Complex optical coherence tomography. What is coherence tomography of the retina. OCT and histology

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. The combination of modern OCT technologies and photoacoustic, spectroscopic, polarization, Doppler and angiographic, elastographic methods made it possible to assess not only tissue morphology, 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 segment 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 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 acquired 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 image quality much less dependent on the distance of the tissue of interest to 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 required. 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. 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 a 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.

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This method of optical diagnostics allows you to visualize the structure of the tissues of a living organism in a cross section. Due to its high resolution, optical coherence tomography (OCT) makes it possible to obtain histological images in vivo, and not after preparation of the section. The OCT method is based on low-coherence interferometry.

In modern medical practice, OCT is used as a non-invasive non-contact technology for studying the anterior and posterior segments of the eye at the morphological level in living patients. This technique allows you to evaluate and record a large number of parameters:

  • condition and optic nerve;
  • thickness and transparency;
  • condition and angle of the anterior chamber.

Due to the fact that the diagnostic procedure can be repeated many times, while recording and saving the results, it is possible to evaluate the dynamics of the process against the background of treatment.

When performing OCT, the depth and magnitude of the light beam is estimated, which is reflected from tissues with different optical properties. With an axial resolution of 10 µm, the most optimal image of the structures is obtained. This technique allows you to determine the echo delay of the light beam, the change in its intensity and depth. During focusing on tissues, the light beam is scattered and partially reflected from microstructures located at different levels in the organ under study.

OCT of the retina (macula)

Optical coherence tomography of the retina, as a rule, is performed for diseases of the central parts of the eye - edema, dystrophies, hemorrhages, etc.

OCT of the optic nerve head (OND)

The optic nerve (its visible part - the disk) is examined for such pathologies of the visual apparatus as swelling of the nerve head, etc.

The mechanism of action of OCT is similar to the principle of obtaining information during A-scanning. The essence of the latter is to measure the time interval required for the passage of an acoustic pulse from the source to the tissues under study and back to the receiving sensor. Instead of a sound wave, OCT uses a beam of coherent light. The wavelength is 820 nm, that is, it is in the infrared range.

OCT does not require special preparation, however, with medical expansion, you can get more information about the structure of the posterior segment of the eye.

Device device

In ophthalmology, a tomograph is used, in which the radiation source is a superluminescent diode. The coherence length of the latter is 5-20 µm. The hardware part of the device contains a Michelson interferometer, a confocal microscope (slit lamp or fundus camera) is located in the object arm, and a time modulation unit is located in the reference arm.

Using a video camera, you can display the image and the scanning path of the study area on the screen. The received information is processed and recorded in the computer memory in the form of graphic files. The tomograms themselves are logarithmic two-color (black and white) scales. To make the result better perceived, with the help of special programs, a black-and-white image is transformed into a pseudo-color one. Areas with high reflectivity are painted white and red, and areas with high transparency are painted black.

Indications for OCT

Based on OCT data, one can judge the structure of the normal structures of the eyeball, as well as identify various pathological changes:

  • , in particular postoperative;
  • iridociliary dystrophic processes;
  • traction vitreomacular syndrome;
  • edema, preruptures and ruptures of the macula;
  • glaucoma;
  • pigmented.

Video about cataracts in diabetes

Contraindications

A limitation to the use of OCT is the reduced transparency of the examined tissues. In addition, difficulties arise in cases where the subject is not able to fix his gaze motionless for at least 2-2.5 seconds. That's how long it takes to scan.

Establishing diagnosis

To make an accurate diagnosis, it is necessary to evaluate the obtained graphs in detail and competently. At the same time, special attention is paid to the study of the morphological structure of tissues (the interaction of various layers with each other and with surrounding tissues) and light reflection (change in transparency or the appearance of pathological foci and inclusions).

With quantitative analysis, it is possible to detect a change in the thickness of a layer of cells or the entire structure, measure its volume and obtain a surface map.

To obtain a reliable result, it is necessary that the surface of the eye be free from foreign fluids. Therefore, after performing with a panfundusscope or, you should first rinse the conjunctiva well from contact gels.

The low-power infrared radiation used in OCT is completely harmless and does not harm the eyes. Therefore, for this study, there are no restrictions on the somatic status of the patient.

Cost of optical coherence tomography

The cost of the procedure in eye clinics in Moscow starts from 1,300 rubles. per eye and depends on the area being examined. You can see all prices for OCT in the ophthalmological centers of the capital. Below we provide a list of institutions where you can do an optical coherence tomography of the retina (macula) or the optic nerve (ON).

5-08-2011, 10:31

Description

Optical coherence tomography (OCT)- an optical research method that allows you to display the structure of the biological tissues of the body in a cross section with a high level of resolution, providing lifetime morphological information at the microscopic level. The operation of OCT is based on the principle of low-coherence interferometry.

The method makes it possible to estimate the magnitude and depth of the light signal reflected from tissues with different optical properties. An axial resolution of about 10 µm provides the best of all existing methods for studying and imaging tissue microstructures. The echo delay of the reflected light wave is determined by the OCT method with the measurement of the intensity and depth of the signal. When a light beam is focused on a target tissue, it is scattered and partially reflected from internal microstructures at various depths of the tissues under study (Fig. 17-1).

The mechanism is similar to that in ultrasonic A-scanning, the essence of which is to measure the time it takes for an acoustic wave pulse to travel from the ultrasound source to the target and back to the receiving device. In OCT, a beam of coherent infrared light with a wavelength of 820 nm is used instead of a sound wave.

Scheme used in ophthalmology optical coherence tomography can be represented as follows. As a radiation source, the device uses a superluminescent diode with a radiation coherence length of 5-20 μm. The Michelson interferometer is built into the hardware of the device, a confocal microscope (fundus camera or slit lamp) is located in the object arm, and a time modulation unit is located in the reference arm.

The visible picture and the trajectory of scanning the area under study by means of a video camera are displayed on the monitor. The computer processes the received information and saves it as graphic files in the database. Optical coherence tomograms are presented as a logarithmic black and white scale. For better perception, the image is transformed into a pseudo-color, where areas with a high degree of light reflection correspond to red and white, optically transparent - black.

Modern OCT- non-contact non-invasive technology that is used to study the morphology of the anterior and posterior segment of the eyeball in vivo. It allows you to identify, record and quantify the state of the retina and the adjacent CT, the optic nerve, as well as measure the thickness and determine the transparency of the cornea, examine the state of the iris and APC. The possibility of multiple repetition of studies and saving the results in the computer memory makes it possible to trace the dynamics of the pathological process.

Indications

OCT allows obtain valuable information both about the state of normal eye structures and the manifestation of pathological conditions, such as various corneal opacities, including those after refractive surgery, iridociliary dystrophies, traction vitreomacular syndrome, macular ruptures and preruptures, macular degeneration, macular edema, retinitis pigmentosa , glaucoma and more.

Contraindications

OCT method it is impossible to obtain a high-quality image with reduced transparency of the media. The study is difficult in patients who cannot provide a fixed fixation of the gaze during the scanning time (2.0-2.5 s).

Training

The procedure does not require additional preparation. However, the expansion of the pupil will allow you to get a better image of the structures of the posterior segment of the eye.

Technique and aftercare

Technically optical coherence tomography carried out as follows. After entering the patient's data (card number, last name, first name, date of birth), they begin the study. The patient fixes his gaze on a flashing object in the lens of the fundus camera. The camera is brought closer to the patient's eye until the retinal image is displayed on the monitor. After that, you should fix the camera by pressing the lock button and adjust the image clarity. If visual acuity is low and the patient does not see a flashing object, then external illumination should be used, and the patient should look straight ahead without blinking. The optimal distance between the examined eye and the camera lens is 9 mm. The study is carried out in the perform scans mode (scanning) and controlled using the control panel, presented in the form of regulatory buttons and manipulators, divided into six functional groups.

Next, the alignment and cleaning of the performed scans from interference is carried out. After data processing, the studied tissues are measured and their optical density is analyzed. The obtained quantitative measurements can be compared with standard normal values ​​or values ​​obtained during previous examinations and stored in the computer's memory.

Interpretation

Establishing a clinical diagnosis should be based primarily on a qualitative analysis of the obtained scans. Attention should be paid to the morphology of tissues (changes in the external contour, the relationship of different layers and departments, relationships with neighboring tissues), changes in light reflection (increase or decrease in transparency, the presence of pathological inclusions). Quantitative analysis makes it possible to identify thickening or thinning of both the cell layer and the entire structure, its volume, and to obtain a map of the surface under study.

Tomography of the cornea. It is important to accurately localize the existing structural changes and calculate their parameters: this makes it possible to more correctly choose the tactics of treatment and objectively evaluate its effectiveness. In some cases, OCT of the cornea is considered the only method that allows you to calculate its thickness (Fig. 17-2). A big advantage for the damaged cornea is the non-contact technique.

Iris tomography makes it possible to isolate the anterior boundary layer, stroma and pigment epithelium. The reflectivity of these layers differs depending on the amount of pigment contained in the layers: on light, weakly pigmented irises, the largest reflected signals come from the posterior pigment epithelium, the anterior boundary layer is not clearly visualized. Early pathological changes in the iris, detected using OCT, are considered significant for making a diagnosis at the preclinical stage in pigment dispersion syndrome, pseudoexfoliative syndrome, essential mesodermal dystrophy, and Frank-Kamenetsky syndrome.

Retinal tomography. Normally, OCT reveals the correct profile of the macula with a depression in the center (Fig. 17-3).

The layers of the retina are differentiated according to their reflective ability, uniform in thickness, without focal changes. The layer of nerve fibers and pigment epithelium has a high reflective ability, the average degree of light reflection is characteristic of the plexiform and nuclear layers of the retina, the layer of photoreceptors is practically transparent. The outer edge of the retina on OCT is limited by a highly photoreflective bright red layer about 70 µm thick, which is a complex of the retinal pigment epithelium (RPE) and choriocapillaries. The darker band (on the tomogram is located directly in front of the "PES/choriocapillaries" complex) is represented by photoreceptors. The bright red line on the inner surface of the retina corresponds to the layer of nerve fibers. ST is normally optically transparent and has a black color on the tomogram. The sharp contrast between tissue staining made it possible to measure the thickness of the retina. In the region of the central fovea of ​​the macula, it averaged about 162 microns, and at the edge of the fovea - 235 microns.

Idiopathic macular holes retinal defects in the area of ​​the macula, occurring without any apparent cause in elderly patients. The use of OCT makes it possible to accurately diagnose the disease at all its stages, determine the tactics of treatment and monitor its effectiveness. Thus, the initial manifestation of an idiopathic macular hole, called a pre-rupture, is characterized by the presence of a foveolar detachment of the neuroepithelium due to vitreofoveolar traction. With lamellar rupture, a defect in the inner surface of the retina is noted, while the layer of photoreceptors is preserved. Through rupture (Fig. 17-4) retinal defect to the full depth.

The second sign influencing visual functions that can be detected using OCT is considered to be degenerative changes in the retina around the gap. Finally, the presence or absence of vitreomacular traction is considered an important prognostic sign. When analyzing a tomogram, one should evaluate the thickness of the retina in the macula, the minimum and maximum diameter of the rupture (at the level of RPE), the thickness of the edema along the edge of the rupture, and the diameter of intraretinal cysts. It is important to pay attention to the safety of the RPE layer, the degree of retinal degeneration around the break (determined by tissue compaction and the appearance of their red staining on the tomogram).

Age-Related Macular Degeneration (AMD) a group of chronic degenerative disorders with unknown etiopathogenesis that affect elderly patients. OCT can be used to diagnose changes in the structures of the posterior pole of the eye at various stages of AMD development. By measuring the thickness of the retina, one can objectively monitor the effectiveness of the therapy. Further, we present clinical cases that allow us to more fully represent the changes in the retina that occur at various stages of AMD development (Fig. 17-5, 17-6).


diabetic macular edema- one of the most severe, prognostically unfavorable and difficult to treat forms of DR. OCT allows assessing the thickness of the retina, the presence of intraretinal changes, the degree of tissue degeneration, as well as the state of the adjacent vitreomacular space (Fig. 17-7).

optic nerve. The high resolution of OCT makes it possible to clearly distinguish the layer of nerve fibers and measure its thickness. The thickness of the nerve fiber layer correlates well with functional parameters, and primarily with visual fields. The nerve fiber layer has a high backscatter and thus contrasts with the intermediate retinal layers as the nerve fiber axons are oriented perpendicular to the OCT tip bundle. Tomography of the ONH can be performed with radial and annular scans. Radial scans through the ONH allow obtaining a cross-sectional image of the disc and assessing excavation, the thickness of the nerve fiber layer in the peripapillary zone, as well as the angle of inclination of the nerve fibers relative to the surface of the ONH and the retina (Fig. 17-8).

3D disk parameter information can be obtained on the basis of a series of tomograms made in different meridians, and allows you to measure the thickness of the layer of nerve fibers in different areas around the ONH and evaluate their structure. The "expanded" tomogram is presented as a flat linear image. The thickness of the layer of nerve fibers and retina can be automatically processed by a computer and presented on the screen as an average value of the entire scan, quadrant (upper, lower, temporal, nasal), hour, or individually for each scan containing an image. These quantitative intentions can be compared with standard normal values ​​or values ​​obtained during previous surveys. This makes it possible to detect both local defects and diffuse atrophy, which can be used for objective diagnosis and monitoring of pathological processes in non-degenerative diseases.

stagnant disc- an ophthalmic symptom of increased intracranial pressure. OCT is considered an objective method that allows you to determine, measure and track the degree of protrusion of the ONH in dynamics. By evaluating the level of light reflection of tissues, it is possible to assess both the hydration of tissues and the degree of their degeneration (Fig. 17-9).

optic fossa- congenital anomaly of development. The most common complication of the optic nerve fossa is retinal detachment (schisis) in the macula. OCT clearly illustrates optic disc defects and retinal detachment, changes occurring in the fovea (Fig. 17-10).

Retinitis pigmentosa or tapetoretinal abiotrophy, - a hereditary progressive disease of the organ of vision with a primary genetically determined lesion of the photoreceptor layer and RPE. The state of the chorioretinal complex and the severity of the development of the disease can be assessed using OCT. On tomograms, the thickness of the layer of photoreceptors, nerve fibers and neuroglia of the retina, the transparency of the layers of the retina relative to the standard color scale of the device, the state of the RPE and the layer of choriocapillaries are assessed. Already in the latent stage of retinitis pigmentosa, in the absence of clinical manifestations and ophthalmoscopic signs of the disease, characteristic changes are found in the form of a decrease in the thickness of the photoreceptor layer, a decrease in its transparency, segments, and an increased metabolism of the pigment epithelium. OCT allows monitoring the pathological process and can be used in the diagnosis of retinitis pigmentosa, including the non-pigmented form, including in children, when it is impossible to conduct functional research methods due to the small age of the child and his inappropriate behavior.

Operating characteristics

The source of the light signal is a superluminescent diode with a wavelength of 820 nm for the retina and 1310 nm for the anterior segment. Signal type - optical scattering from tissue. Image field: 30 mm horizontally and 22 mm vertically for the rear segment, 10-16 mm for the anterior segment. Resolution: longitudinal - 10 microns, transverse - 20 microns. Scanning speed - 500 axial slices per second.

Factors affecting the result

If the patient underwent ophthalmoscopy the day before using a panfundusscope, Goldmann lenses, or gonioscopy, OCT is possible only after the contact medium has been washed out of the conjunctival cavity.

Complications

The low-power infrared radiation used does not have a damaging effect on the examined tissues, has no restrictions on the patient's somatic condition and excludes injury.

Alternative Methods

Part of the information that OCT provides can be obtained using the Heidelberg Retinal Tomograph, FAG, ultrasound biomicroscopy, IOL-Master, etc.

Article from the book: .

Optical coherence tomography is a non-invasive (non-contact) method for examining tissue. It allows you to get higher resolution images compared to the results of ultrasound procedures. In fact, optical coherence tomography of the eye is a kind of biopsy, only for the first one there is no need to take a tissue sample.

A brief excursion into history

The concept on the basis of which modern optical coherence tomography is performed was developed by researchers in the distant 1980s. In turn, the idea of ​​introducing a new principle into ophthalmology was proposed in 1995 by the American scientist Carmen Pouliafito. A few years later, Carl Zeiss Meditec developed a corresponding device, which was called the Stratus OCT.

At present, using the latest model, it is possible not only to study retinal tissues, but also optical coherence tomography of the coronary arteries, the optic nerve at the microscopic level.

Research principles

Optical coherence tomography consists in the formation of graphic images based on the measurement of the delay period when a light beam is reflected from the tissues under study. The main element of devices of this category is a superluminescent diode, the use of which makes it possible to form light beams of low coherence. In other words, when the device is activated, the beam of charged electrons is divided into several parts. One flow is directed to the area of ​​the tissue structure under study, the other - to a special mirror.

Rays reflected from objects are summed up. Subsequently, the data are recorded by a special photodetector. The information generated on the graph allows the diagnostician to draw conclusions about the reflectivity at individual points of the object under study. When evaluating the next section of the fabric, the support is moved to another position.

Optical coherence tomography of the retina makes it possible to generate graphics on a computer monitor that are in many ways similar to the results of an ultrasound examination.

Indications for the procedure

Today, optical coherence tomography is recommended for diagnosing such pathologies as:

  • Glaucoma.
  • Macular tissue ruptures.
  • Thrombosis of the circulatory pathways of the retina.
  • Degenerative processes in the structure of the eye tissue.
  • Cystoid edema.
  • Anomalies in the functioning of the optic nerve.

In addition, optical coherence tomography is prescribed to evaluate the effectiveness of the therapeutic procedures used. In particular, the research method is indispensable in determining the quality of the installation of a drainage device that integrates into the tissues of the eye in glaucoma.

Features of the diagnosis

Optical coherence tomography involves focusing the subject's vision on special marks. In this case, the operator of the device performs a number of sequential tissue scans.

Pathological processes such as edema, abundant hemorrhages, and all kinds of opacities are capable of significantly complicating research and hindering effective diagnosis.

The results of coherence tomography are formed in the form of protocols that inform the researcher about the state of certain tissue areas, both visually and quantitatively. Since the data obtained are recorded in the memory of the device, they can subsequently be used to compare the state of tissues before the start of treatment and after the application of therapies.

3D visualization

Modern optical coherence tomography makes it possible to obtain not only two-dimensional graphs, but also to produce a three-dimensional visualization of the objects under study. High-speed scanning of tissue sections makes it possible to generate more than 50,000 images of the diagnosed material within a few seconds. Based on the information received, special software reproduces the three-dimensional structure of the object on the monitor.

The generated 3D image is the basis for studying the internal topography of the eye tissue. Thus, it becomes possible to determine the clear boundaries of pathological neoplasms, as well as to fix the dynamics of their change over time.

Benefits of coherence tomography

Coherence tomography devices demonstrate the greatest efficiency in the diagnosis of glaucoma. In the case of using devices of this category, specialists get the opportunity to determine with high accuracy the factors in the development of pathology in the early stages, to identify the degree of progression of the disease.

The research method is indispensable in diagnosing such a common disease as macular degeneration of the tissue, in which, as a result of age-related characteristics of the body, the patient begins to see a black spot in the central part of the eye.

Coherence tomography is effective in combination with other diagnostic procedures, such as fluorescein angiography of the retina. By combining procedures, the researcher obtains particularly valuable data that contributes to the correct diagnosis, determination of the complexity of the pathology and the choice of effective treatment.

Where can an optical coherence tomography be performed?

The procedure is possible only with a specialized OCT apparatus. Diagnostics of such a plan can be resorted to in modern research centers. Most often, vision correction rooms and private ophthalmological clinics have such equipment.

Issue price

Carrying out coherence tomography does not require a referral from the attending physician, but even if it is available, diagnostics will always be paid. The cost of the study determines the nature of the pathology, which is aimed at identifying the diagnosis. For example, the definition of macular tissue ruptures is estimated at 600-700 rubles. While tomography of the tissue of the anterior part of the eye can cost the patient of the diagnostic center 800 rubles or more.

As for complex studies aimed at assessing the functioning of the optic nerve, the state of the retinal fibers, the formation of a three-dimensional model of the visual organ, the price for such services today starts at 1,800 rubles.

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