invasiveness of the study, the absence of the need to use fluorescent dyes, the possibility of measuring ocular blood flow in the vessels in quantitative terms.

Optophysiology is a method of non-invasive study of physiological processes in tissues using OCT. OCT is sensitive to spatial changes in the optical reflection or scattering of light by tissues associated with local changes in the refractive index. The physiological processes that take place in 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 the physiological response to retinal light stimulation was demonstrated in 2006. Subsequently, this technique was applied to the study of 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 modalities in ophthalmology. Currently, devices for technology are in the list of products of more than 50 companies in the world. Over the past 20 years, resolution has improved 10 times and scanning speed has increased hundreds of times. Continuous advances in OCT technology have made this method a valuable tool for investigating the structures of the eye in practice. The development over the past decade of new technologies and additions to OCT makes it possible to make an accurate diagnosis, carry out dynamic monitoring and evaluate the results of treatment. This is an example of how new technologies can solve real medical problems. And, as is often the case with new technologies, further application experience and application development may enable a deeper understanding of the pathogenesis of ocular pathology.

Literature

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

It requires high-tech equipment, and allows you to get 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 bring inconvenience either during the diagnosis or after it.

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

Tomograph device for OCT

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

The rays 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 passage of light rays is recorded and analyzed (with 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 body, 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. Rays 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 a high degree of reflection and will turn out to be red.

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

Scanning is carried out in two planes along, as well as across, a lot of planar sections are made. This allows you to simulate an accurate three-dimensional picture 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., To study the anterior section - 1310 nm.

The level of technical equipment today allows you to explore the anterior and posterior pole of the eye. To obtain high-quality diagnostic results, the transparency of the optical media and the tear film are normal (often an artificial tear is used), the pupil must be dilated (special mydriatic preparations are used).

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

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

When is coherence tomography indicated?

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

various forms of glaucoma (examine and evaluate the operation of drainage systems),
corneal ulcers,
complex keratitis.

Coherence 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.

The posterior part of the eye is examined if:

age-related, degenerative changes in the retina;
macular holes or macular cystoid edema.

with suspicion of retinal detachment,
in 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 polyferative vitreoretinopathy or when it is detected.

Often, coherence 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 a biopsy.

Examination procedure on a coherence tomograph

The diagnosis itself is absolutely painless, it takes 2–3 minutes in time, and is carried out in conditions that are comfortable for the patient. The patient is placed in front of the fundus camera lens (the head is fixed) and looks at the flashing 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 first enter the patient's 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 healthy people, digital data is converted into maps, drawings are easy to read. All results will be presented to the subject in the form of maps, tables and protocols.

Results of coherence tomography

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 different layers, structures and departments, connective tissues;
indicators of light reflection: their changes, increase or decrease, pathologies;
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 necessary to accurately indicate the location of damage, their size and quality, and the thickness of the cornea itself. OCT allows you to very accurately determine the desired parameters. Here, non-contact methods are of great importance.

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

Retinal coherence tomography will give a normal profile of the macula with a depression in the center. 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 will 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 will be bright red. Size measurements should be as follows: in the area of the fossa of the macula, a little more than 162 microns, at its edge - 235 microns.

The study 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.

Detection of pathologies on a coherence tomograph

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

Retinal defects, idiopathic tears . They are often found in older people, 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 interaritinal cysts.
Age related macular degeneration. OCT allows you to identify these diseases (typical for the elderly), as well as evaluate the effectiveness of the therapy.
diabetic edema classified as one of the most severe forms of diabetic retinopathy, it is difficult to treat. Coherence 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 fossa . Among them, stratification is the most common.
retinitis pigmentosa . The definition of this progressive hereditary disease is often difficult. The method is very informative for babies, when other methods are powerless in front of the anxiety of the baby.

Advantages and Features

Key benefits 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 making a diagnosis;
in one session, the doctor gets a clear picture 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 studied without a biopsy;
the resolution of OCT is many times higher than that of conventional computed tomography or ultrasound - tissue damage no larger than 4 microns in size is detected, pathological changes are detected at the earliest stages;
no need to administer intravenous contrast stains;
the procedure is non-invasive, therefore it has almost no contraindications, does not require special training and a recovery period.

When performing coherence tomography, the patient does not receive any radiation exposure, which is also a great advantage, given the harmful effects of external factors that every modern person is exposed to without it.

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 decoding and analysis of the received data are carried out.

Retinal oct 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 what the image on the computer monitor looks like, obtained by scanning with OCT, it will require special knowledge and skills of a specialist to decipher it

To obtain a complete, 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, there are opacities of the vitreous body, hemorrhages, and foreign particles.

What can be done with optical tomography:

Determine the thickness of the eye structures.
Determine the size of the optic nerve head.
Detect and evaluate changes in the structure of the retina and nerve fibers.
Assess the condition of the elements of the anterior part of the eyeball.

Indications for carrying out

Optical tomography, in principle, can be prescribed to every patient who has applied to an ophthalmologist with any complaints. But in some cases, this procedure cannot be dispensed with; it replaces CT and MRI and even outstrips them in terms of information content. Indications for OCT are the following symptoms and complaints of patients:

"Flies", cobwebs, lightning and flashes before the eyes.
Blurred visual image.
Sudden and sudden decrease in vision in one or both eyes.
Severe pain in the organs of vision.
A significant increase in intraocular pressure in glaucoma or for other reasons.
Exophthalmos - protrusion of the eyeball from the orbit spontaneously or after injury.

Glaucoma, increased intraocular pressure, changes in the optic nerve head, suspected retinal detachment, and preparation for eye surgery are all indications for optical coherence tomography.

What can be determined and detected using coherence tomography:

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

Thanks to such a diagnostic study, even minor changes and anomalies of the organs of vision can be detected, a correct diagnosis can be made, the degree of damage can be determined and the optimal method of treatment can be determined. OCT actually helps to preserve or restore the patient's visual function. And since the procedure is completely safe and painless, it is often performed as a preventive measure for diseases that can be complicated by eye pathologies - with diabetes mellitus, hypertension, cerebrovascular accidents, after injuries or surgery.

When not to do OCT

The presence of a pacemaker and other implants, conditions in which the patient cannot focus, is unconscious, or is not able to control his emotions and movements, most diagnostic studies are not performed. In the case of coherence tomography, everything is different. A procedure of this kind can be carried out with confusion and an unstable psycho-emotional state of the patient.

The main and, in fact, the only obstacle to performing OCT is the simultaneous conduct of other diagnostic studies. On the day on which OCT is scheduled, no other diagnostic methods for examining the organs of vision can be used. If the patient has already undergone other procedures, then OCT is transferred to another day.

OCT technique

It must be said right away that optical coherence tomography is usually not performed in district clinics, since ophthalmological rooms 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, all you need is a functioning OCT scanner and the patient himself. The subject will be asked to sit on a chair and focus on a specified mark. If the eye whose structure is to be examined is not able to focus, then the gaze is fixed as far as possible with another, healthy eye. It takes no more than two minutes to be stationary - this is enough to pass beams of infrared radiation through the eyeball.

Summary: most ophthalmic diseases are extremely difficult to recognize and diagnose in the early stages, especially to establish the real degree of damage to eye structures. With suspicious symptoms, ophthalmoscopy is usually prescribed, but this method is not enough to get the most accurate picture of the condition of the eyes. Computed tomography and magnetic resonance imaging provide more complete information, 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. To date, this is the only non-invasive way to obtain the most complete information about the condition of the eyes. The only difficulty that may arise is that not all ophthalmological rooms have the equipment necessary for the procedure.