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In April 2019, the US Food and Drug Administration, in conjunction with 11 professional ophthalmic, vision science, and optometric societies, convened a forum on laser-based imaging. The forum brought together the Food and Drug Administration, clinicians, researchers, industry members, and other stakeholders to stimulate innovation and ensure that patients in the US are the first in the world to have access to high-quality, safe, and effective medical devices. This conference focused on the technology, clinical applications, regulatory issues, and reimbursement issues surrounding innovative ocular imaging modalities. Furthermore, the emerging role of artificial intelligence in ophthalmic imaging was reviewed. This article summarizes the presentations, discussion, and future directions.

Purpose: The purpose of the present study was to quantify test-retest reproducibility of measurements of stiffness of the human trabecular meshwork (HTM) by atomic force microscopy (AFM). Methods: Eleven 40 μm radial limbal cryostat sections from a fresh human donor rim were mounted on charged slides and rehydrated at room temperature. Stiffness at four TM locations (anterior to posterior along Schlemm’s canal) was measured by AFM. At each location, a 6 x 6 grid was sampled. Indentation points were evenly distributed over a 20 μm x 20 μm area, with a rate of one load/unload cycle per second. Measurements were then repeated for calculation of test-retest variability. Results: The test-retest coefficients of variation for the four measurement locations (anterior to posterior) were 24.39, 25.28, 12.74, and 14.26%, respectively, with a notable drop in the two posterior locations compared to the anterior. The test-retest coefficient for the sections was 19.17%. For the entire eye, the test-retest coefficient of variation for the measurement of the TM stiffness was 17.13%. Young’s moduli consistently decreased from anterior to posterior location. Conclusions: Wide regional variation suggests that single value does little to fully describe the complex array of TM stiffness levels within the eye, and future studies of TM stiffness assessed by AFM should include multiple tissue samples from each eye, with documentation of the anterior-posterior location of each measurement.

The lamina cribrosa is a primary site of damage in glaucoma. While mechanical distortion is hypothesized to cause reduction of axoplasmic flow, little is known about how the pores, which contains the retinal ganglion cell axons, traverse the lamina cribrosa. We investigated lamina cribrosa pore paths in vivo to quantify differences in tortuosity of pore paths between healthy and glaucomatous eyes. We imaged 16 healthy, 23 glaucoma suspect and 48 glaucomatous eyes from 70 subjects using a swept source optical coherence tomography system. The lamina cribrosa pores were automatically segmented using a previously described segmentation algorithm. Individual pore paths were automatically tracked through the depth of the lamina cribrosa using custom software. Pore path convergence to the optic nerve center and tortuosity was quantified for each eye. We found that lamina cribrosa pore pathways traverse the lamina cribrosa closer to the optic nerve center along the depth of the lamina cribrosa regardless of disease severity or diagnostic category. In addition, pores of glaucoma eyes take a more tortuous path through the lamina cribrosa compared to those of healthy eyes, suggesting a potential mechanism for reduction of axoplasmic flow in glaucoma.

Glaucoma is a leading cause of blindness that leads to characteristic changes in the optic nerve head (ONH) region, such as nasalization of vessels. It is unknown whether the spatial location of this vessel shift inside the ONH occurs within the lamina cribrosa (LC) or the prelaminar tissue. The purpose of this study was to compare the location of the central retinal vessel trunk (CRVT) in the LC and prelaminar tissue in living healthy and glaucomatous eyes. We acquired 3-dimensional ONH scans from 119 eyes (40 healthy, 29 glaucoma suspect, and 50 glaucoma) using optical coherence tomography (OCT). The CRVT location was manually delineated in separate projection images of the LC and prelamina. We found that the CRVT in glaucoma suspect and glaucomatous eyes was located significantly more nasally compared to healthy eyes at the level of the prelamina. There was no detectable difference found in the location of the CRVT at the level of the LC between diagnostic groups. While the nasal location of the CRVT in the prelamina has been associated with glaucomatous axonal death, our results suggest that the CRVT in the LC is anchored in the tissue with minimal variation in glaucomatous eyes.

Aim/Purpose: Previous studies have shown that the trabecular meshwork (TM) is mechanically stiffer in glaucomatous eyes as compared to normal eyes. It is believed that elevated TM stiffness increases resistance to the aqueous humor outflow, producing increased intraocular pressure (IOP).It would be advantageous to measure TM mechanical properties in vivo, as these properties are believed to play an important role in the pathophysiology of glaucoma and could be useful for identifying potential risk factors. The purpose of this study was to develop a method to estimate in-vivo TM mechanical properties using clinically available exams and computer simulations.Design: Inverse finite element simulationMethods: A finite element model of the TM was constructed from optical coherence tomography (OCT) images of a healthy volunteer before and during IOP elevation. An axisymmetric model of the TM was then constructed. Images of the TM at a baseline IOP level of 11, and elevated level of 23 mmHg were treated as the undeformed and deformed configurations, respectively. An inverse modeling technique was subsequently used to estimate the TM shear modulus (G). An optimization technique was used to find the shear modulus that minimized the difference between Schlemm’s canal area in the in-vivo images and simulations.Results: Upon completion of inverse finite element modeling, the simulated area of the Schlemm’s canal changed from 8,889 μm2 to 2,088 μm2, similar to the experimentally measured areal change of the canal (from 8,889 μm2 to 2,100 μm2). The calculated value of shear modulus was found to be 1.93 kPa, (implying an approximate Young’s modulus of 5.75 kPa), which is consistent with previous ex-vivo measurements.Conclusion: The combined imaging and computational simulation technique provides a unique approach to calculate the mechanical properties of the TM in vivo without any surgical intervention. Quantification of such mechanical properties will help us examine the mechanistic role of TM biomechanics in the regulation of IOP in healthy and glaucomatous eyes.

Purpose of review Approximately 10% of patients become blind despite using evidence-based guidelines developed from clinical trials and epidemiology studies. Our purpose is to review opportunities to decrease glaucoma-related blindness using the emerging principles of precision medicine. Recent findings The current review focuses on three topics: first, candidate biomarkers for angle-based surgeries, second, head-mounted display (HMD) technology for vision and testing, and third, glaucoma risk alleles discovered by genome-wide association studies. First, in angle-based surgeries, tracers injected into the anterior chamber or Schlemm's canal have allowed visualization of aqueous veins. We describe an innovative use of optical coherence tomography angiography to visualize aqueous veins in a case with 6-year successful outcome following catheter-based trabeculotomy. Second, HMD technology can augment perceived vision and can be used for perimetry testing. Third, developing genetic risk scores that characterize patients who are at highest risk for blindness is a priority. Such biomarker risk scores will integrate genome-wide association study-based risk alleles for glaucoma along with well known demographic and clinical risk factors. Summary As we gain more knowledge, precision medicine will enable clinicians to decrease glaucoma-related blindness by providing more timely interventions to those patients who are at highest risk for progression to blindness. VIDEO ABSTRACT: http://links.lww.com/COOP/A29.

The aqueous humor outflow (AHO) tract is the pathway by which fluid travels from the eye’s anterior chamber to drain into the venous circulation. This tract plays an important role in the regulation of eye pressure and is likely key in the pathogenesis of glaucoma. As such, the outflow tract is a target for alteration by medical and surgical treatment approaches. To understand this pathway’s anatomy and physiology, numerous structural and functional studies have been conducted. While the ability to conduct such studies has advanced along with modern imaging approaches, many challenges to accurately assessing its structure and function remain.

Medical imaging has revolutionized the diagnosis and management of disease in healthcare. Early integration of computers into medical imaging brought control of devices and data acquisition to new heights of precision. As computing power and sophistication of software evolve, we have reached an era of computer-based image interpretation. Machine learning approaches have been developed to automate certain quantitative measures derived from these modalities. Prospects for machine learning in ophthalmology address its potential role in approaching some of the most common causes of blindness worldwide: diabetic retinopathy, glaucoma, and age-related macular degeneration. As these analysis techniques evolve, concurrent advancements are seen in ophthalmology imaging technologies themselves, and today, the aqueous outflow tract and the optic nerve head can be visualized in more detail than ever before. The optimization and assimilation of these tools may hold clinically significant answers to screening, diagnosing, and managing disease.

Whereas it is known that elevated intraocular pressure (IOP) increases the risk of glaucoma, it is not known why optic nerve heads (ONHs) vary so much in sensitivity to IOP and how this sensitivity depends on the characteristics of the ONH such as tissue mechanical properties and geometry. It is often assumed that ONHs with uncommon or atypical sensitivity to IOP, high sensitivity in normal tension glaucoma or high robustness in ocular hypertension, also have atypical ONH characteristics. Here we address two specific questions quantitatively: Do atypical ONH characteristics necessarily lead to atypical biomechanical responses to elevated IOP? And, do typical biomechanical responses necessarily come from ONHs with typical characteristics. We generated 100,000 ONH numerical models with randomly selected values for the characteristics, all falling within literature ranges of normal ONHs. The models were solved to predict their biomechanical response to an increase in IOP. We classified ONH characteristics and biomechanical responses into typical or atypical using a percentile-based threshold, and calculated the fraction of ONHs for which the answers to the two questions were true and/or false. We then studied the effects of varying the percentile threshold. We found that when we classified the extreme 5% of individual ONH characteristics or responses as atypical, only 28% of ONHs with an atypical characteristic had an atypical response. Further, almost 29% of typical responses came from ONHs with at least one atypical characteristic. Thus, the answer to both questions is no. This answer held irrespective of the threshold for classifying typical or atypical. Our results challenge the assumption that ONHs with atypical sensitivity to IOP must have atypical characteristics. This finding suggests that the traditional approach of identifying risk factors by comparing characteristics between patient groups (e.g. ocular hypertensive vs. primary open angle glaucoma) may not be a sound strategy.

To compare the rate of glaucoma structural and functional progression in American and Korean cohorts.Retrospective longitudinal study.Three hundred thirteen eyes from 189 glaucoma and glaucoma suspects, followed up for an average of 38 months.All subjects were examined semiannually with visual field (VF) testing and spectral-domain optical coherence tomography. All subjects had 5 or more reliable visits.The rates of change of retinal nerve fiber layer (RNFL) thickness, cup-to-disc (C/D) ratios, and VF mean deviation (MD) were compared between the cohorts. Variables affecting the rate of change for each parameter were determined, including ethnicity, refraction, baseline age and disease severity, disease subtype (high- vs. normal-tension glaucoma), clinical diagnosis (glaucoma vs. glaucoma suspect), and the interactions between variables.The Korean cohort predominantly demonstrated normal-tension glaucoma, whereas the American cohort predominantly demonstrated high-tension glaucoma. Cohorts had similar VF parameters at baseline, but the Korean eyes had significantly thicker mean RNFL and larger cups. Korean glaucoma eyes showed a faster thinning of mean RNFL (mean, -0.71 μm/year vs. -0.24 μm/year; P0.01). There were no detectable differences in the rate of change between the glaucoma cohorts for C/D ratios and VF MD and for all parameters in glaucoma suspect eyes. Different combinations of the tested variables significantly impacted the rate of change.Ethnicity, baseline disease severity, disease subtype, and clinical diagnosis should be considered when comparing glaucoma progression studies.

To predict the development of glaucomatous visual field (VF) defects using Fourier-domain optical coherence tomography (FD OCT) measurements at baseline visit.Multicenter longitudinal observational study. Glaucoma suspects and preperimetric glaucoma participants in the Advanced Imaging for Glaucoma Study.The optic disc, peripapillary retinal nerve fiber layer (NFL), and macular ganglion cell complex (GCC) were imaged with FD OCT. VF was assessed every 6 months. Conversion to perimetric glaucoma was defined by VF pattern standard deviation (PSD) or glaucoma hemifield test (GHT) outside normal limits on 3 consecutive tests. Hazard ratios were calculated with the Cox proportional hazard model. Predictive accuracy was measured by the area under the receiver operating characteristic curve (AUC).Of 513 eyes (309 participants), 55 eyes (46 participants) experienced VF conversion during 41 ± 23 months of follow-up. Significant (P.05, Cox regression) FD OCT risk factors included all GCC, NFL, and disc variables, except for horizontal cup-to-disc ratio. GCC focal loss volume (FLV) was the best single predictor of conversion (AUC = 0.753, P.001 for test against AUC = 0.5). Those with borderline or abnormal GCC-FLV had a 4-fold increase in conversion risk after 6 years (Kaplan-Meier). Optimal prediction of conversion was obtained using the glaucoma composite conversion index (GCCI) based on a multivariate Cox regression model that included GCC-FLV, inferior NFL quadrant thickness, age, and VF PSD. GCCI significantly improved predictive accuracy (AUC = 0.783) over any single variable (P = .04).Reductions in NFL and GCC thickness can predict the development of glaucomatous VF loss in glaucoma suspects and preperimetric glaucoma patients.

Canine glaucoma is a group of disorders that are generally associated with increased intraocular pressure (IOP) resulting in a characteristic optic neuropathy. Glaucoma is a leading cause of irreversible vision loss in dogs and may be either primary or secondary. Despite the growing spectrum of medical and surgical therapies, there is no cure, and many affected dogs go blind. Often eyes are enucleated because of painfully high, uncontrollable IOP. While progressive vision loss due to primary glaucoma is considered preventable in some humans, this is mostly not true for dogs. There is an urgent need for more effective, affordable treatment options. Because newly developed glaucoma medications are emerging at a very slow rate and may not be effective in dogs, work toward improving surgical options may be the most rewarding approach in the near term. This Viewpoint Article summarizes the discussions and recommended research strategies of both a Think Tank and a Consortium focused on the development of more effective therapies for canine glaucoma; both were organized and funded by the American College of Veterinary Ophthalmologists Vision for Animals Foundation (ACVO‐VAF). The recommendations consist of (a) better understanding of disease mechanisms, (b) early glaucoma diagnosis and disease staging, (c) optimization of IOP‐lowering medical treatment, (d) new surgical therapies to control IOP, and (e) novel treatment strategies, such as gene and stem cell therapies, neuroprotection, and neuroregeneration. In order to address these needs, increases in research funding specifically focused on canine glaucoma are necessary.

High-resolution adaptive optics (AO) imaging of retinal neurons in living eyes holds promise for improved diagnosis and better assessment of treatment outcomes for retinal diseases. By integrating different imaging modalities, such as scanning laser ophthalmoscopy (SLO) and optical coherence tomography (OCT), AO has enabled microscopic views of different retinal neurons, including recently reported retinal ganglion cells. In this study, we present a novel design of a multimodal adaptive optics imaging system to investigate the microscopic structure of living human retina. The optical system was designed using Zemax ray tracing software. The system performance was evaluated in terms of image quality and beam displacement. Optical performance was predicted to achieve diffraction limited image quality at the retinal plane and beam displacement was predicted to be >7× smaller than the pitch of Shack-Hartmann lenslet at the pupil planes for scan angles over 3.6°×3.6° field of view. The initial human subject images are presented. High quality photoreceptor images were acquired in both AO-SLO channel and AO-OCT channel simultaneously at 3° temporal from the fovea. Individual cones are delineated in AO-SLO image, the corresponding AO-OCT image showed four main reflections from outer retina, namely external limiting membrane, cone inner segment/outer segment junction, cone outer segment tip, and retinal pigment epithelium. The system allows flexibly alternating between AO-SLO and AO-OCT modes, which provides complementary views of retinal cells, and the potential to improve disease diagnosis and treatment.

There is extensive knowledge on the relationship of posterior scleral biomechanics and intraocular pressure (IOP) load on glaucomatous optic neuropathy; however, the role for biomechanical influence of the perilimbal scleral tissue on the aqueous humor drainage pathway, including the distal venous outflow system, and IOP regulation is not fully understood. The purpose of this work is to study the outflow characteristics of perfused porcine eyes relative to the biomechanical properties of the perilimbal sclera, the posterior sclera and the cornea. Enucleated porcine eyes from eleven different animals were perfused with surrogate aqueous at two fixed flow rates while monitoring their IOP. After perfusion, mechanical stress-strain and relaxation tests were conducted on specimens of perilimbal sclera, posterior sclera, and cornea from the same perfused eyes. Statistical analysis of the data demonstrated a strong correlation between increased tangent modulus of the perilimbal sclera tissues and increased perfusion IOP (R2 = 0.74, p = 0.0006 at lower flow rate and R2 = 0.71, p = 0.0011 at higher flow rate). In contrast, there were no significant correlations between IOP and the tangent modulus of the other tissues (Posterior sclera: R2 = 0.17 at lower flow rate and R2 = 0.30 at higher flow rate; cornea: R2 = 0.02 at lower flow rate and R2

There is increasing clinical evidence that the eye is not only affected by intraocular pressure (IOP), but also by intracranial pressure (ICP). Both pressures meet at the optic nerve head of the eye, specifically the lamina cribrosa (LC). The LC is a collagenous meshwork through which all retinal ganglion cell axons pass on their way to the brain. Distortion of the LC causes a biological cascade leading to neuropathy and impaired vision in situations such as glaucoma and idiopathic intracranial hypertension. While the effect of IOP on the LC has been studied extensively, the coupled effects of IOP and ICP on the LC remain poorly understood. We investigated in-vivo the effects of IOP and ICP, controlled via cannulation of the eye and lateral ventricle in the brain, on the LC microstructure of anesthetized rhesus monkeys eyes using the Bioptigen spectral-domain optical coherence tomography (OCT) device (Research Triangle, NC). The animals were imaged with their head upright and the rest of their body lying prone on a surgical table. The LC was imaged at a variety of IOP/ICP combinations, and microstructural parameters, such as the thickness of the LC collagenous beams and diameter of the pores were analyzed. LC microstructure was confirmed by histology. We determined that LC microstructure deformed in response to both IOP and ICP changes, with significant interaction between the two. These findings emphasize the importance of considering both IOP and ICP when assessing optic nerve health.

Purpose: Evaluation of the effect of prelaminar tissue thickness on visualization of the lamina cribrosa (LC) using optical coherence tomography (OCT). Methods: The optic nerve head (ONH) region was scanned using OCT. The quality of visible LC microstructure was assessed subjectively using a grading system and objectively by analyzing the signal intensity of each scan's superpixel components. Manual delineations were made separately and in 3-dimensions quantifying prelaminar tissue thickness, analyzable regions of LC microstructure, and regions with a visible anterior LC (ALC) boundary. A linear mixed effect model quantified the association between tissue thickness and LC visualization. Results: A total of 17 healthy, 27 glaucoma suspect, and 47 glaucomatous eyes were included. Scans with thicker average prelaminar tissue measurements received worse grading scores (P = 0.007), and superpixels with low signal intensity were associated significantly with regions beneath thick prelaminar tissue (P < 0.05). The average prelaminar tissue thickness in regions of scans where the LC was analyzable (214 μm) was significantly thinner than in regions where the LC was not analyzable (569 μm; P < 0.001). Healthy eyes had significantly thicker average prelaminar tissue measurements than glaucoma or glaucoma suspect eyes (both P < 0.001), and glaucoma suspect eyes had significantly thicker average prelaminar tissue measurements than glaucoma eyes (P = 0.008). Significantly more of the ALC boundary was visible in glaucoma eyes (63% of ONH) than in healthy eyes (41%; P = 0.005). Conclusions: Thick prelaminar tissue was associated with impaired visualization of the LC. Healthy subjects generally had thicker prelaminar tissue, which potentially could create a selection bias against healthy eyes when comparing LC structures., United States. National Institutes of Health (R01-EY013178), United States. National Institutes of Health (R01-EY025011), United States. National Institutes of Health (R01-EY011289), United States. National Institutes of Health (P30-EY008098), United States. National Institutes of Health (T32-EY017271)

Dry eye (DE) disease is a multifactorial disease of the outer ocular surface characterized by several ocular symptoms and mainly by tear film instability. We have developed an optical imaging system, the tear film imager (TFI), which is the first instrument that can directly image the muco-aqueous tear layer physical dimension in vivo and evaluate its parameters in a noninvasive mode with nanometer axial resolution. This instrument provides quantified information about many attributes of the tear film, including muco-aqueous layer thickness, lipid layer thickness, thickness change rate, and the break-up time. The TFI performances are based on simultaneous acquisition of large field of view (FOV) imagery and fast spectrometric measurement of the interference from the thin tear film sublayers. Herein, after describing the instrument and the methodology of the measurements, we use a tear film mock-up to quantify device accuracy (2.2 nm) and repeatability (0.25 nm standard deviation). In conclusion, we present a new technology for the assessment of the tear film with an unprecedented axial resolution and excellent accuracy and reproducibility.

The lamina cribrosa (LC) is a sieve-like structure in the sclera where retinal ganglion cell axons exit from the eye. The LC has been known to play a critical role in the pathogenesis of glaucoma. With the advent of imaging technologies, such as enhanced depth imaging, spectral-domain optical coherence tomography (OCT) enables us to unveil the LC in vivo features. The application of adaptive optics technology and a compensatory image-processing algorithm has further improved the visualization of the beams and pores and neural pathways of the LC and the scleral insertion sites. Monitoring the changes of these structures in relation to acute and chronic elevation of intraocular pressure would be germane to decipher the relationship between the stress and strain response of the LC and optic nerve damage and improve our understanding of glaucoma pathophysiology. While the impact of investigating the integrity of LC is substantive, considerable challenges remain for imaging the LC. Nevertheless, with the rapid development of the OCT technology, it is expected that some of these limitations can be overcome and the potentials of LC imaging will be unraveled.

Aim To evaluate the utility of gold nanorods (AuNRs) as a contrast agent for ocular optical coherence tomography (OCT). Methods Mice were intravitreally injected with sterile AuNRs coated with either poly(strenesulfate) (PSS-AuNRs) or anti-CD90.2 antibodies (Ab-AuNRs), and imaged using OCT. After 24 h, eyes were processed for transmission electron microscopy or rendered into single cell suspensions for flow cytometric analysis to determine absolute numbers of CD45 + leukocytes and subsets (T cells, myeloid cells, macrophages, neutrophils). Generalised estimation equations were used to compare cell counts between groups. Results PSS-AuNRs and Ab-AuNRs were visualised in the vitreous 30 min and 24 h post-injection with OCT. At 24 h, a statistically significant increase in leukocytes, comprised primarily of neutrophils, was observed in eyes that received either AuNR in comparison to eyes that received saline. The accumulation of leukocytes was equal in eyes given PSS-AuNR or Ab-AuNR. Endotoxin-resistant C3H/HeJ mice also showed ocular inflammation after injection with AuNRs, indicating that the inflammatory response was not due to lipopolysaccharide contamination of AuNRs. Conclusions Although AuNRs can be visualised in the eye using OCT, they can induce ocular inflammation, which limits their use as a contrast agent.

BACKGROUND AND OBJECTIVE: To evaluate the ability of structural assessment to predict glaucomatous visual field progression. PATIENTS AND METHODS: A total of 119 healthy eyes with suspected glaucoma and glaucomatous eyes with 5 or more optic nerve stereophotographs, optical coherence tomography (OCT), and confocal scanning laser ophthalmoscopy (CSLO) all acquired within 6 months of each other were enrolled. Odds ratios to predict progression were determined by generalized estimating equation models. RESULTS: Median follow-up was 4.0 years (range: 1.5 to 5.7 years). Fifteen eyes progressed by glaucoma progression analysis, 20 by visual field index, and 10 by both. Baseline parameters from stereophotographs (vertical cup-to-disc ratio and Disc Damage Likelihood Scale), OCT (global, superior quadrant, and inferior quadrant retinal nerve fiber layer thickness), and CSLO (cup shape measure and mean cup depth) were significant predictors of progression. Comparing the single best parameter from all models, only the OCT superior quadrant RNFL predicted progression. CONCLUSION: Baseline stereophotographs, OCT, and CSLO measurements may be clinically useful to predict glaucomatous visual field progression.

Aims To evaluate the glaucoma discriminating ability of macular retinal layers as measured by spectral domain optical coherence tomography (SD-OCT). Methods Healthy, glaucoma suspect and glaucomatous subjects had a comprehensive ocular examination, visual field testing and SD-OCT imaging (Cirrus HD-OCT; Carl Zeiss Meditec, Dublin, California, USA) in the macular and optic nerve head regions. OCT macular scans were segmented into macular nerve fibre layer (mNFL), ganglion cell layer with inner plexiform layer (GCIP), ganglion cell complex (GCC) (composed of mNFL and GCIP), outer retinal complex and total retina. Glaucoma discriminating ability was assessed using the area under the receiver operator characteristic curve (AUC) for all macular parameters and mean circumpapillary retinal nerve fibre layer (cpRNFL). Results Analysis was performed on 51 healthy, 49 glaucoma suspect and 63 glaucomatous eyes. The median visual field MD was −2.21 dB (IQR: −6.92 to −0.35) for the glaucoma group, −0.32 dB (IQR: −1.22 to 0.73) for the suspect group and −0.18 dB (IQR: −0.92 to 0.71) for the healthy group. Highest age adjusted AUCs were found for average GCC and GCIP (AUC=0.901 and 0.900, respectively) and their sectoral measurements: infero-temporal (0.922 and 0.913), inferior (0.904 and 0.912) and supero-temporal (0.910 and 0.897). These values were similar to the discriminating ability of the mean cpRNFL (AUC=0.913). Comparison of these AUCs did not yield any statistically significant difference (all p>0.05). Conclusions SD-OCT GCIP and GCC measurements showed similar glaucoma diagnostic ability and were comparable with that of cpRNFL.

Purpose: To investigate how the lamina cribrosa (LC) microstructure changes with distance from the central retinal vessel trunk (CRVT), and to determine how this change differs in glaucoma. Methods: One hundred nineteen eyes (40 healthy, 29 glaucoma suspect, and 50 glaucoma) of 105 subjects were imaged using swept-source optical coherence tomography (OCT). The CRVT was manually delineated at the level of the anterior LC surface. A line was fit to the distribution of LC microstructural parameters and distance from CRVT to measure the gradient (change in LC microstructure per distance from the CRVT) and intercept (LC microstructure near the CRVT). A linear mixed-effects model was used to determine the effect of diagnosis on the gradient and intercept of the LC microstructure with distance from the CRVT. A Kolmogorov-Smirnov test was applied to determine the difference in distribution between the diagnostic categories. Results: The percent of visible LC in all scans was 26 ± 7%. Beam thickness and pore diameter decreased with distance from the CRVT. Glaucoma eyes had a larger decrease in beam thickness (−1.132 ± 0.503 μm, P = 0.028) and pore diameter (−0.913 ± 0.259 μm, P = 0.001) compared with healthy controls per 100 μm from the CRVT. Glaucoma eyes showed increased variability in both beam thickness and pore diameter relative to the distance from the CRVT compared with healthy eyes (P < 0.05). Conclusions: These findings results demonstrate the importance of considering the anatomical location of CRVT in the assessment of the LC, as there is a relationship between the distance from the CRVT and the LC microstructure, which differs between healthy and glaucoma eyes., Research to Prevent Blindness, Inc. (United States), Eye and Ear Foundation (Pittsburgh, Pa.), National Institutes of Health (U.S.) (NIH grant R01-EY013178), National Institutes of Health (U.S.) (NIH grant R01-EY025011), National Institutes of Health (U.S.) (NIH grant R01- EY011289), National Institutes of Health (U.S.) (NIH grant P30-EY008098), National Institutes of Health (U.S.) (NIH grant T32-EY017271)

Aims To determine the retinal nerve fibre layer (RNFL) thickness at which visual field (VF) damage becomes detectable and associated with structural loss. Methods In a prospective cross-sectional study, 72 healthy and 40 glaucoma subjects (one eye per subject) recruited from an academic institution had VF examinations and spectral domain optical coherence tomography (SD-OCT) optic disc cube scans (Humphrey field analyser and Cirrus HD-OCT, respectively). Comparison of global mean and sectoral RNFL thicknesses with VF threshold values showed a plateau of threshold values at high RNFL thicknesses and a sharp decrease at lower RNFL thicknesses. A ‘broken stick’ statistical model was fitted to global and sectoral data to estimate the RNFL thickness ‘tipping point’ where the VF threshold values become associated with the structural measurements. The slope for the association between structure and function was computed for data above and below the tipping point. Results The mean RNFL thickness threshold for VF loss was 75.3 mm (95% CI: 68.9 to 81.8), reflecting a 17.3% RNFL thickness loss from age-matched normative value. Above the tipping point, the slope for RNFL thickness and threshold value was 0.03 dB/mm (CI: � 0.02 to 0.08) and below the tipping point, it was 0.28 dB/mm (CI: 0.18 to 0.38); the difference between the slopes was statistically significant (p

BACKGROUND AND OBJECTIVE: To evaluate the accuracy, reproducibility, and variability of volumetric flow measurements taken by color Doppler imaging ultrasound, using an in vitro “phantom” model to simulate the ophthalmic artery. MATERIALS AND METHODS: An agar flow phantom with two wall-less lumens was constructed to simulate the ophthalmic artery. Velocity and volumetric flow measurements were taken for various flow rates and ultrasound probe positions. The measurements were analyzed for accuracy, reproducibility, and variability. RESULTS: Velocity measurements were more accurate than flow measurements (8 of 24 vs 3 of 24 accurate trials). The average coefficient of variation for volumetric blood flow was 11.4% (n = 120). Volumetric flow significantly correlated with velocity (R 2 = 0.408, n = 600, P < .001). The highest correlation was achieved using the large lumen with the probe held at 75°, offset to the flow (R 2 = 0.862, n = 75). CONCLUSION: Based on an in vitro model, noninvasive color Doppler imaging recordings of volumetric flow measurements in the ophthalmic artery significantly correlated with velocity and higher correlations were found using the larger lumens, although the data showed a lack of high accuracy in measurements of flow and velocity. [Ophthalmic Surg Lasers Imaging 2010;41:375–382.]

Purpose: To evaluate, within ocular imaging scans of acceptable quality as determined by manufacturers’ guidelines, the effects of image quality on glaucoma discrimination capabilities. Methods: One hundred and four healthy and 75 glaucomatous eyes from the Advanced Imaging in Glaucoma Study (AIGS) were imaged with GDx-VCC, HRT II, and StratusOCT. Quality score (QS≥8), pixel standard deviation (SD≤50), and signal strength (SS≥5) were used as quality parameter cutoffs, respectively. GDx nerve fiber indicator (NFI), and HRT Moorfields regression analysis (MRA) classifications and OCT mean retinal nerve fiber layer (RNFL) thickness were used as the discriminatory parameters. Logistic regression models were used to model the dichotomous clinical classification (healthy vs. glaucoma) as a function of image quality parameters and discriminatory parameters. Results: Quality parameter covariates were statistically non¬-significant for GDx and HRT but had an inverse effect on OCT in predicting disease (higher SS had lower probability of glaucoma). Age was a significant covariate for GDx and HRT, but not OCT, while ethnicity and interaction between the image quality and the institute where scans were acquired were significant covariates in the OCT models. Conclusion: Within scans considered to be acceptable based on the manufacturers’ recommended limits, scan quality does not affect the discriminating ability of global discriminatory parameters of GDx, HRT but have an effect on OCT measurements.

BACKGROUND AND OBJECTIVE The Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA) device is a spectral-domain optical coherence tomography system that allows faster data acquisition than the previous generation StratusOCT (Carl Zeiss Meditec, Dublin, CA), which is a time-domain device. The authors compared images from both units to determine the clinical usefulness of spectral-domain optical coherence tomography technology in patients with macular diseases. PATIENTS AND METHODS Six consecutive patients were imaged with both the Cirrus HD-OCT and the StratusOCT devices and the images were compared. RESULTS Cirrus HD-OCT images were typically more useful than StratusOCT images for assessing fine architectural details in macular pathology. The Cirrus HD-OCT software also facilitated a better understanding of three-dimensional data volumes. CONCLUSIONSCommercially available spectral-domain optical coherence tomography is a clinically useful tool for visualizing and understanding macular diseases and offers benefits not inherent in previous generation machines. [Ophthalmic Surg Lasers Imaging 2009;40:25-31.] AUTHORS From the UPMC Eye Center (PMB, LK, TRF), Department of Ophthalmology, University of Pittsburgh School of Medicine, Pittsburgh; and the Department of Bioengineering (LK), Swanson School of Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania. Accepted for publication May 8, 2008. Supported by grant NIH RO1-EY013178-8 from the National Institutes of Health, Bethesda, Maryland. The authors have no financial or proprietary interests in the materials presented herein. The authors thank Hiroshi Ishikawa, MD, for his assistance. Address correspondence to Thomas R. Friberg, MD, UPMC Eye Center, 203 Lothrop Street, Suite 824, Pittsburgh, PA 15213.

Aims: The purpose of this study was to compare the day-to-day reproducibility of optical coherence tomography (OCT; StratusOCT, Carl Zeiss Meditec, Dublin, CA) measurements of retinal nerve-fibre layer (RNFL) measurements at time points 1 year apart. Methods: One eye in each of 11 healthy subjects was examined using the StratusOCT fast RNFL scan protocol. Three fast RNFL scans with signal strength ⩾7 were obtained on each of 3 days within a month. This protocol was repeated after 12 months. A linear mixed effects model fitted to the nested data was used to compute the variance components. Results: The square root of the variance component that was attributed to the differences between subjects was 7.17 μm in 2005 and 7.28 μm in 2006. The square roots of the variance component due to differences between days within a single subject were 1.95 μm and 1.50 μm, respectively, and for within day within a single subject were 2.51 μm and 2.55 μm, respectively. There were no statistically significant differences for any variance component between the two testing occasions. Conclusions: Measurement error variance remains similar from year to year. Day and scan variance component values obtained in a cohort study may be safely applied for prediction of long-term reproducibility.

To compare stereometric parameters and classification results from the Heidelberg Retina Tomograph version 2 (HRT2); HRT3; and HRT3 Glaucoma Probability Score (GPS), an automated method of obtaining optic nerve head analysis without the need for manual definition of disc margin.Retrospective cross-sectional study.Five hundred four eyes from 281 consecutive subjects (glaucoma, glaucoma suspect, and healthy) evaluated in a glaucoma clinic.All participants had HRT2 scanning of the optic nerve head. Inclusion criteria were scans with good centration and focus, even illumination, an overall quality score by HRT3 of acceptable or better, and standard deviation50 mum. A Bland-Altman analysis was used for the comparison of HRT2 and HRT3. From these results, calibration equations were determined to permit conversion of the measurements between devices. The agreement between HRT2 and HRT3 Moorfields regression analysis (MRA) and HRT3 GPS classification methods was measured using kappa statistics.Heidelberg Retina Tomograph version 2 and HRT3 stereometric parameters, MRA, and global GPS.There was a statistically significant difference between HRT2 and HRT3 global disc area, rim area, cup area, rim volume, cup volume, height variation contour, and retinal nerve fiber layer cross-sectional area stereometric parameters. All of those parameters were smaller using HRT3, due to a manufacturer-reported horizontal scaling error of 4% in HRT2 that was corrected in HRT3. kappas for agreement were 0.60 between classifications (within normal limits, borderline, and outside normal limits) of MRA by HRT2 and HRT3 and 0.47 between HRT3 MRA and GPS.The HRT3 generally provided smaller stereometric disc measurements than HRT2. There was no clear conversion between HRT3 and GPS parameters, as the 2 methods for measuring the stereometric parameters differ.

Aims: To assess the effects of brinzolamide and dorzolamide on ocular haemodynamics and retinal oxygen saturation in patients with primary open-angle glaucoma (OAG). Methods: Fifteen patients with OAG were evaluated in a randomised, cross-over, double-blind study. They were treated with either brinzolamide or dorzolamide for 3 months and then crossed-over after a 4-week washout period. They were given timolol during a 4-week run-in period and during washout. The following were performed after run-in, after washout and after each treatment period: adverse events check, measurement of visual acuity, contrast sensitivity, blood pressure, heart rate, and intraocular pressure, and fundus examination. Ocular blood flow was assessed using confocal scanning laser Doppler flowmetry (HRF) and colour Doppler imaging (CDI). Retinal oxygenation levels were determined using a non-invasive measurement of haemoglobin oxygen saturation by digital photographic fundus oximetry. Results: Both brinzolamide and dorzolamide reduced the number of zero-flow pixels in the retina as measured by HRF, suggesting an increase in retinal blood flow (−6.86 and −0.452 respectively) with brinzolamide treatment resulting in fewer zero-flow pixels than dorzolamide (−6.41) (p = 0.024). Both brinzolamide and dorzolamide increased oxygen saturation in the retina as measured by photographic retinal oximetry in the superior (0.82 (p = 0.002) and 0.87 (p = 0.005)) and inferior (0.88 (p = 0.035) and 0.82 (p = 0.002)) retinal veins. No significant changes were found in CDI measurements of the retrobulbar blood supply during either treatment. Conclusion: This pilot study suggests that brinzolamide and dorzolamide may increase retinal oxygen saturation in patients with OAG.

BACKGROUND AND OBJECTIVE To evaluate the difference in arteriovenous passage time calculated from retinal angiograms using indocyanine green and fluorescein sodium dye in healthy individuals. PATIENTS AND METHODS Examinations of eight healthy individuals (mean age ± standard deviation: 27.3 ± 4.2 years) who had four consecutive ophthalmic visits that were 2 weeks apart were analyzed. Indocyanine green and fluorescein angiography were performed sequentially (with the indocyanine green angiography occurring first) using scanning laser ophthalmoscopy. Arteriovenous passage times were calculated offline using digital angiogram video analysis. RESULTS Mean arteriovenous passage times calculated from indocyanine green angiography (1.417 ± 0.136 seconds) recordings were significantly shorter than fluorescein sodium times (2.539 ± 0.421 seconds) (P < .0001) and intra-group variability was similar. There were no significant correlations between indocyanine green and fluorescein sodium arteriovenous passage times. CONCLUSION Arteriovenous passage times calculated using indocyanine green and fluorescein angiography are not interchangeable. [Ophthalmic Surg Lasers Imaging 2008;39:203-208.] AUTHORS From the Department of Ophthalmology (DTS, BS, AH), Indiana University School of Medicine, Indianapolis, Indiana; UPMC Eye Center (LK), Eye and Ear Institute, Ophthalmology and Visual Science Research Center, University of Pittsburgh School of Medicine, and the Department of Bioengineering (LK), Swanson School of Engineering, University of Pittsburgh, Pittsburgh, PA; Goldschleger Eye Institute (ER), Sheba Medical Center, Tel-Hashomer, Israel; and the Department of Ophthalmology (HJG), Bnai Zion Medical Center, The Bruce Rappaport Faculty of Medicine, Technion, Haifa, Israel. Accepted for publication November 12, 2007. Funded by an unrestricted grant from Research to Prevent Blindness, Inc., New York, New York. Address correspondence to Alon Harris, PhD, Department of Ophthalmology, Indiana University School of Medicine, 702 Rotary Circle, Room 137A, Indianapolis, IN 46202.

Purpose To compare the ability of the Heidelberg retina tomograph version 3 (HRT 3) and HRT version 2 (HRT 2) to discriminate between healthy and glaucomatous eyes. Design Retrospective cross-sectional study. Participants Seventy-one eyes of 71 healthy volunteers and 50 eyes of 50 glaucoma patients were studied. The average visual field mean deviation of the glaucoma group was −6.03±5.78 dB. Intervention All participants had comprehensive ocular examinations, perimetry, and HRT scanning within 6 months. HRT 2 data were analyzed using HRT 3 software without modifying the disc margin. Main Outcome Measures Discrimination capabilities between healthy and glaucomatous eyes were determined by areas under the receiver operating characteristics (AROCs) curves. Comparisons between corresponding AROCs obtained by HRT 2 and HRT 3 analyses were performed using the nonparametric DeLong method. Agreement between classifications as defined by the different analysis methods was quantified by κ analysis. Results The individual stereometric parameters with the best discrimination were linear cup/disc ratio (AROC = 0.897; 95% confidence interval [CI], 0.836–0.958) for standard HRT 3 analysis and horizontal retinal nerve fiber layer curvature (0.905) for HRT 3 glaucoma probability score (GPS) analysis. Areas under the receiver operating characteristics for discrimination between glaucomatous and healthy eyes of the overall classification by HRT 2 Moorfields regression analysis (MRA), HRT 3 MRA, and GPS were 0.927 (95% CI, 0.877–0.977), 0.934 (0.888–0.980), and 0.880 (0.812–0.948), respectively. The difference between the 3 AROCs was not significant ( P = 0.44). The agreement between HRT 2 and HRT 3 overall MRA classification was good (κ = 0.70; CI, 0.59–0.80) with HRT 3 tending to report more abnormalities than HRT 2 analysis. The agreement between overall HRT 3 MRA and overall GPS was κ = 0.58 (CI, 0.45–0.70). Conclusions The glaucoma discriminating ability of the new HRT 3 software is similar to that of the previous generation HRT 2. The GPS analysis showed promising results in differentiating between healthy and glaucomatous eyes without the need for subjective operator input.

Purpose: Matrix perimetry is a new iteration of frequency doubling technology (FDT) using a smaller target size in the standard achromatic perimetry presentation pattern. This study compared Matrix and Swedish interactive thresholding algorithm (SITA) perimetry performance in detecting glaucoma diagnosed by structural assessment. Design: Prospective cross-sectional study. Methods: Seventy-six eyes of 76 consecutive healthy subjects, glaucoma suspects and glaucoma patients were included. All patients underwent optic nerve head (ONH) photography, SITA and Matrix perimetry and optical coherence tomography (OCT; Stratus OCT) within a six month interval. Glaucoma diagnosis was established by either glaucomatous optic neuropathy or OCT retinal nerve fiber layer (RNFL) thickness. Mean deviation (MD), pattern standard deviation (PSD), glaucoma hemifield test (GHT) and cluster of abnormal testing locations were recorded from Matrix and SITA. Results: Similar correlations were observed with Matrix and SITA MD and PSD with either cup-to-disc ratio or OCT mean RNFL. The area under the receiver operating characteristic (AROC) curves of MD and PSD for discriminating between healthy and glaucomatous eyes ranged from 0.69 to 0.81 for Matrix and from 0.75 to 0.77 for SITA. There were no statistically significant differences among any corresponding Matrix and SITA AROCs. Conclusions: Matrix and SITA perimetry had similar capabilities for distinguishing between healthy and glaucomatous eyes regardless of whether the diagnosis was established by ONH or OCT RNFL assessment.

The mechanical characteristics of the trabecular meshwork (TM) are linked to outflow resistance and intraocular pressure (IOP) regulation. The rationale behind this technique is the direct observation of the mechanical response of the TM to acute IOP elevation. Prior to scanning, IOP is measured at baseline and during IOP elevation. The limbus is scanned by spectral-domain optical coherence tomography at baseline and during IOP elevation (ophthalmodynamometer (ODM) applied at 30 g force). Scans are processed to enhance visualization of the aqueous humor outflow pathway using ImageJ. Vascular landmarks are used to identify corresponding locations in baseline and IOP elevation scan volumes. Schlemm canal (SC) cross-sectional area (SC-CSA) and SC length from anterior to posterior along its long axis are measured manually at 10 locations within a 1 mm segment of SC. Mean inner to outer wall distance (short axis length) is calculated as the area of SC divided by its long axis length. To examine the contribution of adjacent tissues to the effect IOP elevations, measurements are repeated without and with smooth muscle relaxation with instillation of tropicamide. TM migration into SC is resisted by TM stiffness, but is enhanced by the support of its attachment to adjacent smooth muscle within the ciliary body. This technique is the first to measure the living human TM response to pressure elevation in situ under physiological conditions within the human eye.

PURPOSE Developing a novel image enhancement method so that nonframe-averaged optical coherence tomography (OCT) images become comparable to active eye-tracking frame-averaged OCT images. METHODS Twenty-one eyes of 21 healthy volunteers were scanned with noneye-tracking nonframe-averaged OCT device and active eye-tracking frame-averaged OCT device. Virtual averaging was applied to nonframe-averaged images with voxel resampling and adding amplitude deviation with 15-time repetitions. Signal-to-noise (SNR), contrast-to-noise ratios (CNR), and the distance between the end of visible nasal retinal nerve fiber layer (RNFL) and the foveola were assessed to evaluate the image enhancement effect and retinal layer visibility. Retinal thicknesses before and after processing were also measured. RESULTS All virtual-averaged nonframe-averaged images showed notable improvement and clear resemblance to active eye-tracking frame-averaged images. Signal-to-noise and CNR were significantly improved (SNR: 30.5 vs. 47.6 dB, CNR: 4.4 vs. 6.4 dB, original versus processed, P < 0.0001, paired t-test). The distance between the end of visible nasal RNFL and the foveola was significantly different before (681.4 vs. 446.5 μm, Cirrus versus Spectralis, P < 0.0001) but not after processing (442.9 vs. 446.5 μm, P = 0.76). Sectoral macular total retinal and circumpapillary RNFL thicknesses showed systematic differences between Cirrus and Spectralis that became not significant after processing. CONCLUSION The virtual averaging method successfully improved nontracking nonframe-averaged OCT image quality and made the images comparable to active eye-tracking frame-averaged OCT images. TRANSLATIONAL RELEVANCE Virtual averaging may enable detailed retinal structure studies on images acquired using a mixture of nonframe-averaged and frame-averaged OCT devices without concerning about systematic differences in both qualitative and quantitative aspects.

Purpose To evaluate agreement among experts of Heidelberg retina tomography9s (HRT) topographic change analysis (TCA) printout interpretations of glaucoma progression and explore methods for improving agreement. Methods 109 eyes of glaucoma, glaucoma suspect and healthy subjects with ≥5 visits and 2 good quality HRT scans acquired at each visit were enrolled. TCA printouts were graded as progression or non-progression. Each grader was presented with 2 sets of tests: a randomly selected single test from each visit and both tests from each visit. Furthermore, the TCA printouts were classified with grader9s individual criteria and with predefined criteria (reproducible changes within the optic nerve head, disregarding changes along blood vessels or at steep rim locations and signs of image distortion). Agreement among graders was modelled using common latent factor measurement error structural equation models for ordinal data. Results Assessment of two scans per visit without using the predefined criteria reduced overall agreement, as indicated by a reduction in the slope, reflecting the correlation with the common factor, for all graders with no effect on reducing the range of the intercepts between the graders. Using the predefined criteria improved grader agreement, as indicated by the narrower range of intercepts among the graders compared with assessment using individual grader9s criteria. Conclusions A simple set of predefined common criteria improves agreement between graders in assessing TCA progression. The inclusion of additional scans from each visit does not improve the agreement. We, therefore, recommend setting standardised criteria for TCA progression evaluation.

Background/aim: The Heidelberg retina flowmeter (HRF) is designed to measure retinal capillary blood flow. Previous studies however showed weak reproducibility of data. The intraindividual reproducibility of circadian HRF measurements was examined in healthy subjects in three locations of the retina. Methods: 36 healthy volunteers (27.3 (SD 4.3) years) were examined by HRF seven times a day (t0–t6). Using a default window of 10×10 pixels, three consecutive measurements were performed in three precise focusing planes: superficial, intermediate and deep layer, peripapillary retina, neuroretinal rim and cup, respectively. Images of identical tissue locations identified by capillary landmarks of each layer were selected to quantify the retinal microcirculation of each volunteer. Means and standard deviations of all flow results of a given subject were calculated, at t0–t6 and the coefficients of variation as a measure of reproducibility. Results: The coefficients of variation ranged between 8.4% and 41.0% in the superficial layer (mean 19.8% (SD 8.4%)), 10.6%, and 43.0% in the intermediate layer (mean 24.0% (SD 8.4%)), and 9.9% and 84.0% (mean 29.6% (SD 15.8%)) in the deep layer. Conclusions: These data show the best reproducibility of measurements in the superficial layer followed by the intermediate and the deep layer. Clinically, this is an unsatisfactory intraindividual reproducibility of flow values in each studied layer.

Advances in optical coherence tomography (OCT) have dramatically improved the ability to visualize the optic nerve head (ONH) in vivo. In recent years there has been an increase in the number of articles reporting OCT investigations of the morphology and biomechanics of the ONH, and in particular of the lamina cribrosa. Although many of these studies have resulted in important advances in understanding of the ONH, the relative simplicity of imaging and morphometry also increases the potential for mistakes. Herein, we alert readers to a common and potentially critical error made in OCT image analysis, and describe a simple solution. The error occurs when measuring lengths and angles in OCT images with spatial aspect ratios other than 1:1. An anterior-posterior “stretching” of OCT images was introduced in the original prototype OCT to aid in the visualization of the details of the thin layers of the retina (Fig.), and continues to be used by many current commercial systems. The Spectralis SD-OCT (Heidelberg GmbH, Heidelberg, Germany), for example, presents images stretched 3-fold in the axial scan direction by default. This is indicated on the display and device output by scale bars. Image stretching also can occur because OCT sampling density is higher in the axial direction versus the transverse. Hence, an image presented in a 1:1 pixel aspect ratio is stretched axially. This is how most image-manipulation software presents images. Measurements using such images are likely to result in erroneous quantification of ocular structure. Figure Illustrating the error of measuring lamina cribrosa depth on stretched images. On the left side, a B-scan through the ONH stretched 3:1 in the axial direction, as presented by Spectralis. On the top right, the same B-scan, corrected to a 1:1 spatial aspect ... For ONH morphometry, a typical measure of interest is the depth of the lamina cribrosa relative to Bruch's membrane opening. This depth can be measured by first drawing a straight line connecting Bruch's membrane openings in a longitudinal view of the ONH, typically a B-scan or a radial scan. The depth of a point in the lamina is then determined by the length of a line that passes through this point and is perpendicular to the Bruch's membrane reference line. Failing to consider that the images are stretched will lead to incorrect lamina depth measurements, for the following two reasons. First, computing the length of the depth line must account for differences in the horizontal and vertical scales: n pixels in the horizontal direction mean a different length than the same n pixels in the vertical direction. This is not difficult, but most popular image-manipulation software do not make this consideration, and the measurements obtained without correction are wrong. A second, less conspicuous but just as serious problem, is that lines that are perpendicular in a stretched image (Fig.) are likely not perpendicular in the 1:1 nonstretched, correct, view (Fig.). Similarly, angles used for morphometry, for example, to measure peripapillary sclera bowing, will be incorrect if measured in stretched images. The magnitude of the error varies with the amount that an image is stretched, and the orientation of the measurements. Note that ONH tilt can vary between scans of a patient, and therefore even analysis of relative change in the same patient, such as before versus after an intervention, can be subject to error. The most straightforward solution is to do all marking and morphometry on unstretched images. If the images were saved stretched, unstretching requires knowledge of the stretch applied or the OCT's native image aspect ratio. This is not always easy, because these values can change from one OCT vendor to another, between software versions, and even with the viewing mode. Depending on how it is done, removing the stretch may downgrade the image, due to resampling, and may still require using measurement software that can deal with pixels that are not square. Alternatively, it is possible to mathematically “correct” post hoc measurements done on stretched images, but this again requires knowledge of the stretching factor as well as the angles of the measured and reference lines. Unfortunately, this potential error is difficult or impossible for a journal reviewer or reader to detect. In our experience, the vast majority of articles never mention consideration of the image aspect ratio. This may contribute to the low visibility of the problem, and likely to increase prevalence. It is impossible to know if authors of articles that show stretched OCT images carried out the measurements using correct aspect ratios. However, illustrations displaying “orthogonal” lines on stretched images, and use of image-manipulation software that does not account for stretched images or nonsquare pixels, suggest errant data. Our recommendation is for authors to include in the article details about whether the image aspect ratio was considered, and how. This could include showing the marking and measurement example illustrations using unstretched 1:1 images.

To determine if perfusion per unit tissue volume of retinal nerve fibre layer and optic nerve head in the inferior sector is lower than in the superior sector.Heidelberg retinal tomogram (HRT) for topographic measurement of optic nerve head and retinal nerve fibre layer and Heidelberg retinal flowmeter (HRF) for retinal blood flow were performed on 19 normal healthy subjects. Measurements from the superior and inferior sectors were compared. The perfusion/nerve fibre ratio (PNR); the blood flow per unit retinal nerve fibre tissue volume, was calculated in each sector with a formula; HRF flow measurements divided by HRT measurements.Retinal nerve fibre layer thickness in the inferior retina was significantly higher than in the superior retina (p0.05). There were, however, no differences in retinal blood flow between the superior and inferior retinal sectors. The PNR in the inferior sector were significantly lower than in the superior sector (p=0.047 for HRF mean flow/rim volume and p = 0.0282 for HRF 75th percentile flow/rim volume).The inferior sector of retinal nerve fibre layer and optic nerve head may have lower blood flow per unit nerve tissue volume compared to the superior sector. This result suggests that the inferior sector is more vulnerable to elevated intraocular pressure (IOP) and ischaemic insults in glaucomatous optic neuropathy.