Resolution Optical Coherence Tomography Research Articles

Retinal Optical Coherence Tomography in Neurodegenerative Diseases (Review)

DOI 10.17691/stm2015.7.1.14 Received October 16, 2014S.N. Svetozarskiy, Clinical Resident, Department of Eye Diseases;S.V. Kopishinskaya, PhD, Associate Professor, Department of Neurology, Psychiatry and Addiction Medicine, Postgraduate FacultyNizhny Novgorod State Medical Academy, 10/1 Minin and Pozharsky Square, Nizhny Novgorod, 603005, Russian FederationIncreased aging of the population makes problems of the diagnosis and treatment of neurodegenerative diseases socially more significant. The ability to use the retina as a “window” to the central nervous system has attracted great attention in recent years. Optical coherence tomography (OCT) is a non-invasive method for in vivo studies of various conditions to generate high-resolution images of the tissue cross sections under study. Retinal OCT parameters are considered to be potential surrogate biomarkers of early-stage neurodegenerative disorders, and have already been included in the guidelines for diagnosing neuromyelitis optica. This review summarizes and analyzes the current information on retinal changes according to OCT data in neurodegeneration in vitro and in vivo in Alzheimer’s and Parkinson’s diseases. The application of ultra-high resolution OCT for the diagnosis of the early stages of neurodegeneration is also considered. Morphological and functional links and possible mechanisms for the retinal lesions in Alzheimer’s and Parkinson’s diseases, and their similarities in glaucoma are discussed. The efficacy of using this method in the diagnosis of neurodegenerative processes at an early stage is likely to be increased by the development of instrumentation and improvements in the design study for carrying out investigations in different groups of patients, including those having hereditary diseases of the nervous system.Key words: retina; biomarker; optical coherence tomography; Parkinson’s disease; Alzheimer’s disease; neurodegeneration.Corresponding author: Svetozarskiy Sergey, e-mail: svetozarskij@rambler.ru

Depth-encoded synthetic aperture optical coherence tomography of biological tissues with extended focal depth.

Optical coherence tomography (OCT) has proven to be able to provide three-dimensional (3D) volumetric images of scattering biological tissues for in vivo medical diagnostics. Unlike conventional optical microscopy, its depth-resolving ability (axial resolution) is exclusively determined by the laser source and therefore invariant over the full imaging depth. In contrast, its transverse resolution is determined by the objective's numerical aperture and the wavelength which is only approximately maintained over twice the Rayleigh range. However, the prevailing laser sources for OCT allow image depths of more than 5 mm which is considerably longer than the Rayleigh range. This limits high transverse resolution imaging with OCT. Previously, we reported a novel method to extend the depth-of-focus (DOF) of OCT imaging in Mo et al.Opt. Express 21, 10048 (2013)]. The approach is to create three different optical apertures via pupil segmentation with an annular phase plate. These three optical apertures produce three OCT images from the same sample, which are encoded to different depth positions in a single OCT B-scan. This allows for correcting the defocus-induced curvature of wave front in the pupil so as to improve the focus. As a consequence, the three images originating from those three optical apertures can be used to reconstruct a new image with an extended DOF. In this study, we successfully applied this method for the first time to both an artificial phantom and biological tissues over a four times larger depth range. The results demonstrate a significant DOF improvement, paving the way for 3D high resolution OCT imaging beyond the conventional Rayleigh range.

Effect of hyaluronic acid on tear film thickness as assessed with ultra-high resolution optical coherence tomography.

The aim of this study was to assess the effect of a single drop of hyaluronic acid on tear film thickness (TFT) in healthy subjects. Sixteen healthy subjects (eight male/eight female) aged between 20 and 36 years were included in this randomized, double-masked placebo-controlled study. One eye received a single dose of hyaluronic acid (Olixia pure(®) ; Croma Pharma, Korneuburg, Austria) eye drops, and the fellow eye received physiologic saline solution as placebo control. The study eye was chosen randomly. TFT as measured with a custom-built Fourier-domain optical coherence tomography (FD-OCT) system was the main outcome variable and measured before and every 10 min until 1 hr after topical administration. Baseline TFT was 4.8 ± 0.5 μm in the study eye and 5.0 ± 0.4 μm in the control eyes. Hyaluronic acid significantly increased TFT (p = 0.008 versus placebo) with a maximum effect 10 min after instillation (13.9 ± 11.9%). Post hoc analysis revealed that an increase in TFT was seen until 30 min after administration compared to placebo. Data in the placebo group show high reproducibility with an intraclass correlation coefficient of 0.93 and a coefficient of variation of 5.4 ± 3.3%. The data of this study indicate that hyaluronic acid increases TFT for as long as 30 min in healthy subjects. In addition, our data provide evidence that our custom-built OCT system is capable of measuring residence time of lubricants on the ocular surface.

Noise-compensated homotopic non-local regularized reconstruction for rapid retinal optical coherence tomography image acquisitions.

BackgroundOptical coherence tomography (OCT) is a minimally invasive imaging technique, which utilizes the spatial and temporal coherence properties of optical waves backscattered from biological material. Recent advances in tunable lasers and infrared camera technologies have enabled an increase in the OCT imaging speed by a factor of more than 100, which is important for retinal imaging where we wish to study fast physiological processes in the biological tissue. However, the high scanning rate causes proportional decrease of the detector exposure time, resulting in a reduction of the system signal-to-noise ratio (SNR). One approach to improving the image quality of OCT tomograms acquired at high speed is to compensate for the noise component in the images without compromising the sharpness of the image details.MethodsIn this study, we propose a novel reconstruction method for rapid OCT image acquisitions, based on a noise-compensated homotopic modified James-Stein non-local regularized optimization strategy. The performance of the algorithm was tested on a series of high resolution OCT images of the human retina acquired at different imaging rates.ResultsQuantitative analysis was used to evaluate the performance of the algorithm using two state-of-art denoising strategies. Results demonstrate significant SNR improvements when using our proposed approach when compared to other approaches.ConclusionsA new reconstruction method based on a noise-compensated homotopic modified James-Stein non-local regularized optimization strategy was developed for the purpose of improving the quality of rapid OCT image acquisitions. Preliminary results show the proposed method shows considerable promise as a tool to improve the visualization and analysis of biological material using OCT.

Effects of a tear supplement containing hyaluronic acid (HA) on tear film thickness in healthy subjects

AbstractPurpose We have recently introduced a new technique for the measurement of tear film thickness (TFT) using ultra‐high resolution optical coherence tomography (OCT) in humans. In the present study we hypothesized that the effect of a hyaluronic acid (HA) containing lubricant on TFT can be evaluated using this technique after single instillation.Methods 16 healthy volunteers were included into the present study. The design was randomized, double‐masked and placebo controlled. Subjects randomly received in one eye HA‐containing eye drops (0.15%, Olixia®, Croma Pharma, Austria) and in the other eye NaCl 0.9%. Measurement of TFT was performed at baseline and thereafter every 10 minutes until 60 minutes after start of instillation. A repeated measures ANOVA model was employed to study statistically significant differences.Results 8 female and 8 male subjects with a mean age of 26.8±4.8 years were included in this study. Baseline TFT was 5.0±0.5 μm in placebo and 4.8±0.5 μm in HA treated eyes. A significant effect of HA containing eyedrops on TFT was seen over time (p=0.014). 10 minutes after instillation TFT increased to 5.5±0.8 μm in the HA containing eyedrops and remained almost stable in the placebo group (5.1±0.8 μm). Post hoc analysis revealed that TFT was increased up to 30 minutes after instillation and then returned to baseline.Conclusion The results of the present study indicate that HA containing eyedrops increase pre‐corneal TFT for as long as 30 minutes in healthy subjects. This effect is not seen with NaCl administration. Our data also indicate that measurement of TFT with ultra‐high resolution OCT is a promising technique to study residence time of eye drops at the ocular surface.

Associated factors analysis for predicting anatomical outcomes of idiopathic macular hole after vitrectomy

Objective To evaluate the associated factors for predicting anatomical outcomes of idiopathic macular hole (IMH) after vitrectomy.Methods This is a retrospective study.A total of 165 eyes in 164 IMH patients underwent a successful vitrectomy and ILM peeling surgery were included in this study.The patients included 43 males and 121 females,with the mean age of (64.0±6.7) years.The corrected vision acuity of logarithm of the minimum angle of resolution (logMAR),indirect ophthalmoscope and spectral domain optical coherence tomography (SD-OCT) were measured for all patients.The mean logMAR corrected vision acuity was 1.0.The duration of disease was (8.7±14.9) months.The minimum diameter (MIN),base diameter (BASE) and height (H) were 521,1010,406 μm respectively.The macular hole index (MHI),tractional hole index (THI),diameter hole index (DHI) and macular hole closure index (MHCI) were 0.43,0.82,0.57,0.92 respectively.Anatomical outcomes were divided into 3 levels.A:bridge-shaped healing; B:good healing; C:poor healing.The mean follow-up was (3.6±3.2) months.The multiple factors related with prognosis including age,sex,duration of disease,preoperative logMAR corrected vision acuity,MIN,BASE and H,MHI,THI,DHI,MHCI were analyzed.Results Duration of disease (r=0.141),preoperative logMAR corrected vision acuity (r=0.082),age (r=0.044),sex (r=0.109) was independent of anatomical prognosis (P＞0.05).MIN (r=0.397),BASE (r=0.276),H (r=-0.240),MHI (r=-0.363),THI (r=-0.432),DHI (r=0.272) was weak correlation to anatomical prognosis (P＜0.05).MHCI correlated significantly with anatomical outcomes (r=-0.543,P=0.000).The median MHCI of A,B and C were 1.07,0.91,0.56 respectively.There were significant difference of MHCI among the three levels (H =52.857,P＜0.05).Conclusions MHCI has the best correlation with anatomical outcomes.It can be considered a key factor for predicting anatomical outcomes of IMH after vitrectomy. Key words: Retinal perforations/surgery; Tomography, optical coherence; Prognosis

Variations in optical coherence tomography resolution and uniformity: a multi-system performance comparison.

Point spread function (PSF) phantoms based on unstructured distributions of sub-resolution particles in a transparent matrix have been demonstrated as a useful tool for evaluating resolution and its spatial variation across image volumes in optical coherence tomography (OCT) systems. Measurements based on PSF phantoms have the potential to become a standard test method for consistent, objective and quantitative inter-comparison of OCT system performance. Towards this end, we have evaluated three PSF phantoms and investigated their ability to compare the performance of four OCT systems. The phantoms are based on 260-nm-diameter gold nanoshells, 400-nm-diameter iron oxide particles and 1.5-micron-diameter silica particles. The OCT systems included spectral-domain and swept source systems in free-beam geometries as well as a time-domain system in both free-beam and fiberoptic probe geometries. Results indicated that iron oxide particles and gold nanoshells were most effective for measuring spatial variations in the magnitude and shape of PSFs across the image volume. The intensity of individual particles was also used to evaluate spatial variations in signal intensity uniformity. Significant system-to-system differences in resolution and signal intensity and their spatial variation were readily quantified. The phantoms proved useful for identification and characterization of irregularities such as astigmatism. Our multi-system results provide evidence of the practical utility of PSF-phantom-based test methods for quantitative inter-comparison of OCT system resolution and signal uniformity.

Patient-specific coronary stenoses can be modeled using a combination of OCT and flow velocities to accurately predict hyperemic pressure gradients.

Computational fluid dynamics (CFD) is increasingly being developed for the diagnostics of arterial diseases. Imaging methods such as computed tomography (CT) and angiography are commonly used. However, these have limited spatial resolution and are subject to movement artifact. This study developed a new approach to generate CFD models by combining high-fidelity, patient-specific coronary anatomy models derived from optical coherence tomography (OCT) imaging with patient-specific pressure and velocity phasic data. Additionally, we used a new technique which does not require the catheter to be used to determine the centerline of the vessel. The CFD data were then compared with invasively measured pressure and velocity. Angiography imaging data of 21 vessels collected from 19 patients were fused with OCT visualizations of the same vessels using an algorithm that produces reconstructions inheriting the in-plane (10 μm) and longitudinal (0.2 mm) resolution of OCT. Proximal pressure and distal velocity waveforms ensemble averaged from invasively measured data were used as inlet and outlet boundary conditions, respectively, in CFD simulations. The resulting distal pressure waveform was compared against the measured waveform to test the model. The results followed the shape of the measured waveforms closely (cross-correlation coefficient = 0.898 ± 0.005, ), indicating realistic modeling of flow resistance, the mean of differences between measured and simulated results was -3. 5 mmHg, standard deviation of differences (SDD) = 8.2 mmHg over the cycle and -9.8 mmHg, SDD = 16.4 mmHg at peak flow. Models incorporating phasic velocity in patient-specific models of coronary anatomy derived from high-resolution OCT images show a good correlation with the measured pressure waveforms in all cases, indicating that the model results may be an accurate representation of the measured flow conditions.

Non-invasive detection of early retinal neuronal degeneration by ultrahigh resolution optical coherence tomography.

Optical coherence tomography (OCT) has revolutionises the diagnosis of retinal disease based on the detection of microscopic rather than subcellular changes in retinal anatomy. However, currently the technique is limited to the detection of microscopic rather than subcellular changes in retinal anatomy. However, coherence based imaging is extremely sensitive to both changes in optical contrast and cellular events at the micrometer scale, and can generate subtle changes in the spectral content of the OCT image. Here we test the hypothesis that OCT image speckle (image texture) contains information regarding otherwise unresolvable features such as organelle changes arising in the early stages of neuronal degeneration. Using ultrahigh resolution (UHR) OCT imaging at 800 nm (spectral width 140 nm) we developed a robust method of OCT image analyses, based on spatial wavelet and texture-based parameterisation of the image speckle pattern. For the first time we show that this approach allows the non-invasive detection and quantification of early apoptotic changes in neurons within 30 min of neuronal trauma sufficient to result in apoptosis. We show a positive correlation between immunofluorescent labelling of mitochondria (a potential source of changes in cellular optical contrast) with changes in the texture of the OCT images of cultured neurons. Moreover, similar changes in optical contrast were also seen in the retinal ganglion cell- inner plexiform layer in retinal explants following optic nerve transection. The optical clarity of the explants was maintained throughout in the absence of histologically detectable change. Our data suggest that UHR OCT can be used for the non-invasive quantitative assessment of neuronal health, with a particular application to the assessment of early retinal disease.

Repeatability and Reproducibility of Eight Macular Intra-Retinal Layer Thicknesses Determined by an Automated Segmentation Algorithm Using Two SD-OCT Instruments

PurposeTo evaluate the repeatability, reproducibility, and agreement of thickness profile measurements of eight intra-retinal layers determined by an automated algorithm applied to optical coherence tomography (OCT) images from two different instruments.MethodsTwenty normal subjects (12 males, 8 females; 24 to 32 years old) were enrolled. Imaging was performed with a custom built ultra-high resolution OCT instrument (UHR-OCT, ∼3 µm resolution) and a commercial RTVue100 OCT (∼5 µm resolution) instrument. An automated algorithm was developed to segment the macular retina into eight layers and quantitate the thickness of each layer. The right eye of each subject was imaged two times by the first examiner using each instrument to assess intra-observer repeatability and once by the second examiner to assess inter-observer reproducibility. The intraclass correlation coefficient (ICC) and coefficients of repeatability and reproducibility (COR) were analyzed to evaluate the reliability.ResultsThe ICCs for the intra-observer repeatability and inter-observer reproducibility of both SD-OCT instruments were greater than 0.945 for the total retina and all intra-retinal layers, except the photoreceptor inner segments, which ranged from 0.051 to 0.643, and the outer segments, which ranged from 0.709 to 0.959. The CORs were less than 6.73% for the total retina and all intra-retinal layers. The total retinal thickness measured by the UHR-OCT was significantly thinner than that measured by the RTVue100. However, the ICC for agreement of the thickness profiles between UHR-OCT and RTVue OCT were greater than 0.80 except for the inner segment and outer segment layers.ConclusionsThickness measurements of the intra-retinal layers determined by the automated algorithm are reliable when applied to images acquired by the UHR-OCT and RTVue100 instruments.

Breaking diffraction limit of lateral resolution in optical coherence tomography.

Quantitative imaging of biomedical specimens is essential in biomedical study and diagnosis. Given excellent capability in three-dimensional (3D) imaging, optical coherence tomography (OCT) has been extensively used in ophthalmic imaging, vascular medicine, dermatological study, etc. Lateral resolution of the OCT is light diffraction limited, which precludes the feasibility of quantitative assessment of individual cells. In this paper, we demonstrated the feasibility of breaking diffraction-limit in OCT imaging through virtually structured detection (VSD). OCT examination of optical resolution target verified resolution doubling in the VSD based OCT imaging. Super-resolution OCT identification of individual frog photoreceptors was demonstrated to verify the potential of resolution enhancement in retinal imaging. We anticipate that further development of the VSD based OCT promises an easy, low cost strategy to achieve sub-cellular resolution tomography of the retina and other biological systems.

Utility of optical coherence tomography and intravascular ultrasound for the evaluation of coronary lesions

Optical coherence tomography (OCT) and intravascular ultrasound (IVUS) are imaging methods used in the diagnosis of coronary lesions. IVUS is widely used in interventional cardiology laboratories, but OCT is now increasingly used. Conventional coronary angiography can identify different types of coronary lesions but sometimes is unable to diagnose them correctly. Both intravascular imaging methods are useful for better interpretation of these lesions, and can accurately diagnose ruptured plaques, thrombosis, stent restenosis and hazy images. However, the resolution of OCT is ten times higher than IVUS, and so an accurate diagnosis cannot always be achieved with ultrasound imaging. We present three cases in which IVUS was unable to identify the lesion causing the condition and OCT was required to obtain clearer images that helped to confirm the diagnosis. The advantages and disadvantages of each method are then discussed.

Utility of optical coherence tomography and intravascular ultrasound for the evaluation of coronary lesions

Optical coherence tomography (OCT) and intravascular ultrasound (IVUS) are imaging methods used in the diagnosis of coronary lesions. IVUS is widely used in interventional cardiology laboratories, but OCT is now increasingly used. Conventional coronary angiography can identify different types of coronary lesions but sometimes is unable to diagnose them correctly. Both intravascular imaging methods are useful for better interpretation of these lesions, and can accurately diagnose ruptured plaques, thrombosis, stent restenosis and hazy images. However, the resolution of OCT is ten times higher than IVUS, and so an accurate diagnosis cannot always be achieved with ultrasound imaging. We present three cases in which IVUS was unable to identify the lesion causing the condition and OCT was required to obtain clearer images that helped to confirm the diagnosis. The advantages and disadvantages of each method are then discussed.

High Resolution OCT Quantitative Analysis of the Space Between the IOL and the Posterior Capsule During the Early Cataract Postoperative Period

We quantitatively characterized the space between the IOL and the posterior capsule (IOL-PC space) during the early postphacoemulsification period, using high resolution optical coherence tomography (OCT). We recruited 30 eyes of 30 patients who underwent phacoemulsification and randomly divided them into two groups. Acrysof Natural IQ IOLs were implanted in one group (n = 15), and Adapt-AO IOLs were implanted into the other (n = 15). A custom-built OCT instrument was used to image the IOL-PC space at 1 day, 1 week, and 1 month after surgery. Slit-lamp examination and auto refraction were performed at each visit. The IOL-PC spaces in the IQ group were 0.72 ± 0.35, 0.40 ± 0.24, and 0.23 ± 0.16 mm(2) at 1 day, 1 week, and 1 month after surgery, respectively. At each of these times, the values for the AO group were significantly smaller (P < 0.001). Compared to 1 day after surgery, significant changes in the ACDs and refractive errors occurred up to 1 month postoperatively in the IQ group; however, changes in the ACD and refractive error were significant only at 1 week in the AO group. The decreases in IOL-PC space and in ACD during the early postoperative period were associated with a myopic shift. It appeared that the different IOL designs had a role in closure of the IOL-PC space. High resolution OCT was suitable for quantitative analysis of IOL-PC space. (ClinicalTrials.gov number, NCT01605812.).

In vivo Optical Coherence Tomography of Light-Driven Melanosome Translocation in Retinal Pigment Epithelium

Optical coherence tomography (OCT) may revolutionize fundamental investigation and clinical management of age-related macular degeneration and other eye diseases. However, quantitative OCT interpretation is hampered due to uncertain sub-cellular correlates of reflectivity in the retinal pigment epithelium (RPE) and photoreceptor. The purpose of this study was twofold: 1) to test OCT correlates in the RPE, and 2) to demonstrate the feasibility of longitudinal OCT monitoring of sub-cellular RPE dynamics. A high resolution OCT was constructed to achieve dynamic imaging of frog eyes, in which light-driven translocation of RPE melanosomes occurred within the RPE cell body and apical processes. Comparative histological examination of dark- and light-adapted eyes indicated that the RPE melanin granule, i.e., melanosome, was a primary OCT correlate. In vivo OCT imaging of RPE melanosomes opens the opportunity for quantitative assessment of RPE abnormalities associated with disease, and enables longitudinal investigation of RPE kinetics correlated with visual function.

Multi-penalty conditional random field approach to super-resolved reconstruction of optical coherence tomography images

Improving the spatial resolution of Optical Coherence Tomography (OCT) images is important for the visualization and analysis of small morphological features in biological tissue such as blood vessels, membranes, cellular layers, etc. In this paper, we propose a novel reconstruction approach to obtaining super-resolved OCT tomograms from multiple lower resolution images. The proposed Multi-Penalty Conditional Random Field (MPCRF) method combines four different penalty factors (spatial proximity, first and second order intensity variations, as well as a spline-based smoothness of fit) into the prior model within a Maximum A Posteriori (MAP) estimation framework. Test carried out in retinal OCT images illustrate the effectiveness of the proposed MPCRF reconstruction approach in terms of spatial resolution enhancement, as compared to previously published super resolved image reconstruction methods. Visual assessment of the MPCRF results demonstrate the potential of this method in better preservation of fine details and structures of the imaged sample, as well as retaining the sharpness of biological tissue boundaries while reducing the effects of speckle noise inherent to OCT. Quantitative evaluation using imaging metrics such as Signal-to-Noise Ratio (SNR), Contrast to Noise Ratio (CNR), Equivalent Number of Looks (ENL), and Edge Preservation Parameter show significant visual quality improvement with the MPCRF approach. Therefore, the proposed MPCRF reconstruction approach is an effective tool for enhancing the spatial resolution of OCT images without the necessity for significant imaging hardware modifications.

Characterization of human corneal grafts’ transparency by optical coherence tomography and scattering measurements

AbstractPurpose In clinical and graft sorting applications, cornea’s transparency is only subjectively qualified. The aim of this study is to bring tools to achieve transparency quantification regarding the evolution of edema within the tissue.Methods The samples are human corneal grafts rejected from bank of tissue due to physiologic issues, which are submitted to swelling protocol to study their properties with edema. A multiscale analysis of the microstructure imaged by 1 µm resolved Optical Coherence Tomography (OCT) combined with a detailed characterization of backscattering properties is performed. Electromagnetical modelization is used to numerically link scattering measurements with structural defects observed with OCT.Results Backscattered intensity measurements enable corneal grafts’ transparency evaluation (backscattering level increases with swelling). Moreover, microstructural tissue modifications occurring during swelling (microstructure disorganization and heterogeneities) are highlighted by OCT imagery. We show that the observed heterogeneities imply higher scattering levels and explain the experimental results.Conclusion Combining both techniques allows linking the scattering behavior with the evolution of microstructures within the tissue and permits to quantify corneal grafts transparency. This study has to be extended to tissues eligible for graft but this characterization in the backscattered space could directly be applied to future study of tissues before removal or to in vivo diagnosis.

Focus-extension by depth-encoded synthetic aperture in Optical Coherence Tomography

We present a novel method to extend the depth-of-focus of Optical Coherence Tomography (OCT). OCT is an interferometric imaging technique that provides depth-resolved scattering information. The axial resolution in OCT is provided by the coherence gate and is invariant over the full image depth. The lateral resolution is determined by the beam parameters such as wavelength and numerical aperture. The Rayleigh range determines the depth range over which the lateral resolution can be maintained. The lateral resolution is often sacrificed to maintain relatively long Rayleigh range. In this study, we propose to use a depth-encoded synthetic aperture detection scheme to extend the depth range over which a sharp focus can be maintained beyond the Rayleigh range. An annular phase plate is inserted into the light path in the sample arm, which gives rise to three separate images in a single B-scan, corresponding to three different optical path length encoded apertures. These three images are coherently summed after phase-manipulation to reconstruct a new image with a lateral resolution that is maintained over a five times larger depth range.

Imaging of retinal ganglion cells in glaucoma: pitfalls and challenges

Imaging has gained a key role in modern glaucoma management. Traditionally, interest was directed toward the appearance of the optic nerve head and the retinal nerve fiber layer. With the improvement of the resolution of optical coherence tomography, the ganglion cell complex has also become routinely accessible in the clinic. Further advances have been made in understanding the structure-function relationship in glaucoma. Nevertheless, direct imaging of the retinal ganglion cells in glaucoma would be advantageous. With the currently used techniques, this goal cannot be achieved, because the transversal resolution is limited by aberrations of the eye. The use of adaptive optics has significantly improved transversal resolution, and the imaging of several cell types including cones and astrocytes has become possible. Imaging of retinal ganglion cells, however, still remains a problem, because of the transparency of these cells. However, the visualization of retinal ganglion cells and their dendrites has been achieved in animal models. Furthermore, attempts have been made to visualize the apoptosis of retinal ganglion cells in vivo. Implementation of these techniques in clinical practice will probably improve glaucoma care and facilitate the development of neuroprotective strategies.

Optical Coherence Tomography in the Diagnosis and Monitoring of Retinal Diseases

Optical coherence tomography (OCT) allows the visualization of the retinal microarchitecture as cross-sectional or tomographic volumetric data. The usefulness of OCT in the management of various retinal diseases is validated by the possibility to allow early diagnosis and to help in the decision-making process. OCT is applied by two main methods: time domain (TD-OCT) and spectral domain (SD-OCT). The advantages of SD-OCT over TD-OCT are significant improvement of the image axial resolution, decreased acquisition times, reduction of motion artifacts, increased area of retinal sampling, and the possibility to create topographic maps by the three-dimensional evaluation of tissues. OCT is the most precise method to measure the central macular thickness (which is the most important practical parameter) in vivo. It has been demonstrated that there are differences in the retinal thickness measurements between OCT models, explained by the higher axial and transverse resolutions of the newer devices. Further research has led to significant improvements in OCT technology represented by ultrahigh resolution OCT (UHR-OCT), swept source OCT (SS-OCT), enhanced depth imaging OCT (EDI-OCT), and adaptive optics. Technological progress in OCT imaging offered new perspectives for better understanding the retinal diseases, opening new avenues for the fundamental and clinical research. This is a review of the data in the literature concerning the evolution of OCT technology in the field of retinal imaging.