Non-invasive measurement of choroidal volume change and ocular rigidity through automated segmentation of high-speed OCT imaging

Ocular pulse amplitude (OPA) in canine ADAMTS10-open-angle glaucoma (ADAMTS10-OAG)

Introduction: The role of ocular rigidity and biomechanics remains incompletely understood in glaucoma, including assessing an individual's sensitivity to intraocular pressure (IOP). In this regard, the clinical assessment of ocular biomechanics represents an important need. The purpose of this study was to determine a possible relationship between the G661R missense mutation in the ADAMTS10 gene and the ocular pulse amplitude (OPA), the difference between diastolic and systolic intraocular pressure (IOP), in a well-established canine model of open-angle glaucoma (OAG). Methods: Animals studied included 39 ADAMTS10-mutant dogs with different stages of OAG and 14 unaffected control male and female dogs between 6 months and 12 years (median: 3.2 years). Dogs were sedated intravenously with butorphanol tartrate and midazolam HCl, and their IOPs were measured with the Icare® Tonovet rebound tonometer. The Reichert Model 30™ Pneumotonometer was used to measure OPA. Central corneal thickness (CCT) was measured via Accutome® PachPen, and A-scan biometry was assessed with DGH Technology Scanmate. All outcome measures of left and right eyes were averaged for each dog. Data analysis was conducted with ANOVA, ANCOVA, and regression models. Results: ADAMTS10-OAG-affected dogs displayed a greater IOP of 23.0 ± 7.0 mmHg (mean ± SD) compared to 15.3 ± 3.6 mmHg in normal dogs (p < 0.0001). Mutant dogs had a significantly lower OPA of 4.1 ± 2.0 mmHg compared to 6.5 ± 2.8 mmHg of normal dogs (p < 0.01). There was no significant age effect, but OPA was correlated with IOP in ADAMTS10-mutant dogs. Conclusion: The lower OPA in ADAMTS10-mutant dogs corresponds to the previously documented weaker and biochemically distinct posterior sclera, but a direct relationship remains to be confirmed. The OPA may be a valuable clinical tool to assess ocular stiffness and an individual's susceptibility to IOP elevation.

Ocular rigidity and choroidal thickness changes in response to microgravity: A case study

Purpose

To evaluate ocular rigidity and choroidal thickness changes in response to microgravity and the Valsalva maneuver in a private astronaut.

Methods

Ophthalmological examination and Optical Coherence Tomography were performed before, during, and after space flight. Choroidal thickness was measured at all time points at rest and during the Valsalva maneuver. Ocular rigidity was obtained before and after flight using a non-invasive method enhanced with deep learning-based choroid segmentation.

Results

Ocular rigidity decreased after space flight compared to baseline. There was an increase in average choroidal thickness during the Valsalva maneuver compared to the resting condition before, during, and after space flight, and such increase was greater when the Valsalva maneuver was performed during space flight.

Conclusions and importance

The data indicates biomechanical changes to ocular tissues because of space flight and greater choroidal thickness increase. The findings could lead to a better understanding of space flight-associated neuro-ocular syndrome and may have repercussions for short duration missions in a nascent industry.

Ocular rigidity and neuroretinal damage in patients with vasospasticity: a pilot study

OBJECTIVE

Evidence suggests that ocular blood flow dysregulation in patients with vasospasticity could occur in response to biomechanical stimuli, contributing to optic nerve head susceptibility in glaucoma. We evaluate the role of vasospasticity in the association between ocular rigidity (OR) and neuroretinal damage, hypothesizing that low OR correlates with greater glaucoma damage in patients with vasospasticity.

DESIGN

Cross-sectional study.

PARTICIPANTS

Patients with open-angle glaucoma (OAG), suspect discs, or no glaucoma.

METHODS

OR was measured using a noninvasive, validated method developed by our group. Retinal nerve fibre layer (RNFL) and ganglion cell complex thicknesses were acquired using spectral domain optical coherence tomography. Vasospasticity was assessed by a standardized questionnaire that was based on existing validated questionnaires and adapted to our requirements. Atherosclerosis was evaluated based on Broadway and Drance's (1998) cardiovascular disease score. Correlations between OR and structural parameters were assessed in patients with vasospasticity and those with atherosclerosis.

RESULTS

Of 118 patients with either OAG (n = 67), suspect discs (n = 26), or no glaucoma (n = 25) who were recruited consecutively, 10 were classified as having vasospasticity, and 37 as having atherosclerosis. In the vasospastic group, significant correlations were found between OR and the minimum ganglion cell complex thickness (rs = 0.681, p = 0.030), the average RNFL thickness (rs = 0.745, p = 0.013), and the RNFL in the temporal quadrant (rs = 0.772, p = 0.009), indicating more damage with lower OR. Similar trends were maintained when applying multiple testing correction; however, only the eighth RNFL clock hour corresponding to the inferior-temporal peripapillary region remained significantly correlated with OR in the vasospastic group (p = 0.015). In contrast, no correlation was found in the atherosclerotic group (p > 0.05).

CONCLUSIONS

The findings of the current pilot study indicate a trend for more neuronal structural damage in less-rigid eyes of patients with vasospasticity, meaning that OR may play a greater role in glaucoma in vasospastic patients than in patients with atherosclerosis. Although these results provide interesting insight into the pathophysiology of OAG, further investigation is needed to confirm our observations.

Quantification of Hypotony Maculopathy Using Spectral-Domain Optical Coherence Tomography

PRCIS

We provide a free-to-use, open-source algorithm to quantify macular hypotony based on optical coherence tomography (OCT) images. This numerical approach calculates a metric that measures the deviations of Bruch's membrane from a smooth ideal retinal layer.

PURPOSE

Hypotony maculopathy is a recurrent complication of glaucoma surgical interventions in which extremely low intraocular pressure triggers changes in the shape of retinal layers. Abnormal folds can often be observed in the retina using standard fundoscopy, but OCT is particularly important to appreciate the severity of symptoms at different depths. Despite the need for metrics that could be used for the informed clinical decision to evaluate the progression and resolution of macular hypotony, algorithms that quantify the retinal folds are not available in the literature or included in clinical imaging equipment. The purpose of this work is to introduce a simple algorithm that can be used to assess hypotony maculopathy from OCT B-Scans and volumes and a free, open-source implementation.

METHODS

The pipeline we present is based on a straightforward segmentation of Bruch's membrane complex. The principal idea of quantification is to compute a smoothed version of this complex and analyze the deviations from an ideal interface. Such deviations are then measured and added to create a metric that characterizes each OCT B-Scan. A full OCT volume reconstruction is thus characterized by the average metric obtained from all planes.

RESULTS

We tested the metric we proposed against the assessment of 3 experts and obtained a very good correspondence, with Pearson correlation coefficients higher than 0.8. Furthermore, agreement with automatic analysis seemed better than between experts. We describe the pipeline in detail and illustrate the results with a group of patients, comparing baseline images, severe hypotony maculopathy, and a variety of outcomes.

CONCLUSION

The tool we introduce and openly provide fills a clinical gap to quantitatively grade hypotony maculopathy. It offers a metric of relatively simple interpretation that can be used to help clinicians in cases where the regression of symptoms is not obvious to the naked eye. Our pilot study demonstrates reliable results, and an open-source implementation facilitates easy improvements to our algorithm.

Ocular Rigidity and Current Therapy

Purpose: Ocular rigidity (OR) is an important biomechanical parameter of the eye accounting for the material and geometrical properties of the corneoscleral shell.Methods: This study used a literature search to review the role of ocular rigidity and the application of potential therapies targeting this parameter in glaucoma and myopia.Conclusion: Biomechanical modeling and improved understanding of the biochemistry, and molecular arrangement of sclera and its constituents have yielded important insights. Recent developments, including that of a non-invasive and direct OR measurement method and improved ocular imaging techniques are helping to elucidate the role of OR in healthy and diseased eyes by facilitating large scale and longitudinal clinical studies. Improved understanding of OR at the initial stages of disease processes and its alterations with disease progression will undoubtedly propel research in the field. Furthermore, a better understanding of the determinants of OR is helping to refine novel therapeutic approaches which target and alter the biomechanical properties of the sclera in sight-threatening conditions such as glaucoma and myopia.

Mapping Pulsatile Optic Nerve Head Deformation Using OCT

OBJECTIVE

To develop a noninvasive technique to quantitatively assess the pulsatile deformation due to cardiac contractions of the optic nerve head (ONH).

DESIGN

Evaluation of a diagnostic test or technology.

PARTICIPANTS

Healthy subjects with no history of refractive surgery, divided into 2 cohorts on the basis of their axial length (AL).

METHODS

We present a noninvasive technique to quantitatively assess the pulsatile deformation of the ONH tissue by combining high-frequency OCT imaging and widely available image processing algorithms. We performed a thorough validation of the approach, numerically and experimentally, evaluating the sensitivity of the method to artificially induced deformation and its robustness to different noise levels. We performed deformation measurements in cohorts of healthy (n = 9) and myopic (n = 5) subjects in different physiological strain conditions by calculating the amplitude of tissue displacement in both the primary position and abduction. The head rotation was measured using a goniometer. During imaging in abduction, the head was rotated 40° ± 3°, and subjects were instructed to direct their gaze toward the OCT visual target.

MAIN OUTCOME MEASURES

Pulsatile tissue displacement maps.

RESULTS

The robustness of the method was assessed using artificial deformations and increasing noise levels. The results show acceptable absolute errors before the noise simulations grossly exaggerate image degradation. For the group of subjects with AL of < 25 mm (n = 9), the median pulsatile displacement of the ONH was 7.8 ± 1.3 μm in the primary position and 8.9 ± 1.2 μm in abduction. The Wilcoxon test showed a significant difference (P ≤ 0.005) between the 2 paired measures. Reproducibility was tested in 2 different sessions in 5 different subjects with the same intraocular pressure, and an intraclass correlation coefficient of 0.99 was obtained (P < 0.005).

CONCLUSIONS

The computational pipeline demonstrated good reproducibility and had the capacity to accurately map the pulsatile deformation of the optic nerve. In a clinical setting, we detected physiological changes in normal subjects supporting its translation potential as a novel biomarker for the diagnosis and progression of optic nerve diseases.

Automatic Segmentation and Measurement of Choroid Layer in High Myopia for OCT Imaging Using Deep Learning

Automatic segmentation and measurement of the choroid layer is useful in studying of related fundus diseases, such as diabetic retinopathy and high myopia. However, most algorithms are not helpful for choroid layer segmentation due to its blurred boundaries and complex gradients. Therefore, this paper aimed to propose a novel choroid segmentation method that combines image enhancement and attention-based dense (AD) U-Net network. The choroidal images obtained from optical coherence tomography (OCT) are pre-enhanced by algorithms that include flattening, filtering, and exponential and linear enhancement to reduce choroid-independent information. Experimental results obtained from 800 OCT B-scans of the choroid layers from both normal eyes and high myopia showed that image enhancement significantly increased the performance of ADU-Net, with an AUC of 99.51% and a DSC of 97.91%. The accuracy of segmentation using the ADU-Net method with image enhancement is superior to that of the existing networks. In addition, we describe some algorithms that can measure automatically choroidal foveal thickness and the volume of adjacent areas. Statistical analyses of the choroidal parameters variation indicated that compared with normal eyes, high myopia has a reduction of 86.3% of the choroidal foveal thickness and 90% of the adjacent volume. It proved that high myopia is likely to cause choroid layer attenuation. These algorithms would have wide application in the diagnosis and precaution of related fundus lesions caused by choroid thinning from high myopia in future studies.

Ocular Pulse Amplitude Correlates With Ocular Rigidity at Native IOP Despite the Variability in Intraocular Pulse Volume With Each Heartbeat

Purpose

The purpose of this study was to assess ocular coat mechanical behavior using controlled ocular microvolumetric injections (MVI) of 15 µL of balanced salt solution (BSS) infused over 1 second into the anterior chamber (AC) via a syringe pump.

Methods

Intraocular pressure (IOP) was continuously recorded at 200 Hz with a validated implantable IOP telemetry system in 7 eyes of 7 male rhesus macaques (nonhuman primates [NHPs]) during 5 MVIs in a series at native (3 trials), 15 and 20 mm Hg baseline IOPs, repeated in 2 to 5 sessions at least 2 weeks apart. Ocular rigidity coefficients (K) and ocular pulse volume (PV) were calculated for each trial. Data were averaged across sessions within eyes; PV was analyzed with a three-level nested ANOVA, and parameter relationships were analyzed with Pearson Correlation and linear regression.

Results

After MVI at native baseline IOP of 10.4 ± 1.6 mm Hg, IOP increased by 9.1 ± 2.8 mm Hg (∆IOP) at a 9.6 ± 2.7 mm Hg/s slope, ocular pulse amplitude (OPA) was 0.70 ± 0.13 mm Hg on average; the average K was 0.042 ± 0.010 µL-1 and average PV was 1.16 ± 0.43 µL. PV varied significantly between trials, days, and eyes (P ≤ 0.05). OPA was significantly correlated with K at native IOP: Pearson coefficients ranged from 0.71 to 0.83 (P ≤ 0.05) and R2 ranged from 0.50 to 0.69 (P ≤ 0.05) during the first trial.

Conclusions

The MVI-driven ∆IOP and slope can be used to assess ocular coat mechanical behavior and measure ocular rigidity.

Translational Relevance

Importantly, OPA at native IOP is correlated with ocular rigidity despite the significant variability in PV between heartbeats.

Comparison of Corneal Wave Speed and Ocular Rigidity in Normal and Glaucomatous Eyes

PRECIS

Ocular biomechanics were compared between treated glaucoma patients and healthy subjects matched for age, intraocular pressure (IOP), and axial length. There was no difference in corneal wave propagation speed, but ocular rigidity was lower in glaucomatous eyes.

PURPOSE

Ocular biomechanical properties are important in understanding glaucoma pathogenesis but the affected tissues are unclear. In this study, we compared corneal wave speed (a measure of corneal elasticity) and ocular rigidity coefficient between glaucomatous and normal eyes.

MATERIALS AND METHODS

Twenty glaucomatous eyes from 10 patients and 20 normal eyes from 13 controls, matched for age, IOP, and axial length were included. Ocular rigidity was calculated based on the difference in supine IOP by pneumatonometry with and without a 10-g weight. Corneal wave speed was determined by ultrasound surface wave elastography. A small, 0.1-second harmonic vibration at 100 Hz was generated through the closed eyelids. Wave propagation was captured by an ultrasound transducer, and wave speed was determined from the phase change with distance. Comparisons were performed using generalized estimating equation models.

RESULTS

There were no significant differences in corneal wave speed between glaucomatous and normal eyes (2.16±0.25 vs. 2.07±0.16 m/s, P=0.17). However, ocular rigidity was significantly lower in glaucomatous eyes (0.0218±0.0033 vs. 0.0252±0.0050/μL, P=0.01). Corneal wave speed was not correlated with age and IOP in either group (P≥0.23) but was correlated with ocular rigidity (R=0.48, P=0.02) and inversely correlated with axial length (R=-0.53, P=0.01) in glaucomatous eyes.

CONCLUSION

Glaucomatous eyes tend to have lower ocular rigidity than healthy eyes with similar age, IOP, and axial length. However, the lack of a difference in corneal wave speed suggests that corneal tissue may not be significantly affected, and scleral changes likely play a more important role in glaucoma.

Non-invasive Clinical Measurement of Ocular Rigidity and Comparison to Biomechanical and Morphological Parameters in Glaucomatous and Healthy Subjects

Purpose: To assess ocular rigidity using dynamic optical coherence tomography (OCT) videos in glaucomatous and healthy subjects, and to evaluate how ocular rigidity correlates with biomechanical and morphological characteristics of the human eye.

Methods: Ocular rigidity was calculated using Friedenwald's empirical equation which estimates the change in intraocular pressure (IOP) produced by volumetric changes of the eye due to choroidal pulsations with each heartbeat. High-speed OCT video was utilized to non-invasively measure changes in choroidal volume through time-series analysis. A control-case study design was based on 23 healthy controls and 6 glaucoma cases. Multiple diagnostic modalities were performed during the same visit including Spectralis OCT for nerve head video, Pascal Dynamic Contour Tonometry for IOP and ocular pulse amplitude (OPA) measurement, Corvis ST for measuring dynamic biomechanical response, and Pentacam for morphological characterization.

Results: Combining glaucoma and healthy cohorts (n = 29), there were negative correlations between ocular rigidity and axial length (Pearson R = −0.53, p = 0.003), and between ocular rigidity and anterior chamber volume (R = −0.64, p = 0.0002). There was a stronger positive correlation of ocular rigidity and scleral stiffness (i.e., stiffness parameter at the highest concavity [SP-HC]) (R = 0.62, p = 0.0005) compared to ocular rigidity and corneal stiffness (i.e., stiffness parameter at the first applanation [SP-A1]) (R = 0.41, p = 0.033). In addition, there was a positive correlation between ocular rigidity and the static pressure-volume ratio (P/V ratio) (R = 0.72, p < 0.0001).

Conclusions: Ocular rigidity was non-invasively assessed using OCT video and OPA in a clinic setting. The significant correlation of ocular rigidity with biomechanical parameters, SP-HC and P/V ratio, demonstrated the validity of the ocular rigidity measurement. Ocular rigidity is driven to a greater extent by scleral stiffness than corneal stiffness. These in vivo methods offer an important approach to investigate the role of ocular biomechanics in glaucoma.

The Association Between Ocular Rigidity and Neuroretinal Damage in Glaucoma

Purpose

Ocular rigidity (OR) is an important biomechanical property, thought to be relevant in the pathophysiology of open-angle glaucoma (OAG). This study aims to evaluate the relationship between OR and neuroretinal damage caused by glaucoma.

Methods

One hundred eight subjects (22 with healthy eyes, 23 with suspect discs, and 63 with OAG) were included in this study. OR was measured using a noninvasive optical coherence tomography (OCT)-based method developed by our group. We also measured central corneal thickness (CCT), corneal hysteresis (CH), and corneal resistance factor (CRF). Pearson and partial correlations were performed to evaluate the relationship between OR and glaucomatous damage represented by ganglion cell complex (GCC), retinal nerve fiber layer (RNFL) thicknesses, and neuroretinal rim area.

Results

Significant positive correlations were found between OR and minimum GCC thickness (r = 0.325, P = 0.001), average GCC thickness (r = 0.320, P = 0.002), rim area (r = 0.344, P < 0.001), and RNFL thickness in the superior (r = 0.225, P = 0.023), and inferior (r = 0.281, P = 0.004) quadrants. These correlations were generally greater than those found for CCT, CH, and CRF. Furthermore, no correlation was found between OR and corneal biomechanical parameters. After adjusting for age, sex, and ethnicity, significant correlations were found between OR and minimum and average GCC thickness (r = 0.357, P = 0.001 and r = 0.344, P = 0.001, respectively), rim area (r = 0.327, P = 0.001), average RNFL thickness (r = 0.331, P = 0.001), and RNFL thickness in the superior (r = 0.296, P = 0.003) and inferior (r = 0.317, P = 0.001) quadrants.

Conclusions

In this study, we found a positive correlation between structural OCT-based parameters and OR, indicating more neuroretinal damage in eyes with lower OR. These findings could provide insight into the pathophysiology of OAG.

Heartbeat-Induced Corneal Axial Displacement and Strain Measured by High Frequency Ultrasound Elastography in Human Volunteers

Purpose

The purpose of this study was to establish in vivo data acquisition and processing protocols for repeatable measurements of heartbeat-induced corneal displacements and strains in human eyes, using a high-frequency ultrasound elastography method, termed ocular pulse elastography (OPE).

Methods

Twenty-four volunteers with no known ocular diseases were recruited for this study. Intraocular pressure (IOP) and ocular pulse amplitude (OPA) were measured using a PASCAL Dynamic Contour Tonometer (DCT). An in vivo OPE protocol was developed to measure heartbeat-induced corneal displacements. Videos of the central 5.7 mm of the cornea were acquired using a 50-MHz ultrasound probe at 128 frames per second. The radiofrequency data of 1000 frames were analyzed using an ultrasound speckle tracking algorithm to calculate corneal displacements and quantify spectral and temporal characteristics. The intrasession and intersession repeatability of OPE- and DCT-measured parameters were also analyzed.

Results

The in vivo OPE protocol and setup were successful in tracking heartbeat-induced corneal motion using high-frequency ultrasound. Corneal axial displacements showed a strong cardiac rhythm, with good intrasession and intersession repeatability, and high interocular symmetry. Corneal strain was calculated in two eyes of two subjects, showing substantially different responses.

Conclusions

We demonstrated the feasibility of high-frequency ultrasound elastography for noninvasive in vivo measurement of the cornea's biomechanical responses to the intrinsic ocular pulse. The high intrasession and intersession repeatability suggested a robust implementation of this technique to the in vivo setting.

Translational Relevance

OPE may offer a useful tool for clinical biomechanical evaluation of the cornea by quantifying its response to the intrinsic pulsation.

Correlation of ocular rigidity with intraocular pressure spike after intravitreal injection of bevacizumab in exudative retinal disease

BACKGROUND/AIMS

To evaluate the non-invasive measurement of ocular rigidity (OR), an important biomechanical property of the eye, as a predictor of intraocular pressure (IOP) elevation after anti-vascular endothelial growth factor (anti-VEGF) intravitreal injection (IVI).

METHODS

Subjects requiring IVI of anti-VEGF for a pre-existing retinal condition were enrolled in this prospective cross-sectional study. OR was assessed in 18 eyes of 18 participants by measurement of pulsatile choroidal volume change using video-rate optical coherence tomography, and pulsatile IOP change using dynamic contour tonometry. IOP was measured using Tono-Pen XL before and immediately following the injection and was correlated with OR.

RESULTS

The average increase in IOP following IVI was 19±9 mm Hg, with a range of 7-33 mm Hg. The Spearman correlation coefficient between OR and IOP elevation following IVI was 0.796 (p<0.001), showing higher IOP elevation in more rigid eyes. A regression line was also calculated to predict the IOP spike based on the OR coefficient, such that IOP spike=664.17 mm Hg·µL×OR + 4.59 mm Hg.

CONCLUSION

This study shows a strong positive correlation between OR and acute IOP elevation following IVI. These findings indicate that the non-invasive measurement of OR could be an effective tool in identifying patients at risk of IOP spikes following IVI.

Critical analysis of techniques and materials used in devices, syringes, and needles used for intravitreal injections

Intravitreal injections have become the most commonly performed intraocular treatments worldwide. Because intravitreal injections may induce severe adverse events, such as infectious and noninfectious endophthalmitis, cataract, ocular hypertension, vitreous hemorrhage, or retinal detachment, appropriate awareness of the materials and techniques used are essential to reduce these sight-threatening complications. This review provides insights into the needles, syringes, silicone oil coating, sterilization methods, devices to assist intravitreal injections, scleral piercing techniques using needles, syringe handling, anesthesia, and safety issues related to materials and techniques. It is paramount that physicians be aware of every step involved in intravitreal injections and consider the roles and implications of all materials and techniques used. The ability to understand the theoretical and practical circumstances may definitely lead to state-of-the-art treatments delivered to patients. The most important practical recommendations are: choosing syringes with as little silicone oil as possible, or, preferably, none; avoiding agitation of syringes; awareness that most biologics (e.g., antiangiogenic proteins) are susceptible to changes in molecular properties under some conditions, such as agitation and temperature variation; understanding that improper materials and techniques may lead to complications after intravitreal injections, e.g., inflammation; and recognizing that some devices may contribute to an enhanced, safer, and faster intravitreal injection technique.

Clinical Corneal Optical Coherence Elastography Measurement Precision: Effect of Heartbeat and Respiration

Purpose

Normal physiological movements (e.g., respiration and heartbeat) induce eye motions during clinical measurements of human corneal biomechanical properties using optical coherence elastography (OCE). We quantified the effects of respiratory and cardiac-induced eye motions on clinical corneal OCE measurement precision and repeatability.

Methods

Corneal OCE was performed using low-force, micro-air-pulse tissue stimulation and high-resolution phase-sensitive optical coherence tomography (OCT) imaging. Axial surface displacements of the corneal apex were measured (M-mode) at a 70-kHz sampling rate and three different stimulation pressures (20-60 Pa). Simultaneously, the axial corneal position was tracked with structural OCT imaging, while the heartrate and respiration were monitored over a 90 second period.

Results

Respiratory- and cardiac-induced eye motions have distinctly lower frequency (0.1-1 Hz) and much greater amplitude (up to ± 50 µm movements) than air-pulse-induced corneal tissue deformations (∼250 Hz, <1 µm). The corneal displacements induced during OCE measurements in vivo were -0.41 ± 0.06 µm (n = 22 measurements, coefficient of variation [CV]: 14.6%) and -0.44 ± 0.07 µm (n = 50 measurements, CV: 15.9%), respectively, from two human subjects at 40 Pa stimulation pressure. Observed variation in corneal tissue displacements were not associated with tissue stimulation magnitude, or the amplitude of physiologically induced axial eye motion.

Conclusions

The microsecond timescale and submicron tissue displacements observed during corneal OCE measurements are separable from normal involuntary physiological movements, such as the oculocardiac pulse and respiratory movements.

Translational Relevance

This work advances innovations in biomedical imaging and engineering for clinical diagnostic applications for soft-tissue biomechanical testing.

Novel Anti-Interleukin-1β Therapy Preserves Retinal Integrity: A Longitudinal Investigation Using OCT Imaging and Automated Retinal Segmentation in Small Rodents

Retinopathy of prematurity (ROP) is the leading cause of blindness in neonates. Inflammation, in particular interleukin-1β (IL-1β), is increased in early stages of the disorder, and contributes to inner and outer retinal vasoobliteration in the oxygen-induced retinopathy (OIR) model of ROP. A small peptide antagonist of IL-1 receptor, composed of the amino acid sequence, rytvela, has been shown to exert beneficial anti-inflammatory effects without compromising immunovigilance-related NF-κB in reproductive tissues. We conducted a longitudinal study to determine the efficacy of “rytvela” in preserving the integrity of the retina in OIR model, using optical coherence tomography (OCT) which provides high-resolution cross-sectional imaging of ocular structures in vivo. Sprague–Dawley rats subjected to OIR and treated or not with “rytvela” were compared to IL-1 receptor antagonist (Kineret). OCT imaging and custom automated segmentation algorithm used to measure retinal thickness (RT) were obtained at P14 and P30; gold-standard immunohistochemistry (IHC) was used to confirm retinal anatomical changes. OCT revealed significant retinal thinning in untreated animals by P30, confirmed by IHC; these changes were coherently associated with increased apoptosis. Both rytvela and Kineret subsided apoptosis and preserved RT. As anticipated, Kineret diminished both SAPK/JNK and NF-κB axes, whereas rytvela selectively abated the former which resulted in preserved monocyte phagocytic function. Altogether, OCT imaging with automated segmentation is a reliable non-invasive approach to study longitudinally retinal pathology in small animal models of retinopathy.

Semantic Segmentation of the Choroid in Swept Source Optical Coherence Tomography Images for Volumetrics

The choroid is a complex vascular tissue that is covered with the retinal pigment epithelium. Ultra high speed swept source optical coherence tomography (SS-OCT) provides us with high-resolution cube scan images of the choroid. Robust segmentation techniques are required to reconstruct choroidal volume using SS-OCT images. For automated segmentation, the delineation of the choroidal-scleral (C-S) boundary is key to accurate segmentation. Low contrast of the boundary, scleral canals formed by the vessel and the nerve, and the posterior stromal layer, may cause segmentation errors. Semantic segmentation is one of the applications of deep learning used to classify the parts of images related to the meanings of the subjects. We applied semantic segmentation to choroidal segmentation and measured the volume of the choroid. The measurement results were validated through comparison with those of other segmentation methods. As a result, semantic segmentation was able to segment the C-S boundary and choroidal volume adequately.

Statistical framework for validation without ground truth of choroidal thickness changes detection

Monitoring subtle choroidal thickness changes in the human eye delivers insight into the pathogenesis of various ocular diseases such as myopia and helps planning their treatment. However, a thorough evaluation of detection-performance is challenging as a ground truth for comparison is not available. Alternatively, an artificial ground truth can be generated by averaging the manual expert segmentations. This makes the ground truth very sensitive to ambiguities due to different interpretations by the experts. In order to circumvent this limitation, we present a novel validation approach that operates independently from a ground truth and is uniquely based on the common agreement between algorithm and experts. Utilizing an appropriate index, we compare the joint agreement of several raters with the algorithm and validate it against manual expert segmentation. To illustrate this, we conduct an observational study and evaluate the results obtained using our previously published registration-based method. In addition, we present an adapted state-of-the-art evaluation method, where a paired t-test is carried out after leaving out the results of one expert at the time. Automated and manual detection were performed on a dataset of 90 OCT 3D-volume stack pairs of healthy subjects between 8 and 18 years of age from Asian urban regions with a high prevalence of myopia.

Imaging in myopia: potential biomarkers, current challenges and future developments

Myopia is rapidly increasing in Asia and around the world, while it is recognised that complications from high myopia may cause significant visual impairment. Thus, imaging the myopic eye is important for the diagnosis of sight-threatening complications, monitoring of disease progression and evaluation of treatments. For example, recent advances in high-resolution imaging using optical coherence tomography may delineate early myopic macula pathology, optical coherence tomography angiography may aid early choroidal neovascularisation detection, while multimodal imaging is important for monitoring treatment response. However, imaging the eye with high myopia accurately has its challenges and limitations, which are important for clinicians to understand in order to choose the best imaging modality and interpret the images accurately. In this review, we present the current imaging modalities available from the anterior to posterior segment of the myopic eye, including the optic nerve. We summarise the clinical indications, image interpretation and future developments that may overcome current technological limitations. We also discuss potential biomarkers for myopic progression or development of complications, including basement membrane defects, and choroidal atrophy or choroidal thickness measurements. Finally, we present future developments in the field of myopia imaging, such as photoacoustic imaging and corneal or scleral biomechanics, which may lead to innovative treatment modalities for myopia.

Attenuation correction assisted automatic segmentation for assessing choroidal thickness and vasculature with swept-source OCT

Swept source optical coherence tomography (SS-OCT) is being used more widely in clinical studies to investigate the choroid due to its deeper penetration under the retinal pigment epithelium and improved image quality compared with spectral domain OCT. However, automatic methods to reliably assess choroidal thickness and vasculature are still limited. This paper reports an approach that applies attenuation correction on SS-OCT structural scans to facilitate accurate automatic segmentation of the choroid and provides visualization of the choroidal vasculature without the necessity of OCT angiography. After attenuation correction, enhanced interlayer contrast at the choroidal-scleral interface was observed (from 0.13 ± 0.05 to 0.29 ± 0.10; P < 0.001). An algorithm that segmented the choroid from attenuation compensated B-scans achieved significantly higher accuracy when compared with an automated segmentation performed on regular OCT scans (91.8 ± 3.7% vs.74.5 ± 8.0%; P < 0.01). After attenuation correction, en face images of choroidal vessels were achieved with fewer artifacts from retinal vessels. Measurements of mean choroidal thickness and vessel density showed high repeatability. The attenuation correction assisted segmentation of the choroid and visualization of the choroidal vasculature will be helpful in studying the quantitative changes that occur in a myriad of diseases involving the choroid such as age-related macular degeneration, polypoidal choroidal vasculopathy, pathologic myopia, central serous chorioretinopathy, and inflammatory eye conditions.

Estimating outflow facility through pressure dependent pathways of the human eye

We develop and test a new theory for pressure dependent outflow from the eye. The theory comprises three main parameters: (i) a constant hydraulic conductivity, (ii) an exponential decay constant and (iii) a no-flow intraocular pressure, from which the total pressure dependent outflow, average outflow facilities and local outflow facilities for the whole eye may be evaluated. We use a new notation to specify precisely the meaning of model parameters and so model outputs. Drawing on a range of published data, we apply the theory to animal eyes, enucleated eyes and in vivo human eyes, and demonstrate how to evaluate model parameters. It is shown that the theory can fit high quality experimental data remarkably well. The new theory predicts that outflow facilities and total pressure dependent outflow for the whole eye are more than twice as large as estimates based on the Goldman equation and fluorometric analysis of anterior aqueous outflow. It appears likely that this discrepancy can be largely explained by pseudofacility and aqueous flow through the retinal pigmented epithelium, while any residual discrepancy may be due to pathological processes in aged eyes. The model predicts that if the hydraulic conductivity is too small, or the exponential decay constant is too large, then intraocular eye pressure may become unstable when subjected to normal circadian changes in aqueous production. The model also predicts relationships between variables that may be helpful when planning future experiments, and the model generates many novel testable hypotheses. With additional research, the analysis described here may find application in the differential diagnosis, prognosis and monitoring of glaucoma.

Choroidal vascular changes in age-related macular degeneration

PURPOSE

To assess the choroidal vascular changes using choroidal vascularity index (CVI) in patients with age-related macular degeneration (AMD) compared to controls.

METHODS

Enhanced depth imaging (EDI) optical coherence tomography (OCT) scans of 64 patients with unilateral or bilateral AMD were obtained. Images with a poorly demarcated choroidal-scleral interface (CSI) were excluded from the analysis. Foveal scans of 63 AMD eyes and 35 'normal fellow' eyes were analysed. Images of 30 eyes from 18 age-matched healthy subjects were included as controls. Choroidal vascularity index (CVI) was derived from binarization of EDI OCT images, using fiji software.

RESULTS

The mean age was 56.50 ± 5.50 years for AMD patients and 52.25 ± 6.75 years for controls. All patients were treatment naïve. Subfoveal choroidal thickness (SFCT) in AMD, 'normal fellow' eyes and controls was 314.02 ± 78.80 μm, 300.88 ± 53.85 μm and 278.5 ± 65.31 μm, respectively. Choroidal vascularity index (CVI) in AMD, 'normal fellow' eyes and controls was 64.04 ± 2.43%, 64.66 ± 2.25% and 66.07 ± 1.72%, respectively. Choroidal vascularity index (CVI) of both AMD and 'normal fellow' eyes was significantly lower compared to controls (p < 0.0001 and p = 0.007). The SFCT of AMD eye was not found to be significantly different from 'normal fellow eyes' (p = 0.45).

CONCLUSION

There was no statistical difference in SFCT, but CVI was significantly lower in patients with AMD. Choroidal vascularity index (CVI) was also lower in 'normal fellow' AMD eyes as compared to controls. This suggests possible reduction in choroidal vascularity in eyes with AMD and also to a certain extent in the 'normal fellow' eyes without phenotypical manifestations and may suggest underlying choroidal morphological change leading to wet AMD.

Pilot study of the pulsatile neuro-peripapillary retinal deformation in glaucoma and its relationship with glaucoma risk factors

PURPOSE

To perform a pilot study of the neuro-peripapillary retinal tissue deformation during the cardiac cycle among healthy eyes, ocular hypertensive (OHT), open angle glaucoma suspect (OAG-S), and early open angle glaucoma (EOAG) patients using video rate optical coherence tomography (OCT) image series.

METHODS

OCT line scan sequences of the same region of the optic nerve head (ONH) were obtained from 15 EOAG, 6 OHT, 10 OAG-S, and 10 healthy age-matched eyes. One eye per patient was studied. Changes in the axial distance between the inferotemporal peripapillary retina and the prelaminar tissue, in time, were determined using an automated custom made algorithm. Linear correlations between this neuro-peripapillary retinal (N-PP) deformation and variables measured during the full ophthalmic examination are analyzed.

RESULTS

Healthy eyes showed larger N-PP deformation (4.8 ± 1 µm) than the OHT (3.5 ± 0.3 µm, p = 0.015), OAG-S (3.8 ± 0.8 µm, p = 0.045), and EOAG (3.2 ± 0.7 µm, p < 0.001) groups. Eyes with lower ocular pulse amplitude, thinner RNFL's, or worse visual fields showed smaller N-PP deformation, depending on the diagnosis. A linear model to explain deformation within the EOAG group with intraocular pressure and systolic perfusion pressure as predictors was found to be significant (R² = 0.767, p < 0.001).

CONCLUSIONS

Smaller mean N-PP deformation was observed in the EOAG, OAG-S, and OHT groups compared to healthy eyes in this pilot study. The measured deformation correlated with risk factors for the glaucomatous optic neuropathy, but these correlations varied depending on the diagnosis. The role of pulsatile neuro-peripapillary retinal deformation in the pathophysiology of OAG remains to be determined.

Validation of Macular Choroidal Thickness Measurements from Automated SD-OCT Image Segmentation

PURPOSE

Spectral domain optical coherence tomography (SD-OCT) imaging permits in vivo visualization of the choroid with micron-level resolution over wide areas and is of interest for studies of ocular growth and myopia control. We evaluated the speed, repeatability, and accuracy of a new image segmentation method to quantify choroid thickness compared to manual segmentation.

METHODS

Two macular volumetric scans (25 × 30°) were taken from 30 eyes of 30 young adult subjects in two sessions, 1 hour apart. A single rater manually delineated choroid thickness as the distance between Bruch's membrane and sclera across three B-scans (foveal, inferior, and superior-most scan locations). Manual segmentation was compared to an automated method based on graph theory, dynamic programming, and wavelet-based texture analysis. Segmentation performance comparisons included processing speed, choroid thickness measurements across the foveal horizontal midline, and measurement repeatability (95% limits of agreement (LoA)).

RESULTS

Subjects were healthy young adults (n = 30; 24 ± 2 years; mean ± SD; 63% female) with spherical equivalent refractive error of -3.46 ± 2.69D (range: +2.62 to -8.50D). Manual segmentation took 200 times longer than automated segmentation (780 vs. 4 seconds). Mean choroid thickness at the foveal center was 263 ± 24 μm (manual) and 259 ± 23 μm (automated), and this difference was not significant (p = 0.10). Regional segmentation errors across the foveal horizontal midline (±15°) were ≤9 μm (median) except for nasal-most regions closest to the nasal peripapillary margin-15 degrees (19 μm) and 12 degrees (16 μm) from the foveal center. Repeatability of choroidal thickness measurements had similar repeatability between segmentation methods (manual LoA: ±15 μm; automated LoA: ±14 μm).

CONCLUSIONS

Automated segmentation of SD-OCT data by graph theory and dynamic programming is a fast, accurate, and reliable method to delineate the choroid. This approach will facilitate longitudinal studies evaluating changes in choroid thickness in response to novel optical corrections and in ocular disease.

In-vivo corneal pulsation in relation to in-vivo intraocular pressure and corneal biomechanics assessed in-vitro. An animal pilot study

The aim was to ascertain whether the characteristics of the corneal pulse (CP) measured in-vivo in a rabbit eye change after short-term artificial increase of intraocular pressure (IOP) and whether they correlate with corneal biomechanics assessed in-vitro. Eight New Zealand white rabbits were included in this study and were anesthetized. In-vivo experiments included simultaneous measurements of the CP signal, registered with a non-contact method, IOP, intra-arterial blood pressure, and blood pulse (BPL), at the baseline and short-term elevated IOP. Afterwards, thickness of post-mortem corneas was determined and then uniaxial tensile tests were conducted leading to estimates of their Young's modulus (E). At the baseline IOP, backward stepwise regression analyses were performed in which successively the ocular biomechanical, biometric and cardiovascular predictors were separately taken into account. Results of the analysis revealed that the 3rd CP harmonic can be statistically significantly predicted by E and central corneal thickness (Models: R² = 0.662, p < 0.005 and R² = 0.832, p < 0.001 for the signal amplitude and power, respectively). The 1st CP harmonic can be statistically significantly predicted by the amplitude and power of the 1st BPL harmonic (Models: R² = 0.534, p = 0.015 and R² = 0.509, p < 0.018, respectively). For elevated IOP, non-parametric analysis indicated significant differences for the power of the 1st CP harmonic (Kruskal-Wallis test; p = 0.031) and for the mean, systolic and diastolic blood pressures (p = 0.025, p = 0.019, p = 0.033, respectively). In conclusion, for the first time, the association between parameters of the CP signal in-vivo and corneal biomechanics in-vitro was confirmed. In particular, spectral analysis revealed that higher amplitude and power of the 3rd CP harmonic indicates higher corneal stiffness, while the 1st CP harmonic correlates positively with the corresponding harmonic of the BPL signal.

4D Graph-Based Segmentation for Reproducible and Sensitive Choroid Quantification From Longitudinal OCT Scans

Purpose

Longitudinal imaging is becoming more commonplace for studies of disease progression, response to treatment, and healthy maturation. Accurate and reproducible quantification methods are desirable to fully mine the wealth of data in such datasets. However, most current retinal OCT segmentation methods are cross-sectional and fail to leverage the inherent context present in longitudinal sequences of images.

Methods

We propose a novel graph-based method for segmentation of multiple three-dimensional (3D) scans over time (termed 3D + time or 4D). The usefulness of this approach in retinal imaging is illustrated in the segmentation of the choroidal surfaces from longitudinal optical coherence tomography (OCT) scans. A total of 3219 synthetic (3070) and patient (149) OCT images were segmented for validation of our approach.

Results

The results show that the proposed 4D segmentation method is significantly more reproducible (P < 0.001) than the 3D approach and is significantly more sensitive to temporal changes (P < 0.0001) achieved by the substantial increase of measurement robustness.

Conclusions

This is the first automated 4D method for jointly quantifying choroidal thickness in longitudinal OCT studies. Our method is robust to image noise and produces more reproducible choroidal thickness measurements than a sequence of independent 3D segmentations, without sacrificing sensitivity to temporal changes.

Open-source algorithm for automatic choroid segmentation of OCT volume reconstructions

The use of optical coherence tomography (OCT) to study ocular diseases associated with choroidal physiology is sharply limited by the lack of available automated segmentation tools. Current research largely relies on hand-traced, single B-Scan segmentations because commercially available programs require high quality images, and the existing implementations are closed, scarce and not freely available. We developed and implemented a robust algorithm for segmenting and quantifying the choroidal layer from 3-dimensional OCT reconstructions. Here, we describe the algorithm, validate and benchmark the results, and provide an open-source implementation under the General Public License for any researcher to use (https://www.mathworks.com/matlabcentral/fileexchange/61275-choroidsegmentation).

Choroidal Vascularity Index in Vogt-Koyanagi-Harada Disease: An EDI-OCT Derived Tool for Monitoring Disease Progression

Purpose

We assessed the application of the choroidal vascularity index (CVI) in the follow-up of Vogt-Koyanagi-Harada disease (VKH) patients derived from image binarization of enhanced depth imaging optical coherence tomography (EDI-OCT) images with Fiji software. Our secondary objective was to derive the retinochoroidal vascularity index based on en face fundus fluorescein and indocyanine green angiography (FFA and ICGA).

Methods

In this retrospective cohort study, EDI-OCT scans of 18 eyes of 9 patients with VKH were obtained at baseline within 2 weeks of acute presentation, and again at 6 to 12 months. Images with poor quality were excluded. Choroidal thickness (CT) and CVI were analyzed and compared to 13 eyes of 13 healthy controls. En face FFA and ICGA obtained from 12 eyes of 7 patients were segmented to derive retinochoroidal vascularity index.

Results

There was no statistical difference in age or sex between the study group and controls. Choroidal thickness of patients with VKH was 359.23 ± 57.63 μm at baseline, compared to 274.09 ± 56.98 μm in controls (P = 0.003). Follow-up CT in VKH patients was 282.62 ± 42.51 μm, which was significantly decreased from baseline (P = 0.0001). Choroidal vascularity index in VKH patients was 70.03 ± 1.93% at baseline, compared to 64.63 ± 1.92% in controls (P < 0.001). Choroidal vascularity index was 66.94 ± 1.82% at follow-up, significantly reduced from baseline (P < 0.0001). Fundus fluorescein angiography and ICGA retinochoroidal vascularity indices at baseline were 70.67 ± 2.65% and 66.42 ± 2.16%, respectively.

Conclusions

In this small series of VKH patients, EDI-OCT–derived CVI had a statistically significant reduction over time, similar to CT. We propose that OCT, FFA, and ICGA-derived vascularity indices may be potential novel supportive tools in monitoring disease progression in VKH.

Translational Relevance

Choroidal vascularity index can be used potentially to study and analyze the structural changes in choroid. It can be a useful tool to explain the changes in the CT in different retinochoroidal disorders. Choroidal vascularity index also can be used for longitudinal follow-up in patients with VKH disease and other inflammatory disease involving the choroid.

Influence of Age on Ocular Biomechanical Properties in a Canine Glaucoma Model with ADAMTS10 Mutation

Soft tissue often displays marked age-associated stiffening. This study aims to investigate how age affects scleral biomechanical properties in a canine glaucoma model with ADAMTS10 mutation, whose extracellular matrix is concomitantly influenced by the mutation and an increased mechanical load from an early age. Biomechanical data was acquired from ADAMTS10-mutant dogs (n = 10, 21 to 131 months) and normal dogs (n = 5, 69 to 113 months). Infusion testing was first performed in the whole globes to measure ocular rigidity. After infusion experiments, the corneas were immediately trephined to prepare scleral shells that were mounted on a pressurization chamber to measure strains in the posterior sclera using an inflation testing protocol. Dynamic viscoelastic mechanical testing was then performed on dissected posterior scleral strips and the data were combined with those reported earlier by our group from the same animal model (Palko et al, IOVS 2013). The association between age and scleral biomechanical properties was evaluated using multivariate linear regression. The relationships between scleral properties and the mean and last measured intraocular pressure (IOP) were also evaluated. Our results showed that age was positively associated with complex modulus (p<0.001) and negatively associated with loss tangent (p<0.001) in both the affected and the normal groups, suggesting an increased stiffness and decreased mechanical damping with age. The regression slopes were not different between the groups, although the complex modulus was significantly lower in the affected group (p = 0.041). The posterior circumferential tangential strain was negatively correlated with complex modulus (R = -0.744, p = 0.006) showing consistent mechanical evaluation between the testing methods. Normalized ocular rigidity was negatively correlated with the last IOP in the affected group (p = 0.003). Despite a mutation that affects the extracellular matrix and a chronic IOP elevation in the affected dogs, age-associated scleral stiffening and loss of mechanical damping were still prominent and had a similar rate of change as in the normal dogs.

Automatic segmentation of the optic nerve head for deformation measurements in video rate optical coherence tomography

Optical coherence tomography (OCT) imaging has become a standard diagnostic tool in ophthalmology, providing essential information associated with various eye diseases. In order to investigate the dynamics of the ocular fundus, we present a simple and accurate automated algorithm to segment the inner limiting membrane in video-rate optic nerve head spectral domain (SD) OCT images. The method is based on morphological operations including a two-step contrast enhancement technique, proving to be very robust when dealing with low signal-to-noise ratio images and pathological eyes. An analysis algorithm was also developed to measure neuroretinal tissue deformation from the segmented retinal profiles. The performance of the algorithm is demonstrated, and deformation results are presented for healthy and glaucomatous eyes.