Magnification characteristics of the Optical Coherence Tomograph STRATUS OCT 3000

Three-Dimensional Quantitative Analysis of Internal Limiting Membrane Peeling Related Structural Changes in Retinal Detachment Repair

PURPOSE

To assess macular microstructural changes associated with internal limiting membrane peeling (ILMP) using 3-dimensional optical coherence tomography (3D-OCT) in primary macula-off rhegmatogenous retinal detachment (RRD) repairs with vitrectomy and silicone oil (SO) tamponade.

DESIGN

Retrospective, consecutive, interventional case series.

METHODS

Setting: Institutional practice.

PATIENT POPULATION

Patients who received primary RRD repair by a single experienced surgeon between January 2017 and December 2021.

MAIN OUTCOME MEASURES

In the qualitative comparative analysis, the presence of macular changes among patients who underwent primary RRD repair with (21 eyes) or without ILMP (20 eyes) were observed. Subsequently, a detailed quantitative analysis of ILMP-related microstructural changes in 56 eyes using both 3D and 2D-OCT images were performed.

RESULTS

In the qualitative comparative analysis, macular microstructural changes were observed in 95% of ILMP eyes and 5% of non-ILMP eyes (p < .001). In the quantitative analysis, 4 major macular microstructural changes were detected: dimple (75%), dissociated nerve fiber layer (DONFL) (55%), ILM peeling edge thinning (IPET) (64%), and temporal macular groove (TMG) (23%). Dimples (n = 251, average 4.5 ± 5.8 per eye) could be further classified into type I (confined to the inner plexiform layer [IPL]; 73%) and type II (beyond IPL, 27%). The average depth of the deepest dimples was 58 ± 18 µm. The extent of IPET was 6.0 ± 3.7 clock hours. The average length of TMG was 1.8 ± 0.4 mm. Comparing to unoperated fellow eyes, the eyes after ILMP showed decreased inner temporal over nasal retinal thickness ratio (0.86 ± 0.07 versus 0.96 ± 0.03, p < .001), shorter disc-fovea distance (4.61 ± 0.32 µm versus 4.78 ± 0.37 µm, p = .041), and wider retinal vein trajectories (c' = 2.48 ± 0.84 vs 3.39 ± 1.61, p = .002).

CONCLUSIONS

Macular microstructural changes are common after ILMP in RRD repair, encompassing both focal changes (dimples, DONFL) and zonal changes (IPET, TMG). DONFL and dimples may be part of a continuum of findings stemming from the same mechanism. IPET and TMG are the results of macular tissue shift due to contracture of the optic disc and neurovascular bundle.

Effects of Myopia on Rates of Change in Optical Coherence Tomography Measured Retinal Layer Thicknesses in People with Multiple Sclerosis and Healthy Controls

PURPOSE

To quantify the associations of myopia with longitudinal changes in retinal layer thicknesses in people with multiple sclerosis (PwMS) and healthy controls (HC).

METHODS

A cohort of PwMS and HC with recorded refractive error (RE) prospectively scanned on Cirrus HD-OCT at the Johns Hopkins MS Center was assessed for inclusion. Exclusion criteria included OCT follow-up < 6 months, ocular comorbidities, incidental OCT pathologies, and inadequate scan quality. Eyes were classified as having high myopia (HM) (RE≤ -6 diopters), low myopia (LM) (RE> -6 and ≤ -3 diopters), or no myopia (NM) (RE> -3 and ≤ +2.75). Linear mixed-effects regression models were used in analyses.

RESULTS

A total of 213 PwMS (eyes: 67 HM, 98 LM, 207 NM) and 80 HC (eyes: 26 HM, 37 LM, 93 NM) were included. Baseline average ganglion cell/inner plexiform (GCIPL) and peri-papillary retinal nerve fiber layer (pRNFL) thicknesses were lower in MS HM compared with MS NM (diff: -3.2 µm, 95% CI: -5.5 to -0.8, p = 0.008 and -5.3 µm, 95% CI: -9.0 to -1.7, p = 0.004, respectively), and similarly in HC HM, as compared with HC NM. Baseline superior, inferior, and nasal pRNFL thicknesses were lower in HM compared with NM, while temporal pRNFL thickness was higher, both in MS and HC (MS: 7.1 µm, 95% CI: 2.7-11.6, p = 0.002; HC: 4.7 µm, 95% CI: -0.3 to 9.7, p = 0.07). No longitudinal differences in rates of GCIPL change were noted between HM and LM vs. NM, either in MS or HC.

CONCLUSION

Cross-sectional differences in average GCIPL and pRNFL thicknesses are commonly seen in people with HM as compared to reference normative values from people with NM and can lead to false attribution of pathology if RE is not taken into account. However, our study suggests that longitudinal changes in average GCIPL thickness in PwMS with myopia are similar in magnitude to PwMS with NM, and therefore are appropriate for monitoring disease-related pathology.

Effect of Ocular Magnification on Macular Choroidal Thickness Measurements Made Using Optical Coherence Tomography in Children

PURPOSE

To evaluate the relationship between ocular magnification correction and macular choroidal thickness (ChT) measurements in children, and to demonstrate when ocular magnification correction is necessary.

METHODS

Chinese children aged 6-9 years with various refractive statuses were included. Swept-source optical coherence tomography was used to measure macular ChT. A self-designed program was adopted to simulate ChT changes in each sector of the ETDRS grid in the macula under various simulated axial lengths (ALs).

RESULTS

ChT measurements were not affected for all simulated ALs in over 95% of the individuals in the central fovea. In the inferior, superior, and temporal parafoveal sectors, the extent of AL that may include 95% of the individuals narrowed from approximately 22.0 mm to 27.2 mm. In the nasal parafoveal sector and inferior, superior, and temporal perifoveal sectors, the extent of AL that may include 95% of the individuals became even narrower, from approximately 22.8 mm to 26.0 mm. The narrowest extent was observed in the perifoveal nasal sector, ranging from 23.3 mm to 25.5 mm. The effect of ocular magnification was more significant in hyperopes than in myopes in the inferior parafoveal sector and temporal, superior, and nasal perifoveal sectors.

CONCLUSION

During macular ChT measurements, ocular magnification correction is not necessary in the central fovea. However, ocular magnification should be corrected normally in the nasal perifoveal region and in individuals with ALs shorter than 22.8 mm or longer than 26.0 mm in the remaining macular regions.

The effect of transverse ocular magnification adjustment on macular thickness profile in different refractive errors in community-based adults

Purpose

Changes in retinal thickness are common in various ocular diseases. Transverse magnification due to differing ocular biometrics, in particular axial length, affects measurement of retinal thickness in different regions. This study evaluated the effect of axial length and refractive error on measured macular thickness in two community-based cohorts of healthy young adults.

Methods

A total of 2160 eyes of 1247 community-based participants (18–30 years; 23.4% myopes, mean axial length = 23.6mm) were included in this analysis. Macular thickness measurements were obtained using a spectral-domain optical coherence tomography (which assumes an axial length of 24.385mm). Using a custom program, retinal thickness data were extracted at the 9 Early Treatment of Diabetic Retinopathy Study (ETDRS) regions with and without correction for transverse magnificent effects, with the corrected measurements adjusting according to the participant’s axial length. Linear mixed models were used to analyse the effect of correction and its interaction with axial length or refractive group on retinal thickness.

Results

The raw measures (uncorrected for axial length) underestimated the true retinal thickness at the central macula, while overestimating at most non-central macular regions. There was an axial length by correction interaction effect in all but the nasal regions (all p<0.05). For each 1mm increase in axial length, the central macular thickness is overestimated by 2.7–2.9μm while thicknesses at other regions were underestimated by 0.2–4.1μm. Based on the raw thickness measurements, myopes have thinner retinas than non-myopes at most non-central macular. However, this difference was no longer significant when the corrected data was used.

Conclusion

In a community-based sample, the raw measurements underestimate the retinal thickness at the central macula and overestimate the retinal thickness at non-central regions of the ETDRS grid. The effect of axial length and refractive error on retinal thickness is reduced after correcting for transverse magnification effects resulting from axial length differences.

Induced Refractive Error Changes the Optical Coherence Tomography Angiography Transverse Magnification and Vascular Indices

PURPOSE

To assess the effect of changing anterior eye refractive power with contact lenses on the transverse magnification of en face images and associated vascular indices from optical coherence tomographic angiography (OCT-A).

DESIGN

Prospective crossover study.

METHODS

Spherical soft contact lenses (-6 diopter [D] to +6 D in 2 D steps) were used to induce anterior eye refractive power changes in 11 healthy young adults and 3 × 3-mm macular scans were captured using OCT-A (Zeiss AngioPlex, software version 11.0; Cirrus HD-OCT 5000, Carl Zeiss Meditec Inc). Image transverse magnification was predicted based on refraction and biometry measurements and compared with empirical changes in the en face images measured with image analysis. Linear regression analysis was performed to assess the relationship between induced refractive ametropia and foveal avascular zone (FAZ) area, perimeter, circularity, and vessel density and perfusion density.

RESULTS

The predicted transverse magnification was linearly related to induced refractive ametropia and to the empirical transverse magnification changes (average slope: 1.02, 95% CI: 0.90-1.34). All the OCT-A indices showed linear relationships with induced refractive ametropia (P < .05) with the 12 D tested range altering the indices by 7% to 12%. After correcting for transverse magnification, all OCT-A indices except FAZ area were linearly related to induced refractive ametropia (P < .05) and were reduced to 1% to 9%.

CONCLUSIONS

This study is the first to show that induced refractive ametropia can affect OCT-A image magnification and indices. These changes are clinically important and need to be considered along with biometry effects when interpreting OCT-A indices. Transverse magnification changes can affect the ability of OCT-A to precisely measure linear dimensions of blood vessels.

Evaluation of Peripapillary Choroidal Microvasculature to Detect Glaucomatous Damage in Eyes With High Myopia

PRECIS

Parapapillary choroidal microvasculature dropout (MvD), as observed by optical coherence tomography (OCT) angiography, was useful to detect glaucomatous damage in highly myopic eyes with unreliable OCT results due to segmentation errors.

PURPOSE

The purpose of this study was to evaluate the usefulness of optical coherence tomography angiography (OCTA) imaging of the peripapillary choroidal microvasculature in detecting glaucomatous damage in highly myopic eyes, in cases where evaluating the thickness of the retinal nerve fiber layer (RNFL) is unreliable due to OCT segmentation errors.

MATERIALS AND METHODS

Forty-five highly myopic eyes with primary open-angle glaucoma (POAG) with an axial length >26.5 mm, and 15 age-matched and axial length-matched 15 control eyes were included in this cross-sectional observational study. All participants had a segmentation error in OCT circumpapillary RNFL scanning. The peripapillary choroidal microvasculature was evaluated on en-face images obtained using swept-source OCTA. MvD was defined as a focal sectoral capillary dropout with no visible microvascular network identified in the choroidal layer. The topographic correlation between the MvD and a hemifield visual field (VF) defect was assessed using κ statistics. The MvD size was assessed by measuring both its area and circumferential extent.

RESULTS

Choroidal MvD was observed in 44 of the 45 (97.8%) POAG eyes with high myopia, while none of the control eyes showed a choroidal MvD. There was an excellent topographic relationship between the choroidal MvD and the hemifield VF defect (κ=0.863, P<0.001). The area (R=0.2619, P=0.0006) and circumferential extent (R=0.3088, P=0.0002) of the MvD have significantly associated with the VF mean deviation.

CONCLUSIONS

Choroidal MvDs were observed in most of the highly myopic POAG eyes and were topographically correlated with the location of glaucomatous VF defects despite unreliable OCT RNFL thickness measurements. Using OCTA to investigate the choroidal microvasculature may facilitate diagnoses of glaucoma in highly myopic eyes.

Spectral Domain Optical Coherence Tomography in Awake Rabbits Allows Identification of the Visual Streak, a Comparison with Histology

Purpose

To evaluate visual streak (VS) identification on spectral-domain optical coherence tomography (SD-OCT) scans in awake rabbits. To report thickness measurements in the VS and adjacent retina on OCT B-scans and histologic sections and to assess inter-method bias, precision and repeatability between OCT and histology.

Methods

Vertical SD-OCT B-scan images through the optic nerve head and VS were acquired from 16 awake, ophthalmologically healthy experimental rabbits. Scans were acquired from both eyes, which were later enucleated and processed for light microscopy. Inner retina, inner nuclear layer, outer nuclear layer, outer retina (OR) and photoreceptor outer segment (PROS) thickness were measured on OCT images and digitalized microscopy slides in- and outside of the VS, and compared using linear mixed effects models.

Results

Both SD-OCT and histology allowed retinal layer identification and measurement. On OCT, OR and PROS were thickest in the central VS and thinnest outside the VS. Histology mirrored OCT results for central outer retinal layers but shows discrepancies for other layers likely because of postmortem processing artifacts. The method comparison demonstrated better repeatability for OCT measurements compared with histology.

Conclusions

Increased OR and PROS thickness compared with the adjacent retina allowed identification of the VS on SD-OCT in awake rabbits. OCT allows measurements devoid of processing artifacts in contrast to histology.

Translational Relevance

SD-OCT is possible in awake rabbits. Easy and reliable identification of the VS may facilitate the positioning and use of rabbits as model species in human macular and generalized retinal disease research.

Comparison of optical coherence tomography measurements between high hyperopic and low hyperopic children

Purpose:

To identify the peripapillary retinal nerve fiber layer, total macular, ganglion cell layer, and inner plexiform layer thicknesses in children with high hyperopia using spectral domain optical coherence tomography.

Methods:

Twenty-one children with high hyperopia and 20 controls were enrolled in this study. Subjects with spherical equivalent +5.0 D or higher were evaluated in the study group and subject with spherical equivalent between +0.25 and +2.0 D in the control group. The retinal nerve fiber layer thickness, macular thickness, macular ganglion cell layer and inner plexiform layer thicknesses were measured using a spectral domain optical coherence tomography, and results were compared between groups.

Results:

The nasal and inferior quadrant and the global retinal nerve fiber layer thickness were significantly thicker in the study group. The mean thickness of inferior quadrant of the inner macula was significantly thicker in the study group than those in the control group. The mean thickness of the ganglion cell layer in nasal, temporal and inferior quadrant of outer macula was significantly thinner in the study group than the control group. The mean thickness of the inner plexiform layer in the inferior quadrant of the inner macula and nasal and inferior quadrant of the outer macula were significantly higher in study group than those in control group.

Conclusion:

High hyperopic children had thicker retinal nerve fiber layer when compared to the controls. This difference should be taken into account when evaluating children with glaucoma or other optic disc disorders.

Lessons learned from quantitative fundus autofluorescence

Quantitative fundus autofluorescence (qAF) is an approach that is built on a confocal scanning laser platform and used to measure the intensity of the inherent autofluorescence of retina elicited by short-wavelength (488 nm) excitation. Being non-invasive, qAF does not interrupt tissue architecture, thus allowing for structural correlations. The spectral features, cellular origin and topographic distribution of the natural autofluorescence of the fundus indicate that it is emitted from retinaldehyde-adducts that form in photoreceptor cells and accumulate, under most conditions, in retinal pigment epithelial cells. The distributions and intensities of fundus autofluorescence deviate from normal in many retinal disorders and it is widely recognized that these changing patterns can aid in the diagnosis and monitoring of retinal disease. The standardized protocol employed by qAF involves the normalization of fundus grey levels to a fluorescent reference installed in the imaging instrument. Together with corrections for magnification and anterior media absorption, this approach facilitates comparisons with serial images and images acquired within groups of patients. Here we provide a comprehensive summary of the principles and practice of qAF and we highlight recent efforts to elucidate retinal disease processes by combining qAF with multi-modal imaging.

Structure-Function Relationships in Perimetric Glaucoma: Comparison of Minimum-Rim Width and Retinal Nerve Fiber Layer Parameters

Purpose

To test the hypotheses that: (1) structure–function (SF) relationships between visual fields (VF) and Bruch's membrane opening-based minimum rim width (BMO-MRW) measurements are superior to those for peripapillary retinal nerve fiber layer (pRNFL) in perimetric glaucoma; (2) BMO-MRW measurements may extend the utility of structural measurement across the range of glaucoma severity; and (3) to estimate the influence of Bruch's membrane opening (BMO) size on BMO-MRW measurements.

Methods

One hundred eight perimetric glaucoma eyes (68 patients) with good quality spectral-domain optical coherence tomography images of the optic disc and pRNFL, and reliable VF within 6 months were recruited. Relationship of global and sectoral BMO-MRW and pRNFL thickness with corresponding VF parameters and the influence of normalizing BMO-MRW (on BMO circumference, nBMO-MRW) on SF relationships were investigated. Broken-stick models were used to compare the point at which pRNFL and BMO-MRW parameters reached their measurement floor.

Results

The median (interquartile range) of VF mean deviation was −5.9 (−12.6 to −3.6) dB. Spearman correlation coefficients between pRNFL, BMO-MRW, and nBMO-MRW measures and corresponding VF cluster average deviations ranged between 0.55 to 0.80, 0.35 to 0.66, and 0.38 to 0.65, respectively. Bruch's membrane opening–MRW parameters demonstrated weaker SF relationships compared with pRNFL globally and in temporal, temporal-superior, and nasal-inferior sectors (P < 0.03). Normalization of BMO-MRW did not significantly influence SF relationships.

Conclusions

Structure–function relationships were somewhat weaker with BMO-MRW parameters compared with pRNFL in eyes with perimetric glaucoma. Bruch's membrane opening–MRW normalization did not significantly change SF relationships in this group of eyes with mild departures from average BMO size.

Postnatal maturation of the fovea in Macaca mulatta using optical coherence tomography

Changes in the foveal anatomy during infancy are an important component in early development of spatial vision. The present longitudinal study in rhesus monkeys was undertaken to characterize the postnatal maturation of the fovea. Starting at four weeks after birth, the retinas of the left eyes of sixteen infant monkeys were imaged using spectral domain optical coherence tomography (SD OCT). Retinal scans were repeated every 30 days during the first year of life and every 60 days thereafter. Volume scans through the fovea were registered, scaled using a three surface schematic eye, and analyzed to measure foveal pit parameters. The individual layers of the retina were manually segmented and thicknesses were measured over a transverse distance of 1250 microns from the center of the foveal pit. Based on infrared scanning laser ophthalmoscope (IR SLO) images acquired with the SD OCT system, there were significant changes in the extent of the retina scanned as the eyes matured. Using a three-surface schematic eye, the length of each scan could be computed and was validated using image registration (R² = 0.88, slope = 1.003, p < 0.05). Over the first 18 months of life, the mean retinal thickness at the pit center had increased by 21.4% with a corresponding 20.3% decrease in pit depth. The major changes occurred within the first 120 days, but did not stabilize until a year after birth. In Macaca mulatta infants, the primary anatomical maturation of the fovea occurs within the first few months of life, as determined by longitudinal data from SD OCT measurements. The timelines for maturation of the fovea correspond well with the normal development of the lateral geniculate nucleus, cortical neurophysiology, and spatial resolution in monkeys.

Macular thickness in healthy eyes of adults (N = 4508) and relation to sex, age and refraction: the Tromsø Eye Study (2007-2008)

Purpose

To provide sex‐stratified normative data on retinal thickness and study the relationship with sex, age and refractive status.

Methods

Population‐based study including 2617 women and 1891 men, aged 38–87 (mean 61 ± 8) years, without diabetes, glaucoma and retinal diseases, and spherical equivalent refraction (SER) within ±6 dioptres. Retinal thickness was measured with optical coherence tomography (spectral domain Cirrus HD‐OCT).

Results

Women had thinner retina than men. Retinal thickness was significantly associated with refraction, where mean change in retinal thickness per 1 D increase in SER was −1.3 (0.2) μm in the fovea, 0.7 (0.1) μm in the pericentral ring and 1.4 (0.1) μm in the peripheral ring. In the fovea, there was a non‐monotonic curved relationship between retinal thickness and age in both sexes with a maximum at about 60 years (p < 0.001). In the pericentral ring, the mean reduction in retinal thickness per 10‐year increase was 2.7 (0.3) μm in women and 4.0 (0.4) μm in men and corresponding results in the peripheral ring were 2.3 (0.3) μm in women and 2.6 (0.4) μm in men. In both regions, there was evidence for a nonlinear pattern with an increased rate of change with higher age. There was a significant interaction between sex and age for retinal thickness of the pericentral ring (p = 0.041).

Conclusion

Women had thinner retina than men, and thickness varied with refractive status. Retinal thickness was associated with age in all macular regions, and the rate of change in retinal thickness varied at different ages.

The Association between Foveal Morphology and Macular Pigment Spatial Distribution: An Ethnicity Study

Purpose

Macular pigment (MP) spatial distribution varies considerably among individuals. We investigated ethnic variations in MP spatial distribution in relation to foveal architecture.

Methods

We measured MP optical density (MPOD) using heterochromatic flicker photometry (MAP test, City, University of London) in 76 white, 80 South Asian and 70 black volunteers (18 to 39 years). MPOD spatial profiles were classified objectively as exponential, ring-like or central dip, based on deviations away from an exponential fit. Measurements including total retinal thickness (RT), inner retinal layer (IRL), inner and outer plexiform layer (IPL and OPL) thickness, foveal width and foveal pit slope were taken from Spectralis SD-OCT (Heidelberg, Germany) scans.

Results

Integrated MPOD up to 1.8° (MPODint) was higher in South Asian (0.84±0.26) and black (0.84±0.31) than whites (0.63±0.24, P<0.0005). Ethnicity explained around 10% of the variance while gender played no significant role. MPOD profile phenotypes were associated with ethnicity: 58% with ring profiles were South Asian and 43% with dip profiles were black (χ²(4,226) = 13.4, P = 0.009). MPODint was lower in exponential (0.66±0.21) compared to ring-like (0.96±0.26) and central dip (1.00±0.32, P<0.0005) groups. White subjects had thicker IRL at 0° (130±21μm) than South Asian (123±16μm) and blacks (116±14μm; F(2) = 12.4, P<0.0005), with comparable results for IPL (P<0.0005) and OPL (P = 0.03). There was no significant difference in IRL, IPL or OPL (from 0 to 3.8° retinal eccentricity) or foveal width between MP profile groups (P>0.05).

Conclusion

We report a significant difference in the amount and distribution of MP between ethnicities that is not explained by variations in foveal morphology.

Effect of ocular magnification on macular measurements made using spectral domain optical coherence tomography

Aim:

The aim of the present study was to study the effect of ocular magnification on macular measurements made using spectral domain optical coherence tomography (OCT).

Materials and Methods:

One hundred and fifty-one subjects were included from the normative study of foveal morphology carried out at our hospital. Subjects underwent comprehensive eye examination and macular scanning using Cirrus high-definition OCT and axial length (AXL) measurement. Macular cube 512 × 128 scan protocol was used for scanning the macula. Automated measurements of the fovea namely foveal diameter, foveal slope (lateral measurements) and foveal depth (axial measurement) were taken. A correction factor for ocular magnification was done using the formula t = p × q × s, where “t” is the corrected measurement, “p” is the magnification of OCT, “q” is the ocular magnification, and “s” is the measurement on OCT without correction. The difference between corrected and uncorrected measurements was evaluated for statistical significance.

Results:

Mean AXL was 22.95 ± 0.78 mm. Refractive error ranged from −3D to +4D. Mean difference between measured and corrected foveal diameter, slope and depth was 166.05 ± 95.37 µm (P < 0.001), 0.81° ± 0.53° (P < 0.001) and 0.05 ± 0.49 µm (P = 0.178) respectively. AXL lesser than the OCT calibrated value of 24.46 mm showed an increased foveal diameter (r = 0.961, P < 0.001) and a reduced foveal slope (r = −0.863, P < 0.001) than the corrected value.

Conclusion:

Lateral measurements made on OCT varied with AXL s other than the OCT calibrated value of 24.46 mm. Therefore, to estimate the actual dimensions of a retinal lesion using OCT, especially lateral dimensions, we recommend correction for the ocular magnification factor.

The Relationship of the Clinical Disc Margin and Bruch's Membrane Opening in Normal and Glaucoma Subjects

Purpose

We tested the hypotheses that the mismatch between the clinical disc margin (CDM) and Bruch's membrane opening (BMO) is a function of BMO area (BMOA) and is affected by the presence of glaucoma.

Methods

A total of 45 normal eyes (45 subjects) and 53 glaucomatous eyes (53 patients) were enrolled and underwent radial optic nerve head (ONH) imaging with spectral domain optical coherence tomography. The inner tip of the Bruch's membrane (BM) and the clinical disc margin were marked on radial scans and optic disc photographs, and were coregistered with custom software. The main outcome measure was the difference between the clinical disc area (CDA) and BMOA, or CDA−BMOA mismatch, as a function of BMOA and diagnosis. Multivariate regression analyses were used to explore the influence of glaucoma and BMOA on the mismatch.

Results

Global CDA was larger than BMOA in both groups but the difference was statistically significant only in the normal group (1.98 ± 0.37 vs. 1.85 ± 0.45 mm², P = 0.02 in the normal group; 1.96 ± 0.38 vs. 1.89 ± 0.56 mm², P = 0.08 in the glaucoma group). The sectoral CDA−BMOA mismatch was smaller in superotemporal (P = 0.04) and superonasal (P = 0.05) sectors in the glaucoma group. The normalized CDA−BMOA difference decreased with increasing BMOA in both groups (P < 0.001). Presence or severity of glaucoma did not affect the CDA−BMOA difference (P > 0.14).

Conclusions

Clinical disc area was larger than BMOA in normal and glaucoma eyes but reached statistical significance only in the former group. The CDA−BMOA mismatch diminished with increasing BMOA but was not affected by presence of glaucoma. These findings have important clinical implications regarding clinical evaluation of the ONH.

Effects of Refractive Power on Macular Thickness Measurement Using Spectral-Domain Optical Coherence Tomography

PURPOSE

To investigate the effects of refractive power on macular thickness measurements by spectral-domain optical coherence tomography (SD-OCT).

SUBJECTS AND METHODS

For this prospective cohort study, a total of 50 eyes from 50 normal subjects with no systemic or ocular disease and no history of ocular surgery were studied from February 2014 to May 2014. We used soft contact lenses with a wide range of diopters to change the refractive power. The macular thickness of uncorrected eyes without contact lenses was measured by SD-OCT, and we compared the findings with the results of subsequent investigations in which macular thickness was measured in the presence of soft contact lenses of different diopters (-8, -4, 0, +4, and +8). We divided the patients into three groups according to the axial length (AL) and analyzed the effect of induced refraction change. The main outcome measure was macular thickness measured by OCT.

RESULTS

The average age of the subjects was 28.0 ± 3.4 years (mean ± SD), and included 17 eyes with normal AL, 18 eyes with mid-range AL, and 15 eyes with long AL. The central macular thickness was 254.5 ± 17.5 µm for eyes without contact lenses, which was not significantly different from the measurements in the presence of plano contact lenses (254.2 ± 18.1 µm). Even with soft contact lenses of four different diopters (-8, -4, +4, and +8), central macular thickness (254.4 ± 16.2, 253.7 ± 17.3, 257.3 ± 17.9, and 256.9 ± 17.9 µm, respectively) was not significantly different from that of naked eyes in each group.

CONCLUSION

These results suggest that central macular thickness measured by SD-OCT is unaffected by refractive power.

The effects of ocular magnification on Spectralis spectral domain optical coherence tomography scan length

PURPOSE

The purpose of this study was to assess the effects of incorporating individual ocular biometry measures of corneal curvature, refractive error, and axial length on scan length obtained using Spectralis spectral domain optical coherence tomography (SD-OCT).

METHODS

Two SD-OCT scans were acquired for 50 eyes of 50 healthy participants, first using the Spectralis default keratometry (K) setting followed by incorporating individual mean-K values. Resulting scan lengths were compared to predicted scan lengths produced by image simulation software, based on individual ocular biometry measures including axial length.

RESULTS

Axial length varied from 21.41 to 29.04 mm. Spectralis SD-OCT scan lengths obtained with default-K ranged from 5.7 to 7.3 mm, and with mean-K from 5.6 to 7.6 mm. We report a stronger correlation of simulated scan lengths incorporating the subject's mean-K value (ρ = 0.926, P < 0.0005) compared to Spectralis default settings (ρ = 0.663, P < 0.0005).

CONCLUSIONS

Ocular magnification appears to be better accounted for when individual mean-K values are incorporated into Spectralis SD-OCT scan acquisition versus using the device's default-K setting. This must be considered when taking area measurements and lateral measurements parallel to the retinal surface.

Age-associated changes in the retinal nerve fiber layer and optic nerve head

PURPOSE

Optical coherence tomography (OCT) measures of the retinal nerve fiber layer (RNFL) thickness and neuroretinal rim (NRR) parameters are often used as a surrogate for retinal ganglion cell content. The purpose of this study was to investigate the relationship between these morphological measures and the aging effects on these structures.

METHODS

One hundred thirteen healthy individuals, aged 19 to 76 years, with no prior history of retinal of optic nerve head pathology were recruited. A circumpapillary and radial OCT scan centered on the optic nerve head (ONH) was used for data analysis. Transverse scaling was calculated for each subject using measures from optical biometry. Custom algorithms were used for morphological analysis of the ONH NRR and RNFL that included quantification of major retinal vascular contribution.

RESULTS

There was a significant age-related loss of RNFL thickness (-0.23 μm/y, R(2) = 0.24, P < 0.01), major retinal vascular contribution (-0.03 μm/y, R(2) = 0.07, P = 0.01, neural rim volume (NRV, -0.004 mm(3)/y, R(2) = 0.15, P < 0.01), and minimum rim width (MRW, -1.77 μm/y, R(2) = 0.23, P < 0.01) before, and after, incorporating the Bruch's membrane opening size (sMRW, -1.86 μm/y, R(2) = 0.22, P < 0.01). When normalized, the rates of change for ONH NRR parameters (NRV, 0.69%/y and sMRW, 0.50%/y) exceeded that of RNFL thickness (0.19%/y, P < 0.01).

CONCLUSIONS

Although both RNFL and ONH NRR parameters contain axons of retinal ganglion cells, there are differences in age-related changes in these measures that should be considered in clinical application.

The influence of axial length on confocal scanning laser ophthalmoscopy and spectral-domain optical coherence tomography size measurements: a pilot study

PURPOSE

To investigate the influence of axial length on SD-OCT and cSLO size measurements from the Heidelberg Spectralis.

METHODS

In this pilot study, eight emmetropic pseudophakic eyes with subretinal visual implant were selected. The axial length was measured in three short (<22.5 mm), three medium (22.51-25.50 mm) and two long (>25.52 mm) eyes. The known size of subretinal implant sensor field (2800 × 2800 μm) was measured on 15 images per eye with cSLO and SD-OCT.

RESULTS

The mean axial length was 20.8 ± 0.8 mm in short eyes, 23.3 ± 0.4 mm in medium eyes, and 26.3 ± 0.5 mm in long eyes respectively. We found in short eyes, in medium eyes and in long eyes a mean value of sensor field size measurements from cSLO of 3327 ± 9 μm, 2800 ± 9 μm and 2589 ± 12 μm and from SD-OCT of 3328 ± 9 μm, 2800 ± 12 μm and 2585 ± 19 μm respectively. The size measurements decreased in SD-OCT and cSLO measurements with longer axial lengths significantly (p < 0.0001).

CONCLUSION

The present findings demonstrate accuracy of the scaling in cSLO and SD-OCT measurements of the Heidelberg Spectralis for emmetropic medium eyes. The size measurements from SD-OCT to those from cSLO were approximately equal. Caution is recommended when comparing the measured values of short and long eyes with the normative database of the instrument. Further studies with larger sample sizes are needed to confirm findings.

Development of a rat schematic eye from in vivo biometry and the correction of lateral magnification in SD-OCT imaging

PURPOSE

Optical magnification in optical coherence tomography (OCT) depends on ocular biometric parameters (e.g., axial length). Biometric differences between eyes will influence scan location. A schematic model eye was developed to compensate for lateral magnification in OCT images of the healthy rat.

METHODS

Spectral-domain optical coherence tomography images were acquired in 19 eyes of 19 brown Norway rats. Images were scaled using the OCT instrument's built-in scaling function and by calculating the micron per degree from schematic model eyes developed from in vivo biometry (immersion A-scan and videokeratometry). Mean total retinal thickness was measured 500 μm away from the optic nerve head and optic nerve head diameter was measured. Corneal curvature, lens thickness, and axial length were modified to calculate their effects on OCT scan location and total retinal thickness.

RESULTS

Mean total retinal thickness increased by 21 μm and the SD doubles when images were scaled with the Built-in scaling (222 ± 13 μm) compared with scaling with individual biometric parameters (201 ± 6 μm). Optic nerve head diameter was three times larger when images were scaled with the Built-in scaling (925 ± 97 μm) than the individual biometric parameters (300 ± 27 μm). Assuming no other change in biometric parameters, total retinal thickness would decrease by 37 μm for every millimeter increase in anterior chamber depth due to changes in ocular lateral magnification and associated change in scan location.

CONCLUSIONS

Scaling SD-OCT images with schematic model eyes derived from individual biometric data is important. This approach produces estimates of retinal thickness and optic nerve head size that are in good agreement with previously reported measurements.

Agreement between retinal nerve fiber layer measures from Spectralis and Cirrus spectral domain OCT

PURPOSE

An assessment of the retinal nerve fiber layer (RNFL) provides important information on the health of the optic nerve. There are several non-invasive technologies, including spectral domain optical coherence tomography (SD OCT), that can be used for in vivo imaging and quantification of the RNFL, but often there is disagreement in RNFL thickness between clinical instruments. The purpose of this study was to investigate the influence of scan centration, ocular magnification, and segmentation on the degree of agreement of RNFL thickness measures by two SD OCT instruments.

METHODS

RNFL scans were acquired from 45 normal eyes using two commercially available SD OCT systems. Agreement between RNFL thickness measures was determined using each instrument's algorithm for segmentation and a custom algorithm for segmentation. The custom algorithm included ocular biometry measures to compute the transverse scaling for each eye. Major retinal vessels were identified and removed from RNFL measures in 1:1 scaled images. Transverse scaling was also used to compute the RNFL area for each scan.

RESULTS

Instrument-derived global RNFL thickness measured from the two instruments correlated well (R(2) = 0.70, p < 0.01) but with significant differences between instruments (mean of 6.7 μm; 95% limits of agreement of 16.0 μm to -2.5 μm, intraclass correlation coefficient = 0.62). For recentered scans with custom RNFL segmentation, the mean difference was reduced to 0.1 μm (95% limits of agreement 6.1 to -5.8 μm, intraclass correlation coefficient = 0.92). Global RNFL thickness was related to axial length (R = 0.24, p < 0.01), whereas global RNFL area measures were not (R(2) = 0.004, p = 0.66). Major retinal vasculature accounted for 11.3 ± 1.6% (Cirrus) or 11.8 ± 1.4% (Spectralis) of the RNFL thickness/area measures.

CONCLUSIONS

Sources of disagreement in RNFL measures between SD-OCT instruments can be attributed to the location of the scan path and differences in their retinal layer segmentation algorithms. In normal eyes, the major retinal vasculature accounts for a significant percentage of the RNFL and is similar between instruments. With incorporation of an individual's ocular biometry, RNFL area measures are independent of axial length, with either instrument.

Influence of anterior segment power on the scan path and RNFL thickness using SD-OCT

PURPOSE

Retinal nerve fiber layer (RNFL) thickness measures with spectral domain-optical coherence tomography (SD-OCT) provide important information on the health of the optic nerve. As with most retinal imaging technologies, ocular magnification characteristics of the eye must be considered for accurate analysis. While effects of axial length have been reported, the effects of anterior segment optical power on RNFL thickness measures have not been described fully to our knowledge. The purpose of our study was to determine the influence of the optical power change at the anterior corneal surface, using contact lenses, on the location of the scan path and measurements of RNFL thickness in normal healthy eyes.

METHODS

We recruited 15 normal subjects with less than 6 diopters (D) of ametropia and no ocular pathology. One eye of each subject was selected randomly for scanning. Baseline SD-OCT scans included raster cubes centered on the optic nerve and macula, and a standard 12-degree diameter RNFL scan. Standard 12-degree RNFL scans were repeated with 10 separate contact lenses, (Proclear daily, Omafilcon A/60%) ranging from +8 to -12 D in 2-D steps. The extent of the retinal scan, and RNFL thickness and area measures were quantified using custom MATLAB programs that included ocular biometry measures (IOL Master).

RESULTS

RNFL thickness decreased (0.52 μm/D, r = -0.33, P < 0.01) and the retinal region scanned increased (0.52%/D, r = 0.97, P < 0.01) with increase in contact lens power (-12 to +8). The normalized/percentage rates of change of RNFL thickness (-0.11/mm, r = -0.67, P < 0.01) and image size (0.11/mm, r = 0.96, P < 0.01) were related to axial length. Changes in the retinal region scanned were in agreement with transverse scaling, computed with a three surface schematic eye (R(2) = 0.97, P < 0.01). RNFL area measures, that incorporated the computed transverse scaling, were not related significantly to contact lens power (863 μm(2)/D, r = 0.06, P = 0.47).

CONCLUSIONS

Measurements of RNFL thickness by SD-OCT are dependent on the optics of the eye, including anterior segment power and axial length. The relationships between RNFL thickness measures and optical power are a direct reflection of scan path location with respect to the optic nerve head rim, caused by relative magnification. An incorporation of transverse scaling to RNFL area measures, based on individualized ocular biometry, eliminated the magnification effect.

Optimizing hand-held spectral domain optical coherence tomography imaging for neonates, infants, and children

PURPOSE

To describe age-related considerations and methods to improve hand-held spectral domain optical coherence tomography (HH-SD OCT) imaging of eyes of neonates, infants, and children.

METHODS

Based on calculated optical parameters for neonatal and infant eyes, individualized SD OCT scan parameters were developed for improved imaging in pediatric eyes. Forty-two subjects from 31 weeks postmenstrual age to 1.5 years were imaged with a portable HH-SD OCT system. Images were analyzed for quality, field of scan, magnification, and potential clinical utility.

RESULTS

The axial length of the premature infant eye increases rapidly in a linear pattern during the neonatal period and slows progressively with age. Refractive error shifts from mild myopia in neonates to mild hyperopia in infants. These factors affect magnification and field of view of optical diagnostic tools applied to the infant eye. When SD OCT parameters were corrected based on age-related optical parameters, SD OCT image quality improved in young infants. The field of scan and ease of operation also improved, and the optic nerve, fovea, and posterior pole were successfully imaged in 74% and 87% of individual eye imaging sessions in the intensive care nursery and clinic, respectively. No adverse events were reported.

CONCLUSIONS

SD OCT in young children and neonates should be customized for the unique optical parameters of the infant eye. This customization, not only improves image quality, but also allows control of the density of the optical sampling directed onto the retina.