Ultrahigh-resolution versus speckle noise-reduction in spectral-domain optical coherence tomography

Optical coherence tomography in Optics Express [Invited]

Optical coherence tomography (OCT) is one of the most successful technologies in the history of biomedical optics. Optics Express played an important role in communicating groundbreaking technological achievements in the field of OCT, and, conversely, OCT papers are among the most frequently cited papers published in Optics Express. On the occasion of the 20th anniversary of the journal, this review analyzes the reasons for the success of OCT papers in Optics Express and discusses possible motivations for researchers to submit some of their best OCT papers to the journal.

Diagnostic capability of peripapillary retinal nerve fiber layer parameters in time-domain versus spectral-domain optical coherence tomography for assessing glaucoma in high myopia

AIM

To evaluate and compare the diagnostic capabilities of peripapillary retinal nerve fiber layer (p-RNFL) parameters of Spectralis optical coherence tomography (OCT) versus Stratus OCT to detect glaucoma in patients with high myopia.

METHODS

This is a retrospective, cross-sectional study. Sixty highly myopic eyes of 60 patients were enrolled, with 30 eyes in the glaucoma group and 30 eyes in the control group. All eyes received peripapillary imaging of the optic disc using Stratus and Spectralis OCT. Areas under the receiver operating characteristic curve (AUROC) and the sensitivity at specificity of >80% and >95% for p-RNFL parameters obtained using the two devices to diagnose glaucoma were analysed and compared.

RESULTS

In Spectralis OCT, p-RNFL thickness parameters with the largest AUROC were the temporal-inferior sector (0.974) and the inferior quadrant (0.951), whereas in Stratus OCT, the best parameters were the 7-o'clock sector (0.918) and the inferior quadrant (0.918). Compared to the Stratus OCT parameters, the Spectralis OCT parameters demonstrated generally higher AUROC; however, the difference was not statistically significant.

CONCLUSION

The best p-RNFL parameters for diagnosing glaucoma in patients with high myopia were the temporal-inferior sector on Spectralis OCT and the 7-o'clock sector on Stratus OCT. There were no significant differences between the AUROCs for Spectralis OCT and Stratus OCT, which suggest that the glaucoma diagnostic capabilities of these two devices in patients with high myopia are similar.

Macular Diagnostic Ability in OCT for Assessing Glaucoma in High Myopia

PURPOSE

To compare the diagnostic abilities of spectral-domain optical coherence tomography (SD-OCT; Spectralis OCT) and time-domain OCT (TD-OCT; Stratus OCT). Changes in macular parameters in highly myopic eyes of glaucoma patients and highly myopic eyes of glaucoma suspects were evaluated and compared.

METHODS

We collected data from 72 highly myopic eyes (spherical equivalent, ≤-6.0D). Forty-one eyes had perimetric glaucoma and 31 eyes were suspected to have glaucoma (control group). All eyes underwent SD-OCT and TD-OCT imaging. Area under the receiver operating characteristic (AUROC) curve and sensitivity were examined on macular volume and thickness parameters at a fixed specificity and compared between groups.

RESULTS

The highest TD-OCT AUROC curves were found using outer inferior sector macular thickness (AUROC curve, 0.911) and volume (AUROC curve, 0.909). The highest SD-OCT AUROC curves were found using outer inferior region thickness (AUROC curve, 0.836) and volume (AUROC curve, 0.834). The difference between the two imaging modalities was not statistically significant (thickness, p = 0.141; volume, p = 0.138). The sensitivity of TD-OCT macular outer inferior average thickness was highest and was 88.2%, with a specificity of 80.4%. The sensitivity of TD-OCT average volume measurements in this same region was 76.5%, with a specificity of 91.3%. The SD-OCT average thickness measurements also had the highest sensitivity in this region, which was 78.6%, with a specificity of 82.1%. The SD-OCT volume measurements had a sensitivity of 67.9%, with a specificity of 92.3%.

CONCLUSIONS

Both SD-OCT and TD-OCT measurements of outer inferior macular thickness and volume can differentiate between eyes of glaucoma patients and glaucoma suspects with high myopia. These independent predictors all had good sensitivity. Based on our results, SD-OCT and TD-OCT have similar diagnostic abilities. These parameters may provide useful additional data in highly myopic eyes to complement standard glaucoma diagnosis tools.

Comparison of amplitude-decorrelation, speckle-variance and phase-variance OCT angiography methods for imaging the human retina and choroid

We compared the performance of three OCT angiography (OCTA) methods: speckle variance, amplitude decorrelation and phase variance for imaging of the human retina and choroid. Two averaging methods, split spectrum and volume averaging, were compared to assess the quality of the OCTA vascular images. All data were acquired using a swept-source OCT system at 1040 nm central wavelength, operating at 100,000 A-scans/s. We performed a quantitative comparison using a contrast-to-noise (CNR) metric to assess the capability of the three methods to visualize the choriocapillaris layer. For evaluation of the static tissue noise suppression in OCTA images we proposed to calculate CNR between the photoreceptor/RPE complex and the choriocapillaris layer. Finally, we demonstrated that implementation of intensity-based OCT imaging and OCT angiography methods allows for visualization of retinal and choroidal vascular layers known from anatomic studies in retinal preparations. OCT projection imaging of data flattened to selected retinal layers was implemented to visualize retinal and choroidal vasculature. User guided vessel tracing was applied to segment the retinal vasculature. The results were visualized in a form of a skeletonized 3D model.

Time Course of Macular and Peripapillary Inner Retinal Thickness in Non-arteritic Anterior Ischaemic Optic Neuropathy Using Spectral-Domain Optical Coherence Tomography

To report a time course of the ganglion cell complex (GCC) and circumpapillary retinal nerve fibre layer (cpRNFL) thicknesses using spectral-domain optical coherence tomography in patients with non-arteritic anterior ischaemic optic neuropathy (NAION), five patients with unilateral NAION were studied (the average age of 66.8 ± 7.8 years old). Forty-one age-matched normal controls were also enrolled. The GCC and cpRNFL thicknesses were measured at the initial visit and at 1, 3, 6, and 12 months using RTVue-100. The GCC thickness and the cpRNFL thickness of the patients were compared with those of the normal controls. The GCC thickness in the NAION patients was 96.49 μm at the initial visit, 84.28 μm at 1 month, 74.26 μm at 3 months, 71.23 μm at 6 months, and 69.51 μm at 12 months. The values at 1, 3, 6, and 12 months were significantly reduced (p < 0.01). The cpRNFL thickness at the initial visit was significantly increased, whereas the values at 6 and 12 months were significantly reduced (p < 0.01). The GCC thickness is more useful for the detection of retinal ganglion cell loss at an early stage than the cpRNFL thickness, because the GCC thickness is unaffected by optic disc swelling at the initial visit, unlike the cpRNFL thickness.

Virtual Averaging Making Nonframe-Averaged Optical Coherence Tomography Images Comparable to Frame-Averaged Images

Purpose

Developing a novel image enhancement method so that nonframe-averaged optical coherence tomography (OCT) images become comparable to active eye-tracking frame-averaged OCT images.

Methods

Twenty-one eyes of 21 healthy volunteers were scanned with noneye-tracking nonframe-averaged OCT device and active eye-tracking frame-averaged OCT device. Virtual averaging was applied to nonframe-averaged images with voxel resampling and adding amplitude deviation with 15-time repetitions. Signal-to-noise (SNR), contrast-to-noise ratios (CNR), and the distance between the end of visible nasal retinal nerve fiber layer (RNFL) and the foveola were assessed to evaluate the image enhancement effect and retinal layer visibility. Retinal thicknesses before and after processing were also measured.

Results

All virtual-averaged nonframe-averaged images showed notable improvement and clear resemblance to active eye-tracking frame-averaged images. Signal-to-noise and CNR were significantly improved (SNR: 30.5 vs. 47.6 dB, CNR: 4.4 vs. 6.4 dB, original versus processed, P < 0.0001, paired t-test). The distance between the end of visible nasal RNFL and the foveola was significantly different before (681.4 vs. 446.5 μm, Cirrus versus Spectralis, P < 0.0001) but not after processing (442.9 vs. 446.5 μm, P = 0.76). Sectoral macular total retinal and circumpapillary RNFL thicknesses showed systematic differences between Cirrus and Spectralis that became not significant after processing.

Conclusion

The virtual averaging method successfully improved nontracking nonframe-averaged OCT image quality and made the images comparable to active eye-tracking frame-averaged OCT images.

Translational Relevance

Virtual averaging may enable detailed retinal structure studies on images acquired using a mixture of nonframe-averaged and frame-averaged OCT devices without concerning about systematic differences in both qualitative and quantitative aspects.

Changes in morphology and visual function over time in mouse models of retinal degeneration: an SD-OCT, histology, and electroretinography study

PURPOSE

To examine the long-term natural course of retinal degeneration in rd10 and rd12 mice using serial spectral-domain optical coherence tomography (SD-OCT), electroretinography/electroretinograms (ERGs), and histological analysis.

METHODS

Photoreceptor layer thickness and the ability to visualize photoreceptor ellipsoid zones were analyzed using SD-OCT images, and these images were compared with hematoxylin and eosin-stained sections and electron microscopy images. The a- and b-wave amplitudes of the ERGs were analyzed.

RESULTS

In rd10 mice, the photoreceptor layer thickness rapidly decreased, and the photoreceptor ellipsoid zone was visible on SD-OCT images in 89 and 43 % of eyes of 21 and 33-day-old mice, respectively. In rd12 mice, the photoreceptor layer gradually thinned, and the ellipsoid zone remained visible in 92 % of eyes at 19 months. Electron microscopy revealed that photoreceptor degeneration had occurred on the inner side of the outer nuclear layer in 21-day-old rd10 and 7-month-old rd12 mice, possibly due to autophagy mechanisms. Scotopic ERGs of rd10 mice showed a diminished response at 21 days; at 33 days, no response was detectable. In rd12 mice, scotopic ERGs were undetectable at 28 days (stimulus intensity 3.0 cds/m(2)). Photopic ERGs were nearly undetectable in 28-day-old rd10 mice, but a small b-wave was still recordable in 13-month-old rd12 mice.

CONCLUSIONS

Our results demonstrate that visual function deteriorated with photoreceptor degeneration within 1 month in rd10 mice. In rd12 mice, however, the process of visual function deterioration and photoreceptor degeneration was still in progress at 13 months of age.

Study of Spectral-Domain Optical Coherence Tomography in Children: Normal Values and Influence of Age, Sex, and Refractive Status

Purpose

To establish normal values for retinal nerve fiber layer (RNFL), macular thickness, and macular volume in children using spectral-domain optical coherence tomography (OCT).

Methods

A descriptive cross-sectional study was performed in healthy children between 3 and 14 years of age. Each child underwent a comprehensive ophthalmic examination that included 3 OCT scans with Spectralis (Heidelberg Engineering, Heidelberg, Germany). All measurements were performed by the same operator, and one eye was randomly selected. The influence of age, sex, and refraction on OCT measurements was analyzed.

Results

The sample consisted of 162 Caucasian subjects, mean age 8.1 ± 3.03 years. The spherical equivalent was 0.03 ± 0.19 D (range ±4 D, astigmatism <1 D). The average values were 263.69 ± 4.54 µm for central macular thickness, 0.21 ± 0.01 mm3 for central macular volume, and 100.45 ± 1.98 µm for RNFL. A significant correlation between RNFL and spherical equivalent was found for the nasal (p = 0.001), inferior (p = 0.009), and inferior nasal (p = 0.005) sectors. No differences were found with regard to sex (p>0.05). However, central macular thickness and central macular volume were correlated with age (p = 0.027, p = 0.02).

Conclusions

This study provides reference values for macular thickness, macular volume, and RNFL in healthy children.

Asymmetry analysis of macular inner retinal layers for glaucoma diagnosis

PURPOSE

To determine if asymmetry in thickness of the retinal nerve fiber layer (RNFL), ganglion cell layer, ganglion cell complex, and total retina between upper and lower macula halves can predict glaucoma.

DESIGN

Retrospective case-control series.

METHODS

One hundred twenty-two eyes of 122 patients (30 normal eyes and 30 preperimetric, 31 early, and 31 advanced glaucoma eyes) were studied. The RNFL, ganglion cell layer, ganglion cell complex, and total retina were segmented and measured on 10 vertical B-scans over a 30 × 15 degree macular area. The equation asymmetry index =|log10 (lower hemiretinal thickness/upper hemiretinal thickness)| was used to calculate asymmetry indices for 8 pairs of upper and lower 0.5-mm segments equidistant from the fovea on each scan. Areas under the receiver operating characteristic curve (AROCs) for mean thickness and mean asymmetry index of 10 B-scans were compared.

RESULTS

The overlap in values for normal and glaucomatous eyes was minimal for the ganglion cell layer asymmetry index. Thickness parameters decreased with the severity of glaucoma, whereas asymmetry indices did not. AROCs for thickness measurements tended to increase with increasing glaucoma severity (preperimetric, 0.746-0.808; early, 0.842-0.940; advanced, 0.943-0.995), whereas AROCs for asymmetry indices did not have distinct ranges according to glaucoma severity (advanced, 0.819-0.996; early, 0.861-0.998; preperimetric, 0.773-0.994). The AROC for the ganglion cell layer asymmetry index remained almost perfect regardless of glaucoma severity (0.994-0.998).

CONCLUSIONS

Macular retinal layer thickness asymmetry indices, particularly for the ganglion cell layer, show promise as early indicators of glaucomatous retinal damage.

Glaucoma progression detection using nonlocal Markov random field prior

Glaucoma is neurodegenerative disease characterized by distinctive changes in the optic nerve head and visual field. Without treatment, glaucoma can lead to permanent blindness. Therefore, monitoring glaucoma progression is important to detect uncontrolled disease and the possible need for therapy advancement. In this context, three-dimensional (3-D) spectral domain optical coherence tomography (SD-OCT) has been commonly used in the diagnosis and management of glaucoma patients. We present a new framework for detection of glaucoma progression using 3-D SD-OCT images. In contrast to previous works that use the retinal nerve fiber layer thickness measurement provided by commercially available instruments, we consider the whole 3-D volume for change detection. To account for the spatial voxel dependency, we propose the use of the Markov random field (MRF) model as a prior for the change detection map. In order to improve the robustness of the proposed approach, a nonlocal strategy was adopted to define the MRF energy function. To accommodate the presence of false-positive detection, we used a fuzzy logic approach to classify a 3-D SD-OCT image into a "non-progressing" or "progressing" glaucoma class. We compared the diagnostic performance of the proposed framework to the existing methods of progression detection.

Relationship between retinal lesions and inward choroidal bulging in Vogt-Koyanagi-Harada disease

PURPOSE

To investigate the correlation between choroidal and retinal lesions in eyes with acute Vogt-Koyanagi-Harada disease (VKH) using optical coherence tomography (OCT) by using a new parameter, retinal pigment epithelium (RPE) undulation index, which quantitatively describes choroidal deformations.

DESIGN

Retrospective, observational, cross-sectional study.

METHODS

Spectral-domain OCT (SD OCT) and swept-source OCT images from a consecutive series of 42 eyes in 22 patients with acute VKH who underwent steroid therapy and 20 healthy eyes in 20 volunteers were analyzed retrospectively. Correlations between best-corrected visual acuity (BCVA), axial length change, and OCT parameters were examined. The RPE undulation index was defined as RPE line length to the total scan length ratio on a foveal-centered scan in the SD OCT image.

RESULTS

Eyes with acute VKH showed increased RPE undulation index, choroidal thickness, and retinal thickness compared to normal subjects, which were reduced following steroidal treatment (P < .0001, P = .0003, and P < .0001, respectively). RPE undulation index was related to choroidal thickness (r = 0.624, P = .0043), retinal thickness (r = 0.483, P = .0028), and BCVA (r = 0.588, P = .0002). Meanwhile, no statistically significant relationship was observed between choroidal thickness and retinal thickness. Axial length changes were significantly correlated with both choroidal thickness (r = 0.842, P < .0001) and RPE undulation index (r = 0.600, P = .0139).

CONCLUSIONS

This study demonstrated that the choroid was diffusely undulated and bulged inward in eyes with acute VKH. Correlations between RPE undulation index and choroid morphology, retinal thickness, and poor BCVA suggest that choroidal folding, quantified by RPE undulation index, is useful in assessing VKH disease severity.

Intraretinal hyperreflective foci on spectral-domain optical coherence tomographic images of patients with retinitis pigmentosa

Background

The purpose of this study was to observe the characteristic findings of spectral-domain optical coherence tomography (SD-OCT) images in the retinas of patients with retinitis pigmentosa and to evaluate their distribution patterns in the early and advanced stages of the disease.

Methods

A total of 184 patients (368 eyes) with retinitis pigmentosa were observed using SD-OCT. We studied the presence or absence of continuous inner/outer segment (IS/OS) lines, presence of thinning of the retinal pigment epithelium-Bruch’s membrane complex, and distribution patterns of hyperreflective foci in the inner and outer nuclear layers (INL and ONL).

Results

The IS/OS junction had partially disappeared in 275 eyes, which were at the early stage of retinitis pigmentosa (group X), whereas the junction had totally disappeared in 93, which were at the advanced stage of retinitis pigmentosa (group Y). Hyperreflective foci in the INL were observed in a significantly larger proportion of the eyes in group X than in group Y (90% versus 61%, P<0.001), but hyperreflective foci in the ONL were observed in a significantly larger proportion of eyes in group Y than in group X (100% versus 69%, P<0.001).

Conclusion

Hyperreflective foci in the INL were more frequently observed in retinas with the early stage of retinitis pigmentosa and hyperreflective foci in the ONL were more frequently observed in the advanced stage. Hyperreflective foci may be indicative of changes in the retinal structure at each stage of retinitis pigmentosa.

Motion artifact and speckle noise reduction in polarization sensitive optical coherence tomography by retinal tracking

We present a novel polarization sensitive optical coherence tomography (PS-OCT) system with an integrated retinal tracker. The tracking operates at up to 60 Hz, correcting PS-OCT scanning positions during the acquisition to avoid artifacts caused by eye motion. To demonstrate the practical performance of the system, we imaged several healthy volunteers and patients with AMD both with B-scan repetitions for frame averaging and with 3D raster scans. Under large retinal motions with up to 1 mm amplitude at 0.5 ~a few Hz frequency range, motion artifact suppression in the PS-OCT images as well as standard deviation noise reduction in the frame averaged retardation images are presented.

Image quality improvement in adaptive optics scanning laser ophthalmoscopy assisted capillary visualization using B-spline-based elastic image registration

PURPOSE

To investigate the effect of B-spline-based elastic image registration on adaptive optics scanning laser ophthalmoscopy (AO-SLO)-assisted capillary visualization.

METHODS

AO-SLO videos were acquired from parafoveal areas in the eyes of healthy subjects and patients with various diseases. After nonlinear image registration, the image quality of capillary images constructed from AO-SLO videos using motion contrast enhancement was compared before and after B-spline-based elastic (nonlinear) image registration performed using ImageJ. For objective comparison of image quality, contrast-to-noise ratios (CNRS) for vessel images were calculated. For subjective comparison, experienced ophthalmologists ranked images on a 5-point scale.

RESULTS

All AO-SLO videos were successfully stabilized by elastic image registration. CNR was significantly higher in capillary images stabilized by elastic image registration than in those stabilized without registration. The average ratio of CNR in images with elastic image registration to CNR in images without elastic image registration was 2.10 ± 1.73, with no significant difference in the ratio between patients and healthy subjects. Improvement of image quality was also supported by expert comparison.

CONCLUSIONS

Use of B-spline-based elastic image registration in AO-SLO-assisted capillary visualization was effective for enhancing image quality both objectively and subjectively.

Assessment of frame-averaging algorithms in OCT image analysis

BACKGROUND AND OBJECTIVE

To evaluate frame registration and averaging algorithms for optical coherence tomography.

PATIENTS AND METHODS

Twenty normal and 20 glaucomatous eyes were imaged. Objective differences were measured by comparing noise variance, spread of edge, and contrast-to-noise ratio. Subjective image quality was also evaluated.

RESULTS

Two frame-averaging algorithms (FA400 and FA407) had better noise variance and contrast-to-noise ratio but worse spread of edge than did single frames (P < .01). Both algorithms provided better subjective assessments of structure boundaries than did single images (P < .001). FA407 had significantly lower spread of edge and better internal limiting membrane visualization than FA400.

CONCLUSION

Frame averaging significantly suppressed speckle noise and increased the visibility of retinal structures, but imperfect image registration caused edge blurring that could be detected by the spread of edge parameter. In frame-averaging algorithms, higher contrast-to-noise ratio and lower noise variance indicated better noise suppression, but spread of edge was most sensitive in comparing edge preservation between algorithms.

Tomographic fundus features in pseudoxanthoma elasticum: comparison with neovascular age-related macular degeneration in Japanese patients

PURPOSE

To determine the retinal and subretinal features characteristic to pseudoxanthoma elasticum (PXE) compared with age-related macular degeneration by using spectral-domain optical coherence tomography (SD-OCT) in Japanese patients.

METHODS

We reviewed colour fundus photographs, fluorescein angiograms, and SD-OCT images of 52 eyes (27 Japanese patients) with angioid streaks (AS) due to PXE. Then we compared the incidence of tomographic features between 24 eyes (24 patient) with choroidal neovascularization (CNV) secondary to AS and 44 eyes (44 patients) with CNV secondary to age-related macular degeneration (AMD).

RESULTS

Secondary CNV was found in 44 eyes (84.6%) of 52 patients with PXE during follow-up. We found characteristic round or ovoid tubular structures with highly reflective annular lines (termed 'outer retinal tubulation' (ORT)) in 31 (70.5%) of 44 eyes with CNV, but none were found in eyes without CNV. We also found characteristic undulations of Bruch's membrane in 38 (73.1%) eyes with AS. The incidence of ORT was significantly greater in eyes with CNV secondary to AS (70.8%; P=0.005) compared with eyes with CNV secondary to AMD (34.1%). The incidence of Bruch's membrane undulation was significantly greater in eyes with CNV secondary to AS (70.8%; P<0.0001) than in eyes with CNV secondary to AMD (11.4%).

CONCLUSION

SD-OCT imaging clearly revealed a greater incidence of unique lesions, including ORT and Bruch's membrane undulation, in eyes in PXE patients with CNV secondary to AS than in eyes with CNV secondary to AMD.

New directions in ophthalmic optical coherence tomography

The rapid development of optical coherence tomography (OCT) and its ophthalmic applications has resulted in the emergence of new laboratory and commercial systems that vary in performance and functionality. The introduction of high-speed imaging capabilities has abrogated the primary limitation of early OCT technology by providing in vivo three-dimensional volumetric reconstructions of both anterior and posterior segments of the human eye within reasonable time constraints. Currently, high-speed swept source OCT technology has made it possible to achieve OCT acquisition speeds of several million A-scans/s. Another direction of OCT development includes the introduction of adaptive optics to imaging of the posterior segment of the eye that allows correction of the eye's static and dynamic aberrations, resulting in the achievement of volumetric cellular resolution retinal imaging. In this review, we introduce readers to various aspects of the development of OCT technology within the context of its ophthalmic applications. We point out directions for future development and indicate different perspectives on this dynamically expanding method. We give a few examples of how OCT has been used over the past few years and describe how high-speed OCT imaging may be used in the future in clinical practice.

Real-time imaging of rabbit retina with retinal degeneration by using spectral-domain optical coherence tomography

BACKGROUND

Recently, a transgenic rabbit with rhodopsin Pro 347 Leu mutation was generated as a model of retinitis pigmentosa (RP), which is characterized by a gradual loss of vision due to photoreceptor degeneration. The purpose of the current study is to noninvasively visualize and assess time-dependent changes in the retinal structures of a rabbit model of retinal degeneration by using speckle noise-reduced spectral-domain optical coherence tomography (SD-OCT).

METHODOLOGY/PRINCIPAL FINDINGS

Wild type (WT) and RP rabbits (aged 4-20 weeks) were investigated using SD-OCT. The total retinal thickness in RP rabbits decreased with age. The thickness of the outer nuclear layer (ONL) and between the external limiting membrane and Bruch's membrane (ELM-BM) were reduced in RP rabbits around the visual streak, compared to WT rabbits even at 4 weeks of age, and the differences increased with age. However, inner nuclear layer (INL) thickness in RP rabbits did not differ from that of WT during the observation period. The ganglion cell complex (GCC) thickness in RP rabbits increased near the optic nerve head but not around the visual streak in the later stages of the observation period. Hyper-reflective change was widely observed in the inner segments (IS) and outer segments (OS) of the photoreceptors in the OCT images of RP rabbits. Ultrastructural findings in RP retinas included the appearance of small rhodopsin-containing vesicles scattered in the extracellular space around the photoreceptors.

CONCLUSIONS/SIGNIFICANCE

In the current study, SD-OCT provided the pattern of photoreceptor degeneration in RP rabbits and the longitudinal changes in each retinal layer through the evaluation of identical areas over time. The time-dependent changes in the retinal structure of RP rabbits showed regional and time-stage variations. In vivo imaging of RP rabbit retinas by using SD-OCT is a powerful method for characterizing disease dynamics and for assessing the therapeutic effects of experimental interventions.

Efficient reduction of speckle noise in Optical Coherence Tomography

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

Clinical significance of B-scan averaging with SD-OCT

Averaging multiple scans is a potential advantage of optical coherence tomography. The authors evaluate the qualitative benefits of B-scan averaging on the visualization of outer retinal structures. A retrospective analysis was performed on Cirrus OCT (Carl Zeiss Meditec, Dublin, CA) B-scans from 1 eye of 35 patients referred to the imaging unit who underwent three types of scan acquisitions: no averaging, 4× averaging, and 20× averaging. Masked assessment of quality was made using a qualitative scale of 0 (worst) to 3 according to the ability to identify structure, brightness, and continuity. Quality scores consistently improved with 4× averaging, but improved only slightly further with 20×. Averaging appeared to have a statistically significant beneficial effect for the assessment of the external limiting membrane and outer nuclear layer (P < .05), with no significant benefit for visualization of the retinal pigment epithelium and inner segment/outer segment junction. The benefits of oversampling or averaging B-scans for visualizing outer retinal substructures are apparent even when averaging relatively few frames. These findings may be helpful when designing acquisition protocols in clinical trials and clinical practice.

Macular ganglion cell layer imaging in preperimetric glaucoma with speckle noise-reduced spectral domain optical coherence tomography

OBJECTIVE

To visualize the macular ganglion cell layer (GCL) and measure its thickness in normal eyes and eyes with preperimetric glaucoma, using speckle noise-reduced spectral domain optical coherence tomography (SD-OCT).

DESIGN

Retrospective consecutive case series.

PARTICIPANTS

Thirty-seven eyes of 37 patients with preperimetric glaucoma and 39 normal eyes of 39 volunteers.

METHODS

Vertical and horizontal SD-OCT B-scan images were acquired with minimal speckle noise by using eye-tracking to obtain and average 50 B-scans at each identical location of interest. B-scan images were manually analyzed for GCL, retinal nerve fiber layer (RNFL), and inner plexiform layer shapes and thicknesses in the macula.

MAIN OUTCOME MEASURES

Macular GCL images and thickness in normal eyes and in eyes with preperimetric glaucoma.

RESULTS

The macular GCL was clearly seen on speckle noise-reduced SD-OCT images in normal eyes and eyes with preperimetric glaucoma. In each eye with preperimetric glaucoma, thinning of the macular GCL was visually apparent, particularly on vertical scans. The mean regional macular GCL was most severely thinned in the inferior perifoveal region, where its thickness was <70% of its normal thickness in 30 (81.1%) of the 37 eyes and <50% of its normal thickness in 13 (35.1%) of the 37 eyes. When the sensitivity and specificity for detecting abnormal thinning (outside the lower limit of 99% confidence interval [CI] for the means in the 39 normal eyes) in at least one 0.5-mm segment or sector were compared, the macular GCL on vertical B-scans exhibited higher sensitivity (81.1%) than the other layers on vertical B-scans (99% CI, 5.4%-59.5%; P = 0.00075-0.02100), the macular GCL (99% CI, 40.5%; P = 0.00027) on horizontal B-scans, the other layers (99% CI, 5.4%-48.6%; P<0.00048-0.00400) on horizontal B-scans, and circumpapillary RNFL automatically measured on SD-OCT (54.1%; P = 0.021), and scanning laser polarimetry with variable corneal compensation (24.3%; P = 0.00095). All the macular layers on both the vertical and horizontal B-scans and circumpapillary RNFL thickness exhibited comparable specificity (91.4-100.0%, statistically not different).

CONCLUSIONS

Speckle noise-reduced SD-OCT imaging allowed clear visualization and measurement of the macular GCL, which was severely thinned in eyes with preperimetric glaucoma.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found after the references.

Human chorioretinal layer thicknesses measured in macula-wide, high-resolution histologic sections

PURPOSE

To provide a comprehensive description of chorioretinal layer thicknesses in the normal human macula, including two-layer pairs that can produce a combined signal in some optical coherence tomography (OCT) devices (ganglion cell [GCL] and inner plexiform [IPL] layers and outer plexiform [OPL] and outer nuclear [ONL] layers).

METHODS

In 0.8-μm-thick, macula-wide sections through the foveola of 18 donors (age range, 40-92 years), 21 layers were measured at 25 locations by a trained observer and validated by a second observer. Tissue volume changes were assessed by comparing total retinal thickness in ex vivo OCT and in sections.

RESULTS

Median tissue shrinkage was 14.5% overall and 29% in the fovea. Histologic laminar boundaries resembled those in SD-OCT scans, but the shapes of the foveolar OPL and ONL differed. Histologic GCL, IPL, and OPLHenle were thickest at 0.8. to 1, 1.5, and 0.4 mm eccentricity, respectively. ONL was thickest in an inward bulge at the foveal center. At 1 mm eccentricity, GCL, INL, and OPLHenle represented 17.3% to 21.1%, 18.0% to 18.5%, and 14.2% to 16.6% of total retinal thickness, respectively. In donors ≥ 70 years of age, the RPE and choroid were 17.1% and 29.6% thinner and OPLHenle was 20.8% thicker than in donors <70 years.

CONCLUSIONS

In this study, the first graphic representation and thickness database of chorioretinal layers in normal macula were generated. Newer OCT systems can separate GCL from IPL and OPLHenle from ONL, with good agreement for the proportion of retinal thickness occupied by OPLHenle in OCT and histology. The thickening of OPLHenle in older eyes may reflect Müller cell hypertrophy associated with rod loss.

Improved visualization of Henle fiber layer by changing the measurement beam angle on optical coherence tomography

PURPOSE

The outer plexiform layer (OPL) includes the photoreceptor synapse layer and Henle fiber layer (HFL) in the macular area. We attempted to improve optical coherence tomography imaging of the HFL by changing the angle of the measurement beam.

METHODS

Thirteen normal eyes underwent spectral domain optical coherence tomography. To focus the angle of the measurement beam perpendicular to the obliquely oriented HFL, the beam in the optical coherence tomography system was aimed at the periphery of the pupil.

RESULTS

The cross-sectional image of the macular area was inclined right or left if the beam entered from the periphery of the pupil. The reflectivity of the OPL increased because of strong backscattering of light in the half of the macula that was tilted down in cross-sectional images. On the opposite side, the reflectivity of HFL (the outer two-thirds of the OPL) decreased compared with that of the outer nuclear layer, but the photoreceptor synapse layer (the inner one-third of the OPL) remained highly reflective. The mean thicknesses of the OPL that could be visualized 1.0 mm and 1.5 mm from the central fovea were 73.7 ± 7.0 μm and 64.1 ± 8.2 μm, respectively.

CONCLUSION

Positioning the measurement beam perpendicular to HFL enhanced visualization performance.

Axial length and outcomes of macular hole surgery assessed by spectral-domain optical coherence tomography

PURPOSE

To use spectral-domain optical coherence tomography (SD OCT) to evaluate macular hole surgery outcomes and features predicting anatomic failure.

DESIGN

Retrospective, interventional case series.

METHODS

Fifty-two eyes of 50 consecutive patients with macular holes were examined. All eyes underwent 3-port pars plana vitrectomy with internal limiting membrane peeling. Eyes were examined after surgery by dense serial SD OCT scanning over the macula.

RESULTS

Eyes with initial anatomic failure were significantly more likely to have greater axial length and refractive error and more posterior staphyloma compared with eyes with initial anatomic success (P = .031 to .0060, < .0001). Overall initial and final anatomic success rates were 92.3% (48 of 52 eyes). In highly myopic eyes with axial lengths of 26.0 mm or more, initial and final success rates were 73.3% (11 of 15 eyes) compared with 100.0% (37 of 37 eyes) of eyes with axial lengths of less than 26.0 mm (P = .0050). In highly myopic eyes, initial and final success rates were 0% (0 of 3 eyes) of eyes with axial lengths of 30.0 mm or more compared with 91.7% (11 of 12 eyes) of eyes with axial lengths of 26.0 mm or more and of less than 30.0 mm (P < .0001). Retinoschisis-like thickening of the outer retina was seen in 3 (75.0%) of 4 eyes with initial failure compared with 3 (6.3%) of 48 eyes with initial success (P = .0030).

CONCLUSIONS

Axial length of 30.0 mm or more may increase the risk of anatomic failure of macular hole surgery.

Features associated with foveal retinal detachment in myopic macular retinoschisis

PURPOSE

To determine the retinal features seen on enhanced spectral-domain optical coherence tomography (SD-OCT) associated with foveal retinal detachment in eyes with myopic macular retinoschisis.

DESIGN

Retrospective case series.

METHODS

We reviewed fundus photographs and conventional and enhanced SD-OCT images of 21 eyes of 19 patients with myopic macular retinoschisis. Features seen on enhanced SD-OCT images were identified and compared between eyes with and without foveal detachment.

RESULTS

On enhanced SD-OCT images, the retinoschisis in the outer retina was accompanied by splitting in the inner plexiform layer (IPL) in 11 eyes (50.0%) and a detachment of the inner limiting membrane (ILM) in 11 eyes (50.0%) in the superior and/or inferior peripheral macula; 9 eyes (42.9%) had both features. Multiple columnar structures were seen throughout the outer retinoschisis in 20 of 21 eyes (95.2%), and in ILM detachment/IPL retinoschisis in all eyes. Thirteen eyes had or progressed to a foveal detachment; an ILM detachment developed in 10 eyes (76.9%) with a foveal detachment and in 1 eye (12.5%) without a foveal detachment (P = .008). IPL retinoschisis was detected in 8 eyes (61.5%) with a foveal detachment and 3 eyes (37.5%) without a foveal detachment, but this difference was not significant.

CONCLUSIONS

An ILM detachment in the superior and/or inferior peripheral macula was associated with a foveal retinal detachment in highly myopic eyes with macular retinoschisis. This feature may indicate strong tractional forces on the ILM that are transmitted to the outer retina through the dense columnar structures, resulting in a foveal retinal detachment.

In vivo Fourier-domain full-field OCT of the human retina with 1.5 million A-lines/s

In vivo full-field (FF) optical coherence tomography (OCT) images of human retina are presented by using a rapidly tunable laser source in combination with an ultra-high-speed camera. Fourier-domain FF-OCT provided a way to increase the speed of retinal imaging by parallel acquisition of A-scans. Reduced contrast caused by cross talk was observed only below the retinal pigment epithelium. With a 100Hz sweep rate, FF-OCT was fast enough to acquire OCT images with acceptable motion artifacts. FF-OCT allows ultrafast retinal imaging, boosting image speed by a lack of moving parts and a considerably higher irradiation power.

Slit-lamp-adapted fourier-domain OCT for anterior and posterior segments: preliminary results and comparison to time-domain OCT

PURPOSE

To evaluate the diagnostic potential of a slit-lamp (SL)-adapted Fourier-domain (= spectral radar, SR) optical coherence tomography (OCT)-SL-SR-OCT-instrument as an in vivo imaging device for use in examinations of the anterior and posterior segments.

MATERIALS AND METHODS

In a pilot study, 88 eyes from 70 healthy volunteers and patients were examined using a prototype Fourier-domain SL-SR-OCT system. Results were compared to those from the following commercially available systems: the 1310-nm SL-OCT (Heidelberg Engineering, Heidelberg, Germany) for anterior segment and the Stratus OCT (Zeiss Meditec, Jena, Germany) for posterior segment imaging. Our SL-SR-OCT provides 1025 axial scans, 5000 Hz line-scan frequency, scan length of up to 8 mm, axial depth in air of 3.5 mm, and resolution of 9 mum. For posterior visualization, a hand-held 78-diopter ophthalmoscopic lens was used.

RESULTS

Our SL-SR-OCT system allowed simultaneous scanning with direct biomicroscopic and SL imaging of anterior and posterior segment structures. Anatomical structures and pathological changes were displayed with high resolution and excellent contrast. Measurements of corneal and retinal thickness were possible. In comparison to images obtained by the SL-OCT, our SL-SR-OCT boasted a higher resolution, thus providing more clinically relevant details of the corneal epithelium, internal structure of filtering blebs, etc. Complete imaging of the chamber angle was limited, however, due to the backscattering properties of the sclera at 830 nm. For posterior segment imaging, excellent delineation of the macula and optic nerve head details, with a distinct portrayal of macular pathology and retinal edema, was possible with SL-SR-OCT.

CONCLUSION

SL-SR-OCT enables detailed imaging of physiological and pathological anterior and posterior segment structures. As a multi-purpose device, it offers a wide spectrum of applications, with high-quality OCT-imaging, in a comfortable setting without the need to move the patient.

Spectral-domain optical coherence tomography: a comparison of modern high-resolution retinal imaging systems

PURPOSE

To provide a review of commercially available spectral-domain optical coherence tomography (SD OCT) systems in clinical ophthalmology.

DESIGN

Perspective.

METHODS

Review of current manufacturer information, selected articles from the literature, and the authors' clinical experience.

RESULTS

Because the premise of SD OCT technology is the nonproprietary mathematical formula of Fourier transformation, 9 different SD OCT systems currently are or soon will be commercially available. Also demonstrated are Cirrus en face C-scan visualization of photoreceptor attenuation resulting from acute zonal occult outer retinopathy and Spectral OCT/scanning laser ophthalmoscopy microperimetric analysis of a macular caldera lesion resulting from North Carolina macular dystrophy.

CONCLUSIONS

Advances in high-resolution imaging of the anterior and posterior segment have revealed new in vivo details of anatomic, physiologic, and pathologic data for the practice of ophthalmology. Compared with time-domain OCT, SD OCT tends to derive increased retinal thickness and decreased nerve fiber layer thickness measurements. This is likely because of increased depth of resolution and greater volume of data acquired with each scan. Interdevice comparison is not practical because of differences in individual segment boundary algorithms. Improvements in photoreceptor inner segment-outer segment layer visualization should aid our understanding of visual dysfunction in a variety of retinal pathologic features. As the technology develops, SD OCT will continue to provide new insights about ocular structure and disease.

Impact of scanning density on measurements from spectral domain optical coherence tomography

PURPOSE

To investigate the relationship between B-scan density and retinal thickness measurements obtained by spectral domain optical coherence tomography (SDOCT) in eyes with retinal disease.

METHODS

Data were collected from 115 patients who underwent volume OCT imaging with Cirrus HD-OCT using the 512 x 128 horizontal raster protocol. Raw OCT data, including the location of the automated retinal boundaries, were exported from the Cirrus HD-OCT instrument and imported into the Doheny Image Reading Center (DIRC) OCT viewing and grading software, termed "3D-OCTOR." For each case, retinal thickness maps similar to those produced by Cirrus HD-OCT were generated using all 128 B-scans, as well as using less dense subsets of scans, ranging from every other scan to every 16th scan. Retinal thickness measurements derived using only a subset of scans were compared to measurements using all 128 B-scans, and differences for the foveal central subfield (FCS) and total macular volume were computed.

RESULTS

The mean error in FCS retinal thickness measurement increased as the density of B-scans decreased, but the error was small (<2 microm), except at the sparsest densities evaluated. The maximum error at a density of every fourth scan (32 scans spaced 188 microm apart) was <1%.

CONCLUSIONS

B-scan density in volume SDOCT acquisitions can be reduced to 32 horizontal B-scans (spaced 188 microm apart) with minimal change in calculated retinal thickness measurements. This information may be of value in design of scanning protocols for SDOCT for use in future clinical trials.