Conventional Spectral-domain Optical Coherence Research Articles

Retinal Nerve Fiber Layer Damage Assessment in Glaucomatous Eyes Using Retinal Retardance Measured by Polarization-Sensitive Optical Coherence Tomography.

To assess the diagnostic performance and structure-function association of retinal retardance (RR), a customized metric measured by a prototype polarization-sensitive optical coherence tomography (PS-OCT), across various stages of glaucoma. This cross-sectional pilot study analyzed 170eyes from 49 healthy individuals and 68 patients with glaucoma. The patients underwent PS-OCT imaging and conventional spectral-domain optical coherence tomography (SD-OCT), as well as visual field (VF) tests. Parameters including RR and retinal nerve fiber layer thickness (RNFLT) were extracted from identical circumpapillary regions of the fundus. Glaucomatous eyes were categorized into early, moderate, or severe stages based on VF mean deviation (MD). The diagnostic performance of RR and RNFLT in discriminating glaucoma from controls was assessed using receiver operating characteristic (ROC) curves. Correlations among VF-MD, RR, and RNFLT were evaluated and compared within different groups of disease severity. The diagnostic performance of both RR and RNFLT was comparable for glaucoma detection (RR AUC = 0.98, RNFLT AUC = 0.97; P = 0.553). RR showed better structure-function association with VF-MD than RNFLT (RR VF-MD = 0.68, RNFLT VF-MD = 0.58; z = 1.99; P = 0.047) in glaucoma cases, especially in severe glaucoma, where the correlation between VF-MD and RR (r = 0.73) was significantly stronger than with RNFLT (r = 0.43, z = 1.96, P = 0.050). In eyes with early and moderate glaucoma, the structure-function association was similar when using RNFLT and RR. RR and RNFLT have similar performance in glaucoma diagnosis. However, in patients with glaucoma especially severe glaucoma, RR showed a stronger correlation with VF test results. Further research is needed to validate RR as an indicator for severe glaucoma evaluation and to explore the benefits of using PS-OCT in clinical practice. We demonstrated that PS-OCT has the potential to evaluate the status of RNFL structural damage in eyes with severe glaucoma, which is currently challenging in clinics.

Wavelength-encoded multi-channel spectral domain optical coherence tomography (SD-OCT) for spatiotemporally resolved imaging

In conventional spectral domain optical coherence tomography (SD-OCT), a beam is focused to one spot on the surface of the sample to acquire depth information beneath that spot. Adjacent spots are serially scanned to obtain a tomographic image of the sample; thus, available OCT techniques can take significant time to scan large objects. This paper illustrates an SD-OCT system involving spectral-slicing of a broadband spectrum to enable the simultaneous acquisition from multiple lateral positions of the sample. Using a wavelength encoding of spatial information, where each lateral location is associated with a specific portion of a source spectrum, multiple lateral spots are interrogated concurrently to improve the imaging speed of SD-OCT. A triple-channel SD-OCT system was devised using a broadband source of centre wavelength 840 nm and full width half maximum (FWHM) bandwidth of 50 nm. Both sample and reference arm beams were split into three channels using two dichroic splitters of 50% cut-on wavelength at 825 nm and 848 nm, resulting in a FWHM spectral distribution of 13.5 nm, 19 nm and 17.5 nm in the three channels, respectively. The imaging performance of the technique was demonstrated by concurrently acquiring tomographic images of a 3D-printed phantom. Despite the relative drop in the per-channel axial resolution and depth sensitivity, this new approach presents a straightforward way of interrogating a sample simultaneously in multiple locations, provides the additional advantage of allowing different delay lines for each spectral section and offers the potential of such a system to be used in a wide range of applications that demand a small product foot print.

Measurable Range of Subfoveal Choroidal Thickness With Conventional Spectral Domain Optical Coherence Tomography.

PurposeWe investigate the measurable range of subfoveal choroidal thickness (SFCT) with conventional spectral domain optical coherence tomography (SD-OCT) in normal healthy eyes.MethodsAll subjects underwent an SD-OCT horizontal line scan centered at the fovea with standard and enhanced depth imaging (EDI) techniques. Two independent observers manually measured SFCT on standard and EDI-OCT images, if two choroidal borders were identified. The rate of successful measurement with standard OCT was evaluated. Inter- and intraclass correlation coefficients and Bland-Altman plots were used for analysis.ResultsOCT images of 724 normal healthy eyes from 374 subjects were evaluated. Although the inner choroidal border was identified on all OCT images, the outer choroidal border was identified on 378 of 724 (52.2%) conventional OCT images. Mean SFCT of all study eyes measured by EDI-OCT images was 292.6 ± 94.0. The number of successful measurements with conventional OCT was 100%, 87.5%, 48.1%, 33.0%, and 0%, in the SFCT ranges of <241, 241–280, 281–320, 321–360, and >360 μm, respectively. The accumulated rate of successful measurement with conventional OCT was 100%, 96.4%, and 82.2% in the SFCT ranges of ≤240, ≤280, and ≤320 μm, respectively. Two protocols showed good inter-correlation of SFCT, when SFCT were measurable in both scans.ConclusionsSFCT can be measured by conventional OCT in eyes with thin choroid, and the measured value shows high agreement with those measured by EDI-OCT. In such eyes, conventional OCT can be used potentially to evaluate the retina and choroid.Translational RelevanceIn eyes with a thin choroid, conventional OCT can be used potentially to evaluate the retina and choroid with high agreement with EDI-OCT.

Long ranging swept-source optical coherence tomography-based angiography outperforms its spectral-domain counterpart in imaging human skin microcirculations.

There is an increasing demand for imaging tools in clinical dermatology that can perform in vivo wide-field morphological and functional examination from surface to deep tissue regions at various skin sites of the human body. The conventional spectral-domain optical coherence tomography-based angiography (SD-OCTA) system is difficult to meet these requirements due to its fundamental limitations of the sensitivity roll-off, imaging range as well as imaging speed. To mitigate these issues, we demonstrate a swept-source OCTA (SS-OCTA) system by employing a swept source based on a vertical cavity surface-emitting laser. A series of comparisons between SS-OCTA and SD-OCTA are conducted. Benefiting from the high system sensitivity, long imaging range, and superior roll-off performance, the SS-OCTA system is demonstrated with better performance in imaging human skin than the SD-OCTA system. We show that the SS-OCTA permits remarkable deep visualization of both structure and vasculature (up to ∼2 mm penetration) with wide field of view capability (up to 18×18 mm2), enabling a more comprehensive assessment of the morphological features as well as functional blood vessel networks from the superficial epidermal to deep dermal layers. It is expected that the advantages of the SS-OCTA system will provide a ground for clinical translation, benefiting the existing dermatological practice.

Design of a k-space spectrometer for ultra-broad waveband spectral domain optical coherence tomography

Nonlinear sampling of the interferograms in wavenumber (k) space degrades the depth-dependent signal sensitivity in conventional spectral domain optical coherence tomography (SD-OCT). Here we report a linear-in-wavenumber (k-space) spectrometer for an ultra-broad bandwidth (760 nm–920 nm) SD-OCT, whereby a combination of a grating and a prism serves as the dispersion group. Quantitative ray tracing is applied to optimize the linearity and minimize the optical path differences for the dispersed wavenumbers. Zemax simulation is used to fit the point spread functions to the rectangular shape of the pixels of the line-scan camera and to improve the pixel sampling rates. An experimental SD-OCT is built to test and compare the performance of the k-space spectrometer with that of a conventional one. Design results demonstrate that this k-space spectrometer can reduce the nonlinearity error in k-space from 14.86% to 0.47% (by approximately 30 times) compared to the conventional spectrometer. The 95% confidence interval for RMS diameters is 5.48 ± 1.76 μm—significantly smaller than both the pixel size (14 μm × 28 μm) and the Airy disc (25.82 μm in diameter, calculated at the wavenumber of 7.548 μm−1). Test results demonstrate that the fall-off curve from the k-space spectrometer exhibits much less decay (maximum as −5.20 dB) than the conventional spectrometer (maximum as –16.84 dB) over the whole imaging depth (2.2 mm).

Effect of high myopia on glaucoma diagnostic parameters measured with optical coherence tomography

We examined the influence of high myopia on conventional spectral-domain optical coherence tomographic parameters and assessed the macular ganglion cell complex thickness to macular outer retinal thickness ratio as a new optical coherence tomography parameter. Prospective cross-sectional study. Sixty normal and 30 highly myopic eyes (refractive error more than -6 D). We used the RTVue-100 to measure macular ganglion cell complex and circumpapillary retinal nerve fibre layer thickness, global loss volume, and focal loss volume and then calculated the ganglion cell complex thickness to macular outer retinal thickness ratio. Each parameter was compared between the two groups. Using the area under receiver operating characteristics curve, the classification abilities of optical coherence tomography parameters were examined in highly myopic eyes. Between normal and highly myopic eyes, we found significant differences in ganglion cell complex and retinal nerve fibre layer thickness, global loss volume and focal loss volume. The new ratio parameter was not significantly different between groups (55.74% vs. 54.50%). The area under receiver operating characteristics curve was 0.775 (P < 0.01) for retinal nerve fibre layer thickness, 0.721 (P < 0.01) for ganglion cell complex thickness and 0.588 (P > 0.05) for the new ratio parameter. Although refractive status significantly affected conventional optical coherence tomography parameters, the new ratio parameter may not be influenced by refractive error. Therefore, a normative database for healthy highly myopic eyes may not be necessary if ratio parameter is used.

SELECTIVE CONE PHOTORECEPTOR INJURY IN ACUTE MACULAR NEURORETINOPATHY

To evaluate retinal structural and functional abnormalities in a patient with acute macular neuroretinopathy. An adaptive optics scanning light ophthalmoscope was used to image the photoreceptor mosaic and assess rod and cone structure. Spectral-domain optical coherence tomography was used to examine retinal lamination. Microperimetry was used to assess function across the macula. Microperimetry showed reduced function of localized areas within retinal lesions corresponding to subjective scotomas. Spectral-domain optical coherence tomography imaging revealed attenuation of two outer retinal bands typically thought to reflect photoreceptor structure. Adaptive optics scanning light ophthalmoscope images of the photoreceptor mosaic revealed a heterogeneous presentation within these lesions. There were areas containing non-waveguiding cones and other areas of decreased cone density where the remaining rods had expanded to fill in the vacant space. Within these lesions, cone densities were shown to be significantly lower than eccentricity-matched areas of normal retina, as well as accepted histologic measurements. A 6-month follow-up revealed no change in rod or cone structure. Imaging of acute macular neuroretinopathy using an adaptive optics scanning light ophthalmoscope shows a preferential disruption of cone photoreceptor structure within the region of decreased retinal sensitivity (as measured by microperimetry). Adaptive optics-based imaging tools provide a noninvasive way to assess photoreceptor structure at a level of detail that is not resolved by use of conventional spectral-domain optical coherence tomography or other clinical measures.

Clinical Analysis of Macular Edema with New Software for SD-OCT Imaging

To evaluate the clinical efficacy of user-friendly software for the measurement of intraretinal hyporeflective spaces expression of macular edema. Fifteen consecutive patients with diabetic retinopathy with clinically significant macular edema were examined using conventional spectral domain optical coherence tomography (OCT). A new composite software application, OCT-measurement analysis tool (OCT-MAT), was developed to automatically process and analyze OCT B-scans by means of image acquisition, filtering, and elaboration, together with hyporeflective area recognition and measurement in µm2. The same macular areas were measured manually, and then compared to the measurements obtained by the automated OCT-MAT software. A statistical t test analysis was applied (statistical significance level at p<0.05). The repeatability and reproducibility coefficient for the automated software was computed using the Wilcoxon matched pair test (5% significance level). In all patients, the software effectively measured the number and extension of intraretinal hyporeflective spaces in µm2. The comparison between mean manual measurements and OCT-MAT measurements (0.478 ± 0.300 × 10(6) µm2 vs 0.471 ± 0.321 × 10(6) µm2) showed correct correspondence (p>0.05). Moreover, the OCT-MAT software showed good repeatability and reproducibility (coefficient below 3%). OCT-MAT allows the precise measurement of macular edema in terms of the number of empty spaces and their size in all patients. Its daily clinical application might give precise information regarding the evolution of macular edema and the efficacy of therapy.

Spectral-Domain Optical Coherence Tomography Enhanced Depth Imaging of the Normal and Glaucomatous Nonhuman Primate Optic Nerve Head

To test whether the enhanced depth imaging (EDI) modality improves anterior and posterior lamina cribrosa surface (ALCS and PLCS) visibility compared with conventional spectral-domain optical coherence tomography (SD-OCT). Conventional and EDI SD-OCT scans were obtained 30 minutes after IOP was manometrically lowered to 10 mm Hg in both eyes of 14 nonhuman primates (NHPs) with unilateral experimental glaucoma (EG). Thirteen horizontal and seven vertical radial B-scans of each SD-OCT data set were delineated by one operator masked to image type. Delineated ALCS and PLCS points were projected to 1 of 100 equal-sized subregions of the neural canal opening (NCO) reference plane, and the number of delineated subregions (≥2 points) was counted. Poisson regression was used to analyze the effects of image type, treatment, and quadrant. Two additional delineations were performed for three NHPs to compare reproducibility. EDI increased the number of subregions delineated for both the ALCS (by 28%; P < 0.0001) and PLCS (by 225%; P < 0.0001). EDI improvement in ALCS visibility was significant in the superior quadrant only and was not different in EG versus control eyes, whereas EDI improvement in PLCS visibility was significant in all four quadrants (P < 0.005) and greater in EG eyes (P < 0.001), nasally and temporally. Intradelineator reproducibility was not different between image types. EDI and standard ONH parameter values were similar except for PLCS depth which was deeper in the EDI data sets (P = 0.0002). ALCS and PLCS visibility within control and EG NHP ONHs increased in EDI compared to conventional SD-OCT data sets. Further study of EDI effects on PLCS parameterization is required.

Pulsed-source and swept-source spectral-domain optical coherence tomography with reduced motion artifacts

Significant motion artifacts may arise in conventional spectral-domain optical coherence tomography due to sample or probe motion during the exposure time of a CCD array. We show, for the first time to our knowledge, that the motion artifacts can be greatly reduced by short illumination of individual CCD pixels and that this can be accomplished by use of two distinct classes of light sources: broadband pulsed sources and cw wavelength-swept sources. We experimentally demonstrate the benefit of these techniques in terms of the reduction of signal fading due to an axially moving sample and fiber-optic catheter at a high rotational speed.