Analysis of posterior retinal layers in spectral optical coherence tomography images of the normal retina and retinal pathologies

Optical coherence tomography

Optical coherence tomography (OCT) is a non-contact method for imaging the topological and internal microstructure of samples in three dimensions. OCT can be configured as a conventional microscope, as an ophthalmic scanner, or using endoscopes and small diameter catheters for accessing internal biological organs. In this Primer, we describe the principles underpinning the different instrument configurations that are tailored to distinct imaging applications and explain the origin of signal, based on light scattering and propagation. Although OCT has been used for imaging inanimate objects, we focus our discussion on biological and medical imaging. We examine the signal processing methods and algorithms that make OCT exquisitely sensitive to reflections as weak as just a few photons and that reveal functional information in addition to structure. Image processing, display and interpretation, which are all critical for effective biomedical imaging, are discussed in the context of specific applications. Finally, we consider image artifacts and limitations that commonly arise and reflect on future advances and opportunities.

Artificial intelligence in OCT angiography

Optical coherence tomographic angiography (OCTA) is a non-invasive imaging modality that provides three-dimensional, information-rich vascular images. With numerous studies demonstrating unique capabilities in biomarker quantification, diagnosis, and monitoring, OCTA technology has seen rapid adoption in research and clinical settings. The value of OCTA imaging is significantly enhanced by image analysis tools that provide rapid and accurate quantification of vascular features and pathology. Today, the most powerful image analysis methods are based on artificial intelligence (AI). While AI encompasses a large variety of techniques, machine-learning-based, and especially deep-learning-based, image analysis provides accurate measurements in a variety of contexts, including different diseases and regions of the eye. Here, we discuss the principles of both OCTA and AI that make their combination capable of answering new questions. We also review contemporary applications of AI in OCTA, which include accurate detection of pathologies such as choroidal neovascularization, precise quantification of retinal perfusion, and reliable disease diagnosis.

Reliability and Reproducibility of Pigment Epithelial Detachment Volume Measurements in AMD Using a New Tool: ReVAnalyzer

BACKGROUND AND OBJECTIVE

To describe and present the reliability and reproducibility of a new software, Retinal Volume Analyzer (ReVAnalyzer), for pigment epithelium detachment (PED) volume quantification.

PATIENTS AND METHODS

This is a retrospective study including patients with PEDs secondary to exudative age-related macular degeneration (AMD). Macular volume scans on spectral-domain optical coherence tomography on enhanced depth imaging mode were performed in all eyes. Image batches were then exported in .xml format to the ReVAnalyzer software. A semiautomated PED volume measurement was performed by three independent readers (RBG, VC, OS) twice, at the beginning and end of a 15-day period. Bland-Altman assessment for agreement was used to compare intra- and interobserver observations.

RESULTS

Twenty eyes of 20 patients presenting with PED were analyzed. Bland-Altman analysis indicated a good agreement between inter- and intraobserver measurements. The intraclass correlation coefficient for intraobserver PED volume measurements and between the three observers (interobserver) was greater than 0.99, demonstrating high reproducibility and consistency of the methodology.

CONCLUSIONS

ReVAnalyzer is a reliable tool that can assist in the analysis of PED volume with high reproducibility. This type of specific retinal volume analysis can be of help for monitoring disease activity and therapeutic response in AMD. [Ophthalmic Surg Lasers Imaging Retina. 2019;50:e242-e249.].

Fast retinal layer segmentation of spectral domain optical coherence tomography images

An approach to segment macular layer thicknesses from spectral domain optical coherence tomography has been proposed. The main contribution is to decrease computational costs while maintaining high accuracy via exploring Kalman filtering, customized active contour, and curve smoothing. Validation on 21 normal volumes shows that 8 layer boundaries could be segmented within 5.8 s with an average layer boundary error <2.35 μm. It has been compared with state-of-the-art methods for both normal and age-related macular degeneration cases to yield similar or significantly better accuracy and is 37 times faster. The proposed method could be a potential tool to clinically quantify the retinal layer boundaries.

Analysis of fundus shape in highly myopic eyes by using curvature maps constructed from optical coherence tomography

Purpose

To evaluate fundus shape in highly myopic eyes using color maps created through optical coherence tomography (OCT) image analysis.

Methods

We retrospectively evaluated 182 highly myopic eyes from 113 patients. After obtaining 12 lines of 9-mm radial OCT scans with the fovea at the center, the Bruch’s membrane line was plotted and its curvature was measured at 1-µm intervals in each image, which was reflected as a color topography map. For the quantitative analysis of the eye shape, mean absolute curvature and variance of curvature were calculated.

Results

The color maps allowed staphyloma visualization as a ring of green color at the edge and as that of orange-red color at the bottom. Analyses of mean and variance of curvature revealed that eyes with myopic choroidal neovascularization tended to have relatively flat posterior poles with smooth surfaces, while eyes with chorioretinal atrophy exhibited a steep, curved shape with an undulated surface (P<0.001). Furthermore, eyes with staphylomas and those without clearly differed in terms of mean curvature and the variance of curvature: 98.4% of eyes with staphylomas had mean curvature ≥7.8×10⁻⁵ [1/µm] and variance of curvature ≥0.26×10⁻⁸ [1/µm].

Conclusions

We established a novel method to analyze posterior pole shape by using OCT images to construct curvature maps. Our quantitative analysis revealed that fundus shape is associated with myopic complications. These values were also effective in distinguishing eyes with staphylomas from those without. This tool for the quantitative evaluation of eye shape should facilitate future research of myopic complications.

Automated segmentation algorithm for detection of changes in vaginal epithelial morphology using optical coherence tomography

We have explored the use of optical coherence tomography (OCT) as a noninvasive tool for assessing the toxicity of topical microbicides, products used to prevent HIV, by monitoring the integrity of the vaginal epithelium. A novel feature-based segmentation algorithm using a nearest-neighbor classifier was developed to monitor changes in the morphology of vaginal epithelium. The two-step automated algorithm yielded OCT images with a clearly defined epithelial layer, enabling differentiation of normal and damaged tissue. The algorithm was robust in that it was able to discriminate the epithelial layer from underlying stroma as well as residual microbicide product on the surface. This segmentation technique for OCT images has the potential to be readily adaptable to the clinical setting for noninvasively defining the boundaries of the epithelium, enabling quantifiable assessment of microbicide-induced damage in vaginal tissue.

"En face" OCT imaging of the IS/OS junction line in type 2 idiopathic macular telangiectasia

PURPOSE

We investigated abnormalities of the photoreceptor inner/outer segment (IS/OS) junction layer viewed "en face" and their functional correlates in type 2 idiopathic macular telangiectasia (type 2 MacTel).

METHODS

Segmentation and "en face" imaging of the IS/OS lines in spectral domain optical coherence tomographic (SD-OCT) volumes were performed manually. Mesopic retinal sensitivity thresholds were determined using a Nidek MP1 microperimeter. "En face" SD-OCT images and microperimetric data were superimposed over images of the fundus. Retinal structure and characteristics of type 2 MacTel were analyzed, and associations of structural changes with function were investigated.

RESULTS

We examined 49 eyes of 28 patients (mean age 62.6 ± 9.4 years). Total IS/OS break area ranged from 0.04 to 2.23 mm² (mean 0.63 mm², SD 0.53 mm²) and 0.03 to 1.49 mm² (mean 0.49 mm², SD 0.42 mm²) in right and left eyes, respectively. A correlation between fellow eyes was present (Spearman correlation ρ = 0.770, P < 0.01). An assessment of the repeatability of IS/OS lesion area measurements (n = 19 eyes) revealed an intra-class correlation coefficient of 0.99 (95% confidence interval [CI] of 0.975-0.996). Retinal areas corresponding to an IS/OS break showed a mean retinal sensitivity of 8.3 ± 5.8 and 8.7 ± 5.7 decibels (dB) in right and left eyes, respectively. Mean sensitivity over retinal areas outside the lesion was significantly higher, 17.0 ± 3.3 and 16.7 ± 3.6 dB in right and left eyes, respectively (paired t-test, P < 0.01). Mean aggregate retinal sensitivity loss was 33.5 ± 30.4 dB (n = 40), correlating well with IS/OS lesion area (Pearson correlation coefficient = 0.848, P < 0.01).

CONCLUSIONS

"En face" OCT imaging of the IS/OS junction layer provides a functionally relevant method for assessing disease severity in type 2 MacTel.

Association of outer retinal layer morphology with visual acuity in patients with retinal vein occlusion: SCORE Study Report 13

PURPOSE

To assess associations between visual acuity (VA) and the status of the photoreceptor inner segment-outer segment (IS-OS) junction in a subset of patients in the Standard Care vs COrticosteroid for REtinal Vein Occlusion (SCORE) Study.

METHODOLOGY

High-resolution time domain optical coherence tomography (OCT) scans of study eyes from a single site participating in the SCORE Study were evaluated. Integrity of the IS-OS junction in the central subfield was evaluated using a three-step scale: absent, abnormal or normal. Associations of the IS-OS status with ETDRS VA letter score and center point thickness (CPT) were investigated.

RESULTS

Baseline OCTs of 42 eyes were evaluated. The IS-OS junction was absent in 30 (71%) and abnormal in 12 (29%). At month 12, the IS-OS junction was absent in 18 (43%), abnormal in 12 (28%), and normal in 12 (28%) eyes. At baseline, IS-OS status was significantly associated with CPT, but not with VA. At month 12, IS-OS status was significantly associated with CPT and VA, that is, absent or abnormal IS-OS was associated with increased CPT and worse VA. Change in IS-OS status was not associated with change in CPT (P=0.8). Worsening of IS-OS status was associated with loss of VA and improvement in IS-OS status to normal was associated with gain in VA (P=0.03).

CONCLUSION

In this data set with long-term follow-up of OCTs as part of the SCORE Study, there is a correlation between change in IS-OS status and VA. This supports further evaluation of outer retinal morphology in larger data sets.

Quantitative imaging of retinal pigment epithelial detachments using spectral-domain optical coherence tomography

PURPOSE

To evaluate the reproducibility of area and volume measurements of retinal pigment epithelium detachments (PEDs) in eyes of patients with age-related macular degeneration using spectral-domain optical coherence tomography imaging and a novel automated, quantitative algorithm.

DESIGN

Prospective study to evaluate a diagnostic technology.

METHODS

Patients with PEDs associated with age-related macular degeneration underwent spectral-domain optical coherence tomography imaging. Each eye was imaged 5 times, and each scan consisted of a raster pattern comprising 40 000 uniformly spaced A-scans organized as a 200 × 200 A-scan array. Each raster scan covered a retinal area of 6 × 6 mm encompassing the entire PED. A novel algorithm was used to create PED maps that permitted both qualitative and quantitative assessment of PED area and volume. Test-retest standard deviations of PED area and volume measurements were calculated for each eye.

RESULTS

Sixty-three eyes of 58 patients were enrolled in this study. The qualitative appearance and the quantitative measurements of PED area and volume were highly reproducible over the 5 different datasets obtained from each eye. The intraclass correlation coefficient was more than 0.99 for both area and volume measurements obtained using the entire dataset.

CONCLUSIONS

A novel algorithm for the qualitative and quantitative assessment of PEDs imaged using spectral-domain optical coherence tomography was shown to be highly reproducible. The ability to measure PED area and volume reliably represents a novel strategy for following disease progression, especially when assessing the response of vascularized PEDs to antiangiogenic therapy.

Efficient reduction of speckle noise in Optical Coherence Tomography

Speckle pattern, which is inherent in coherence imaging, influences significantly axial and transversal resolution of Optical Coherence Tomography (OCT) instruments. The well known speckle removal techniques are either sensitive to sample motion, require sophisticated and expensive sample tracking systems, or involve sophisticated numerical procedures. As a result, their applicability to in vivo real-time imaging is limited. In this work, we propose to average multiple A-scans collected in a fully controlled way to reduce the speckle contrast. This procedure involves non-coherent averaging of OCT A-scans acquired from adjacent locations on the sample. The technique exploits scanning protocol with fast beam deflection in the direction perpendicular to lateral dimension of the cross-sectional image. Such scanning protocol reduces the time interval between A-scans to be averaged to the repetition time of the acquisition system. Consequently, the averaging algorithm is immune to bulk motion of an investigated sample, does not require any sophisticated data processing to align cross-sectional images, and allows for precise control of lateral shift of the scanning beam on the object. The technique is tested with standard Spectral OCT system with an extra resonant scanner used for rapid beam deflection in the lateral direction. Ultrahigh speed CMOS camera serves as a detector and acquires 200,000 spectra per second. A dedicated A-scan generation algorithm allows for real-time display of images with reduced speckle contrast at 6 frames/second. This technique is applied to in vivo imaging of anterior and posterior segments of the human eye and human skin.

Spectral domain optical coherence tomography imaging of drusen in nonexudative age-related macular degeneration

PURPOSE

To measure drusen area and volume in eyes with nonexudative age-related macular degeneration (AMD) using spectral domain optical coherence tomography imaging (SD-OCT).

DESIGN

Evaluation of diagnostic technology.

PARTICIPANTS

One hundred three eyes from 74 patients with drusen.

METHODS

Patients with drusen secondary to nonexudative AMD were enrolled in this study. Five separate SD-OCT scans, each consisting of 40 000 uniformly spaced A-scans organized as 200 A-scans in each B-scan and 200 horizontal B-scans, were performed on each eye. Each scan covered a retinal area of 6×6 mm centered on the fovea. A novel algorithm was used to quantitatively assess drusen area and volume. Measurements from the entire scans, as well as in regions contained within 3- and 5-mm circles centered on the fovea, were analyzed. Test-retest standard deviations of drusen area and volume measurements were calculated for each eye.

MAIN OUTCOME MEASURES

Drusen area and volume.

RESULTS

The algorithm created drusen maps that permitted both qualitative and quantitative assessment of drusen area and volume. Both the qualitative appearance and the quantitative measurements of drusen area and volume were highly reproducible over the 5 different datasets. The intraclass correlation coefficient was >0.99 for both area and volume measurements on the entire dataset as well as the 3- and 5-mm circles. The correlation between lesion size and the test-retest standard deviations can be eliminated by performing a square root transformation of the area measurements and a cube root transformation of the volume measurements. These transformed data allowed for the inclusion of all drusen sizes in the calculation of an estimated single pooled test-retest standard deviation, which will be useful for longitudinal studies of drusen natural history.

CONCLUSIONS

A novel algorithm for the qualitative and quantitative assessment of drusen imaged using SD-OCT was shown to be highly reproducible. The ability to assess drusen volume reliably represents a new quantitative parameter to measure in AMD and may be useful when assessing disease progression, particularly in trials for treatments of nonexudative AMD.

Retinal Nerve Fiber Layer Segmentation on FD-OCT Scans of Normal Subjects and Glaucoma Patients

Automated measurements of the retinal nerve fiber layer thickness on circular OCT B-Scans provide physicians additional parameters for glaucoma diagnosis. We propose a novel retinal nerve fiber layer segmentation algorithm for frequency domain data that can be applied on scans from both normal healthy subjects, as well as glaucoma patients, using the same set of parameters. In addition, the algorithm remains almost unaffected by image quality. The main part of the segmentation process is based on the minimization of an energy function consisting of gradient and local smoothing terms. A quantitative evaluation comparing the automated segmentation results to manually corrected segmentations from three reviewers is performed. A total of 72 scans from glaucoma patients and 132 scans from normal subjects, all from different persons, composed the database for the evaluation of the segmentation algorithm. A mean absolute error per A-Scan of 2.9 µm was achieved on glaucomatous eyes, and 3.6 µm on healthy eyes. The mean absolute segmentation error over all A-Scans lies below 10 µm on 95.1% of the images. Thus our approach provides a reliable tool for extracting diagnostic relevant parameters from OCT B-Scans for glaucoma diagnosis.

Segmentation and quantification of retinal lesions in age-related macular degeneration using polarization-sensitive optical coherence tomography

We present polarization-sensitive optical coherence tomography (PS-OCT) for quantitative assessment of retinal pathologies in age-related macular degeneration (AMD). On the basis of the polarization scrambling characteristics of the retinal pigment epithelium, novel segmentation algorithms were developed that allow one to segment pathologic features such as drusen and atrophic zones in dry AMD as well as to determine their dimensions. Results from measurements in the eyes of AMD patients prove the ability of PS-OCT for quantitative imaging based on the retinal features polarizing properties. Repeatability measurements were performed in retinas diagnosed with drusen and geographic atrophy in order to evaluate the performance of the described methods. PS-OCT appears as a promising imaging modality for three-dimensional retinal imaging and ranging with additional contrast based on the structures' tissue-inherent polarization properties.

Longitudinal study of retinal degeneration in a rat using spectral domain optical coherence tomography

Rodent models of retinal degenerative diseases are used by vision scientists to develop therapies and to understand mechanisms of disease progression. Measurement of changes to the thickness of the various retinal layers provides an objective metric to evaluate the performance of the therapy. Because invasive histology is terminal and provides only a single data point, non-invasive imaging modalities are required to better study progression, and to reduce the number of animals used in research. Optical Coherence Tomography (OCT) has emerged as a dominant imaging modality for human ophthalmic imaging, but has only recently gained significant attention for rodent retinal imaging. OCT provides cross section images of retina with micron-scale resolution which permits measurement of the retinal layer thickness. However, in order to be useful to vision scientists, a significant fraction of the retinal surface needs to be measured. In addition, because the retinal thickness normally varies as a function of distance from optic nerve head, it is critical to sample all regions of the retina in a systematic fashion. We present a longitudinal study of OCT to measure retinal degeneration in rats which have undergone optic nerve axotomy, a well characterized form of rapid retinal degeneration. Volumetric images of the retina acquired with OCT in a time course study were segmented in 2D using a semi-automatic segmentation algorithm. Then, using a 3D algorithm, thickness measurements were quantified across the surface of the retina for all volume segmentations. The resulting maps of the changes to retinal thickness over time represent the progression of degeneration across the surface of the retina during injury. The computational tools complement OCT retinal volumetric acquisition, resulting in a powerful tool for vision scientists working with rodents.

Improving image segmentation performance and quantitative analysis via a computer-aided grading methodology for optical coherence tomography retinal image analysis

We demonstrate quantitative analysis and error correction of optical coherence tomography (OCT) retinal images by using a custom-built, computer-aided grading methodology. A total of 60 Stratus OCT (Carl Zeiss Meditec, Dublin, California) B-scans collected from ten normal healthy eyes are analyzed by two independent graders. The average retinal thickness per macular region is compared with the automated Stratus OCT results. Intergrader and intragrader reproducibility is calculated by Bland-Altman plots of the mean difference between both gradings and by Pearson correlation coefficients. In addition, the correlation between Stratus OCT and our methodology-derived thickness is also presented. The mean thickness difference between Stratus OCT and our methodology is 6.53 microm and 26.71 microm when using the inner segment/outer segment (IS/OS) junction and outer segment/retinal pigment epithelium (OS/RPE) junction as the outer retinal border, respectively. Overall, the median of the thickness differences as a percentage of the mean thickness is less than 1% and 2% for the intragrader and intergrader reproducibility test, respectively. The measurement accuracy range of the OCT retinal image analysis (OCTRIMA) algorithm is between 0.27 and 1.47 microm and 0.6 and 1.76 microm for the intragrader and intergrader reproducibility tests, respectively. Pearson correlation coefficients demonstrate R(2)>0.98 for all Early Treatment Diabetic Retinopathy Study (ETDRS) regions. Our methodology facilitates a more robust and localized quantification of the retinal structure in normal healthy controls and patients with clinically significant intraretinal features.

Combined image-processing algorithms for improved optical coherence tomography of prostate nerves

Cavernous nerves course along the surface of the prostate gland and are responsible for erectile function. These nerves are at risk of injury during surgical removal of a cancerous prostate gland. In this work, a combination of segmentation, denoising, and edge detection algorithms are applied to time-domain optical coherence tomography (OCT) images of rat prostate to improve identification of cavernous nerves. First, OCT images of the prostate are segmented to differentiate the cavernous nerves from the prostate gland. Then, a locally adaptive denoising algorithm using a dual-tree complex wavelet transform is applied to reduce speckle noise. Finally, edge detection is used to provide deeper imaging of the prostate gland. Combined application of these three algorithms results in improved signal-to-noise ratio, imaging depth, and automatic identification of the cavernous nerves, which may be of direct benefit for use in laparoscopic and robotic nerve-sparing prostate cancer surgery.

Scanning protocols dedicated to smart velocity ranging in spectral OCT

We introduce a new type of scanning protocols, called segmented protocols, which enable extracting multi-range flow velocity information from a single Spectral OCT data set. The protocols are evaluated using a well defined flow in a glass capillary. As an example of in vivo studies, we demonstrate two- and three-dimensional imaging of the retinal vascular system in the eyes of healthy volunteers. The flow velocity detection is performed using a method of Joint Spectral and Time domain OCT. Velocity ranging is demonstrated in imaging of retinal vasculature in the macular region and in the optic disk area characterized by different flow velocity values. Additionally, an enhanced visualization of retinal capillary network is presented in the close proximity to macula.

Intra-retinal layer segmentation in optical coherence tomography images

Retinal layer thickness, evaluated as a function of spatial position from optical coherence tomography (OCT) images is an important diagnostics marker for many retinal diseases. However, due to factors such as speckle noise, low image contrast, irregularly shaped morphological features such as retinal detachments, macular holes, and drusen, accurate segmentation of individual retinal layers is difficult. To address this issue, a computer method for retinal layer segmentation from OCT images is presented. An efficient two-step kernel-based optimization scheme is employed to first identify the approximate locations of the individual layers, which are then refined to obtain accurate segmentation results for the individual layers. The performance of the algorithm was tested on a set of retinal images acquired in-vivo from healthy and diseased rodent models with a high speed, high resolution OCT system. Experimental results show that the proposed approach provides accurate segmentation for OCT images affected by speckle noise, even in sub-optimal conditions of low image contrast and presence of irregularly shaped structural features in the OCT images.

Tissue discrimination in anterior eye using three optical parameters obtained by polarization sensitive optical coherence tomography

We developed a tissue discrimination algorithm of polarization sensitive optical coherence tomography (PS-OCT) based on the optical properties of tissues. We calculated the three-dimensional (3D) feature vector from the parameters intensity, extinction coefficient, birefringence, which were obtained by PS-OCT. The tissue type of each pixel was determined according to the position of the feature vector in the 3D feature space. The algorithm was applied for discriminating tissues of the human anterior eye segment. The conjunctiva, sclera, trabecular meshwork (TM), cornea, and uvea were well separated in the 3D feature space, and we observed them with good contrast. The TM line can be observed in the 3D discriminated volume, as observed by gonioscopy.We validated our method by applying our algorithm and histological data to porcine eyes. A marker was injected into sub-Tenon's space and the tissues that were anterior to the marker and posterior to the marker were successfully segmented by our algorithm.

Comparison of reflectivity maps and outer retinal topography in retinal disease by 3-D Fourier domain optical coherence tomography

We demonstrate and compare two image processing methods for visualization and analysis of three-dimensional optical coherence tomography (OCT) data acquired in eyes with different retinal pathologies. A method of retinal layer segmentation based on a multiple intensity thresholding algorithm was implemented in order to generate simultaneously outer retinal topography maps and reflectivity maps. We compare the applicability of the two methods to the diagnosis of retinal diseases and their progression. The data presented in this contribution were acquired with a high speed (25,000 A-scans/s), high resolution (4.5 microm) spectral OCT prototype instrument operating in the ophthalmology clinic.

Projection OCT fundus imaging for visualising outer retinal pathology in non-exudative age-related macular degeneration

AIMS

To demonstrate ultrahigh-resolution, three-dimensional optical coherence tomography (3D-OCT) and projection OCT fundus imaging for enhanced visualisation of outer retinal pathology in non-exudative age-related macular degeneration (AMD).

METHODS

A high-speed, 3.5 mum resolution OCT prototype instrument was developed for the ophthalmic clinic. Eighty-three patients with non-exudative AMD were imaged. Projection OCT fundus images were generated from 3D-OCT data by selectively summing different retinal depth levels. Results were compared with standard ophthalmic examination, including fundus photography and fluorescein angiography, when indicated.

RESULTS

Projection OCT fundus imaging enhanced the visualisation of outer retinal pathology in non-exudative AMD. Different types of drusen exhibited distinct features in projection OCT images. Photoreceptor disruption was indicated by loss of the photoreceptor inner/outer segment (IS/OS) boundary and external limiting membrane (ELM). RPE atrophy can be assessed using choroid-level projection OCT images.

CONCLUSIONS

Projection OCT fundus imaging facilities rapid interpretation of large 3D-OCT data sets. Projection OCT enhances contrast and visualises outer retinal pathology not visible with standard fundus imaging or OCT fundus imaging. Projection OCT fundus images enable registration with standard ophthalmic diagnostics and cross-sectional OCT images. Outer retinal alterations can be assessed and drusen morphology, photoreceptor impairment and pigmentary abnormalities identified.