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OPTICAL COHERENCE TOMOGRAPHY IN DIAGNOSTICS OF OPTIC NERVE
PATHOLOGY
Jalalova Dilfuza Zuhridinovna
Department of Ophthalmology, Samarkand State Medical University.
Hatamova Oʻgʻiloy
Samarkand State Medical University, Department of Ophthalmology, 1st year clinical Ordinator.
https://doi.org/10.5281/zenodo.15106589
Abstract. Optical coherence tomography (OCT) is a modern non-invasive, non-contact
method for intravital visualization and analysis of the morphological properties of the structures
of the optically transparent tissues of the eye, based on the principle of light interferometry. The
first OCT devices used a sequential (time) imaging method (time-domain optical coherence
tomography, TD-OCT).
Keywords: optic nerve, OCT, OCT, OST.
Introduction:
The principle of operation of the interferometer is based on Michelson. The
low-coherence light beam of a superluminescent light-emitting diode is split into two beams, one
of which is reflected by the object under study, and the other passes along the reference path inside
the device and is reflected by a mirror whose position is adjusted by the researcher. When the
length of the beam reflected from the tissue under study and the beam on the mirror are equal, an
interference phenomenon occurs, which is recorded by the LED, which in turn is analyzed by the
software. The scanning results are presented as a gray or pseudo-color image. Each measurement
point corresponds to one A-scan. The resulting single A-scans are combined to form a two-
dimensional image. The axial resolution of the first generation devices is 8-10 μm, the scanning
speed is 400 A-scans per second. The appearance of artifacts and the decrease in scan quality are
due to eye movements that occur during the examination.
Research methods and materials:
With the advent of high-speed cameras, spectral
tomographs have replaced time-lapse cameras [7, 10]. The light source in spectral tomographs is
a broadband superluminescent diode, which allows obtaining low-coherence light containing
several wavelengths. As in time-domain OCT, in spectral OCT, the light beam is split into two
beams, one of which is reflected from the object under examination (the eye) and the other 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 camera. Then, using the mathematical Fourier
transform, the interference spectrum is processed and a linear A-scan is formed.
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Unlike time-domain OCT, where a linear A-scan is obtained by sequentially measuring the
reflectance properties of each individual point, spectral OCT produces a linear A-scan by
simultaneously measuring the reflected rays from each individual point [4, 7]. Modern spectral
OCT scanners have an axial resolution of 3–7 μm and scan rates exceeding 26,000 A-scans per
second. High scan rates can significantly improve the quality of the acquired images by reducing
artifacts that occur during eye movement during the examination [4].
High-resolution spectral tomographs (collective source OCT, SS-OCT) use laser sources
whose emission frequency is tuned at high speed in a specific spectral range (frequency sweep).
During the frequency tuning cycle, the amplitude of the reflected signal is not recorded,
but rather the frequency [15]. By using two parallel photodetectors, the scanning speed increases
to 70-100 thousand A-scans. SS-OCT technology (due to the increase in wavelength and scanning
speed) allows imaging of deep structures such as the choroid and lamina cribrosa of the optic disc.
The development of the method, the creation and implementation of new generation
devices have helped to expand our knowledge of the structures of the eye and improve the
interpretation of the obtained data. Modern optical coherence tomographs allow obtaining images
comparable to microscopic examination.
Given the current div of knowledge, in 2014 an international panel of experts (the
International Nomenclature for Optical Coherence Tomography Panel) presented an improved
version of the interpretation of normal OCT anatomy of the retina. Most of the layers retained their
previous names, but some underwent fundamental changes.
The rate of evolution of tomographs began to exceed the capabilities of specialists in
analyzing and interpreting data. The amount of data that needs to be described, classified and
evaluated has increased. In ophthalmological practice, the capabilities of modern OCT scanners
are widely used in the diagnosis and monitoring of pathologies of the retina and the anterior
segment of the optic nerve and the macular region. Modern optical coherence tomographs allow
you to build three-dimensional models of the studied area and maps of the thickness of individual
layers of the retina, which allows you to objectively assess the dynamics of the pathological
process and the effectiveness of treatment. The latest generation of tomographs, thanks to the use
of innovative technologies, allow you to visualize the blood flow of the retina and optic nerve
head, which opens up fundamentally new opportunities for the diagnosis and treatment of retinal
diseases.
In the diagnosis of anterior segment diseases, OCT is used to assess: the precorneal tear
film and tear meniscus profile; the cornea before and after refractive surgery and keratoplasty; the
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state of newly created pathways for the outflow of intraocular fluid at different times after UPC,
iridocrystalline lens contacts, antiglaucoma operations; iridocorneal adhesions due to trauma,
inflammation and dystrophic diseases of the cornea and the anterior segment in general; the state
of lenses and intraocular implants (IOLs, drains, corneal rings, artificial iris).
Results:
The diagnostic value of the method has been reliably proven in pathologies of the
macular region (tear, edema, degeneration, retinoschisis); focal changes in the vitreoretinal
interface and vitreoretinal traction syndrome; epiretinal membranes; serous and hemorrhagic
detachment of the retina and pigment epithelium; diabetic retinopathy; retinal neovascularization;
dystrophic changes in the retina; glaucoma, etc.
There are no absolute contraindications to the examination. However, to obtain high-
quality images, the transparency of the optical medium and the normal state of the tear film are
necessary. In severe disorders of the tear film, it is recommended to instill artificial tears into the
conjunctival cavity immediately before the examination. The examination is complicated by any
degree of turbidity of the optical medium, pronounced nystagmus, head tremor, lack of central
fixation and low visual acuity, the patient does not see the fixation sign. High astigmatism and
IOL decentration reduce the quality of the study. In case of pronounced miosis (less than 3 mm),
the study is performed in conditions of drug-induced mydriasis. If the patient underwent
ophthalmoscopy using a panfundus lens, Goldman lens or gonioscopy the day before the
examination, then scanning is possible only after washing the contact lens from the conjunctival
cavity.
It should be noted that today OCT technology is developing faster than its detailed
standardized analysis capabilities. In this regard, the task of clinical interpretation of the obtained
tomograms, which consists of qualitative analysis (morphology, structure and reflectance analysis)
and quantitative assessment of the parameters of the studied area (thickness, area, volume, surface
mapping), is becoming increasingly urgent.
It is better to study the scanners in detail in black and white, rather than in pseudo-color.
The shades of color OCT images are determined by the system software, each shade is
associated with a certain level of reflectance. Therefore, in the color image we see different shades
of color, while in reality there are consistent changes in the reflectance of the tissue. The black and
white image allows you to detect small deviations in the optical density of the tissues and examine
details that may not be noticeable in the color image. Some structures may be better visible in
negative images.
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In recent years, there has been a growing interest among ophthalmologists in exploring the
diagnostic potential of OCT in examining the anterior segment of the eye. This technique allows
for imaging of the anterior segment of the eyeball, safe and accurate biometric measurements of
the structures of the cornea, anterior segment, and lens, and assessment of IOL status.
Currently, the ophthalmological equipment market offers various models for examining
the anterior segment of the eye: a specialized device for examining only the anterior segment of
the eyeball - Visante OST (Carl Zeiss Meditec), as well as tomographs equipped with a module
for examining the structures of the anterior segment of the eye ( Optovue, C, Canon , etc.). New
opportunities have opened up for the study of lacrimal fluid using optical coherence tomography
(LM) of the lacrimal meniscus - OCT menisometry [2, 8]. The lower meniscus is examined, the
result is recorded 3-4 seconds after blinking. The method allows for detailed visualization of the
lacrimal meniscus and accurate measurement of its geometric parameters.
The height of the lacrimal meniscus is measured as the distance from the meniscocorneal
contact point to the lower eyelid margin to the point of contact of the meniscus. The radius of the
lacrimal meniscus is calculated as the distance between the line connecting the edges of the
meniscus and the point of its greatest concavity. The radius of the meniscus determines the nature
of the interaction of the lacrimal fluid with the conjunctiva and cornea of the eye, indirectly reflects
the qualitative and quantitative state of the lacrimal fluid and the ocular surface. The wetting angle
of the lacrimal meniscus reflects the degree of proximity of the lacrimal fluid to the corneal
epithelium and conjunctiva of the eye and is an indicator of the stability of the tear film. High
diagnostic sensitivity and specificity of OCT menisometry in the diagnosis of various pathological
conditions of the tear film were noted.
The possibility of non-contact visualization of the anterior segment structures of the eye is
of particular importance in patients with destructive-inflammatory diseases of the cornea.
The diagnostic value of OCT in refractive surgery is high for preoperative examination of
the cornea and for postoperative assessment of the corneal flap and stroma, since all modern
methods of keratorefractive surgery lead to changes in the morphological structure of the cornea
to one degree or another.
In recent years, due to the development of new technologies in refractive surgery, much
attention has been paid to the study of the epithelium, the influence of changes in the epithelial
layer and its thickness on refractive indices after surgical interventions [1]. The state of the lacrimal
meniscus of the right (a) and left (b) eyes in a patient with anisometropia corrected by contact
lenses (AVANTIRT VueX R, Optovue)
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Discussion:
Modern tomographs allow obtaining pachymetric maps and mapping the
epithelium. The obtained data include the thickness of the cornea and the map of the epithelial
layer corresponding to the scanning area. they are constructed on the basis of interpolation of radial
scanning data (16,320 points at 22.5 ° along 8 meridians). The parameters of the statistical value
reflect the average values of the corneal and epithelial thickness in the central zone and in the
radial segments of the central and peripheral zones, the minimum and maximum values, the
difference between them, the standard deviation of the thickness value over the measurement area.
When examining patients with IOLs, visualization of the lens is possible only in the visible
range. Limitations in visualization of the anterior segment structures are associated with the
absorption of optical rays by the pigment layer of the iris.
OCT allows to determine the depth of the anterior chamber of the eye in any area of interest,
to measure the internal diameter and width of the anterior chamber, and to objectively assess the
degree of opening of the anterior chamber. These measurements are more accurate than ultrasound
A-mode and UBM [6, 9, 19, 23]. OCT of the anterior segment of the eye is an informative and
safe method for assessing the morphofunctional state of the antiglaucoma surgical area, allowing
to visualize all the structures of the surgical area, to assess their morphometric characteristics and
the optical density of the tissues. The non-contact nature of the method allows to examine the
surgical field from the early postoperative stages, which allows to identify prognostic signs of the
effectiveness of the intervention.
Objective measurement of the degree of opening of the anterior chamber angle (OCT
goniometry). The trabecular-iris distance (AOD) and the irido-trabecular space area (TISA), which
characterize the degree of opening of the anterior chamber, are determined from the corneal
endothelium at a distance of 500 and 750 μm from the scleral junction perpendicular to the anterior
surface of the iris (Cirruss50)
Conclusion:
Examination of the iris is limited, since, as mentioned above, the iris pigment
epithelium makes it difficult for light to penetrate [6]. Visualization of the posterior surface of the
iris and ciliary div with OCT is insufficient. The study is limited to assessing the size and depth
of defects and pathological formations (cysts, neoplasms, synechiae). The optical coherence
tomography method is of the greatest importance in the diagnosis of diseases of the posterior pole
of the eye for detecting pathological changes in the retina and optic nerve, for objective assessment
of the dynamics of the pathological process during the natural course of the disease, for assessing
the effectiveness of surgical or drug treatment.
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Analysis of the scan results, as well as for interpreting the data of the anterior segment
study, consists in studying the morphology of the retina and choroid, their reflective ability, and
quantitative assessment. Morphological analysis allows you to identify the entire deformation of
the retina, its surface and individual layers, changes in the profile of the macula and foveola,
irregularities on the surface of the retina, folds, tears, vitreoretinal strands, preretinal and epiretinal
membranes, exudates, drusen, chorioretinal membranes.
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