Showing papers on "Optical coherence tomography published in 2011"

Imaging the subcellular structure of human coronary atherosclerosis using micro–optical coherence tomography

Abstract: Progress in understanding, diagnosis, and treatment of coronary artery disease (CAD) has been hindered by our inability to observe cells and extracellular components associated with human coronary atherosclerosis in situ. The current standards for microstructural investigation, histology and electron microscopy are destructive and prone to artifacts. The highest-resolution intracoronary imaging modality, optical coherence tomography (OCT), has a resolution of ~10 μm, which is too coarse for visualizing most cells. Here we report a new form of OCT, termed micro-optical coherence tomography (μOCT), whose resolution is improved by an order of magnitude. We show that μOCT images of cadaver coronary arteries provide clear pictures of cellular and subcellular features associated with atherogenesis, thrombosis and responses to interventional therapy. These results suggest that μOCT can complement existing diagnostic techniques for investigating atherosclerotic specimens, and that μOCT may eventually become a useful tool for cellular and subcellular characterization of the human coronary wall in vivo.

Macular choroidal thickness and volume in normal subjects measured by swept-source optical coherence tomography.

Abstract: PURPOSE. To study the choroidal thickness in healthy subjects by swept-source optical coherence tomography (SS-OCT) at longer wavelength. METHODS. The macular area of 31 eyes (31 healthy volunteers) was studied with an SS-OCT prototype system, which uses a tunable laser as a light source operated at 100,000 Hz A scan repetition rate in the 1-m wavelength region. Three-dimensional volumetric measurement comprised of 512 128 A scans was acquired in 0.8 second. From a series of OCT images, a chroidal thickness map of the macular area was created by manual segmentation. To evaluate interoperator reproducibility, the choroidal thickness in each section from 10 subjects was determined independently by two observers. RESULTS. SS-OCT at the 1-m wavelength region allowed visualization of the fine structure of the choroid as well as that of the retina. Mean choroidal thickness and volume in the macula area were, respectively, 191.5 74.2 m and 5.411 2.097 mm 3 . The mean choroidal thickness of the outer nasal area was significantly thinner than that of all other areas (P 0.05). The measurements by the two independent observers were significantly identical; the intraclass correlation coefficient in mean choroidal thickness was between 0.945 and 0.980 in each area. The macular choroidal thickness was significantly correlated with axial length after adjustment for age (P 0.001), and with age after adjustment for axial length (P 0.001). CONCLUSIONS. SS-OCT system at 1 m provides macular choroidal thickness maps and allows one to evaluate the choroidal thickness more accurately. (Invest Ophthalmol Vis Sci. 2011; 52:4971‐4978) DOI:10.1167/iovs.11-7729

Megahertz OCT for ultrawide-field retinal imaging with a 1050nm Fourier domain mode-locked laser

Abstract: We demonstrate ultrahigh speed swept source retinal OCT imaging using a Fourier domain mode locked (FDML) laser. The laser uses a combination of a semiconductor optical amplifier and an ytterbium doped fiber amplifier to provide more than 50mW output power. The 1050nm FDML laser uses standard telecom fiber for the km long delay line instead of two orders of magnitude more expensive real single mode fiber. We investigate the influence of this “oligo-mode” fiber on the FDML laser performance. Two design configurations with 684,400 and 1,368,700 axial scans per second are investigated, 25x and 50x faster than current commercial instruments and more than 4x faster than previous single spot ophthalmic results. These high speeds enable the acquisition of densely sampled ultrawide-field data sets of the retina within a few seconds. Ultrawide-field data consisting of 1900 x 1900 A-scans with ~70° degrees angle of view are acquired within only 3 and 6 seconds using the different setups. Such OCT data sets, more than double as large as previously reported, are collapsed to a 4 megapixel high definition fundus image. We achieve good penetration into the choroid by hardware spectral shaping of the laser output. The axial resolution in tissue is 12µm (684kHz) and 19µm (1.37MHz). A series of new data processing and imaging extraction protocols, enabled by the ultrawide-field isotropic data sets, are presented. Dense isotropic sampling enables both, cross-sectional images along arbitrary coordinates and depth-resolved en-face fundus images. Additionally, we investigate how isotropic averaging compares to the averaging of cross-sections along the slow axis.

Primary retinal pathology in multiple sclerosis as detected by optical coherence tomography

Abstract: Optical coherence tomography studies in multiple sclerosis have primarily focused on evaluation of the retinal nerve fibre layer. The aetiology of retinal changes in multiple sclerosis is thought to be secondary to optic nerve demyelination. The objective of this study was to use optical coherence tomography to determine if a subset of patients with multiple sclerosis exhibit primary retinal neuronopathy, in the absence of retrograde degeneration of the retinal nerve fibre layer and to ascertain if such patients may have any distinguishing clinical characteristics. We identified 50 patients with multiple sclerosis with predominantly macular thinning (normal retinal nerve fibre-layer thickness with average macular thickness < 5th percentile), a previously undescribed optical coherence tomography defined phenotype in multiple sclerosis, and compared them with 48 patients with multiple sclerosis with normal optical coherence tomography findings, 48 patients with multiple sclerosis with abnormal optical coherence tomography findings (typical for multiple sclerosis) and 86 healthy controls. Utilizing a novel retinal segmentation protocol, we found that those with predominant macular thinning had significant thinning of both the inner and outer nuclear layers, when compared with other patients with multiple sclerosis (P < 0.001 for both), with relative sparing of the ganglion cell layer. Inner and outer nuclear layer thicknesses in patients with non-macular thinning predominant multiple sclerosis were not different from healthy controls. Segmentation analyses thereby demonstrated extensive deeper disruption of retinal architecture in this subtype than may be expected due to retrograde degeneration from either typical clinical or sub-clinical optic neuropathy. Functional corroboration of retinal dysfunction was provided through multi-focal electroretinography in a subset of such patients. These findings support the possibility of primary retinal pathology in a subset of patients with multiple sclerosis. Multiple sclerosis-severity scores were also significantly increased in patients with the macular thinning predominant phenotype, compared with those without this phenotype (n = 96, P=0.006). We have identified a unique subset of patients with multiple sclerosis in whom there appears to be disproportionate thinning of the inner and outer nuclear layers, which may be occurring as a primary process independent of optic nerve pathology. In vivo analyses of retinal layers in multiple sclerosis have not been previously performed, and structural demonstration of pathology in the deeper retinal layers, such as the outer nuclear layer, has not been previously described in multiple sclerosis. Patients with inner and outer nuclear layer pathology have more rapid disability progression and thus retinal neuronal pathology may be a harbinger of a more aggressive form of multiple sclerosis.

Revealing Henle's Fiber Layer Using Spectral Domain Optical Coherence Tomography

Abstract: Henle's fiber layer (HFL) contains bundles of unmyelinated cone and rod photoreceptor axons terminating in the pedicles and spherules that synapse in the retinal outer plexiform layer (OPL).1 These fibers are intermingled with Muller cell processes and are obliquely oriented as a result of foveal pit development where photoreceptors migrate inward and ganglion cells migrate outward.2,3 Like axons elsewhere in the central nervous system, the axons of HFL contain microtubules and are long, cylindrical structures.1–3 Their average length is 558 μm,4 and the first synapses occur with dendrites of bipolar and horizontal cells approximately 350 μm from the foveal center.4 Given the large number of central foveal photoreceptor nuclei and this marked displacement, HFL constitutes a significant fraction of the thickness of retinal layers within the macula, as is evident histologically (Fig. 1). HFL is oriented radially about the fovea and is indirectly visible ophthalmoscopically in patients with macular star formation in neuroretinitis.5 HFL also demonstrates the optical property of form birefringence, a property that can be exploited to infer the location of the foveal center as a direct consequence of its consistent effects on polarized light.6 Figure 1. Mammalian foveal histology, courtesy of Roger C. Wagner, Professor Emeritus of Biological Sciences, University of Delaware, http://dspace.udel.edu:8080/dspace/handle/19716/1884. Photoreceptor components are indicated by the rectangle, showing the substantial ... Optical coherence tomography (OCT) uses infrared light to interferometrically derive optical reflectivity information varying by depth in living tissues.7 The use of broadband light sources, a spectrometer, and the application of signal processing techniques has culminated in commercial spectral domain (SD) OCT systems that are capable of imaging with a 5-μm axial resolution in retinal tissue.8,9 These improvements, along with acquisition speeds fast enough to permit frame averaging without significant motion artifact, contribute to improved image quality in SD-OCT devices capable of identifying structures that could not previously be resolved.10 Despite advances in SD-OCT hardware and software, HFL visualization has remained elusive. A seminal paper published in 2004 comparing in vitro OCT images of monkey fovea to histology recognized HFL as a major layer of the retina that could be visualized in vitro and that should be accounted for in vivo.11 However, since that publication, HFL has not been included in diagrams of retinal layers visualized by OCT, likely because of the inability to distinguish a change in reflectivity at the interface between the HFL and the outer nuclear layer (ONL).12–14 Furthermore, segmentation algorithms, normative thickness data, and ONL measurements overlying drusen have recently been published that do not recognize the contribution to macular thickness provided by HFL to the ONL.15–19 Although the need to account for the presence of HFL in OCT images has recently been recognized, the proposed means to measure the contribution of the HFL was an inferential normative model based on high-quality pathologic specimens rather than by a direct means of visualization (Curcio CA, et al. IOVS. 2010;51:ARVO E-Abstract 2286). The only explicit mention of the optical conditions in which HFL may be visualized with OCT can be found in a letter to the editor and was not experimentally validated.20 In our study we describe a systematic method that can be applied to commercial SD-OCT systems to directly visualize and quantify HFL. Additionally, we provide an optical explanation of this phenomenon and demonstrate its clinical relevance.

Reflective afocal broadband adaptive optics scanning ophthalmoscope.

Abstract: A broadband adaptive optics scanning ophthalmoscope (BAOSO) consisting of four afocal telescopes, formed by pairs of off-axis spherical mirrors in a non-planar arrangement, is presented. The non-planar folding of the telescopes is used to simultaneously reduce pupil and image plane astigmatism. The former improves the adaptive optics performance by reducing the root-mean-square (RMS) of the wavefront and the beam wandering due to optical scanning. The latter provides diffraction limited performance over a 3 diopter (D) vergence range. This vergence range allows for the use of any broadband light source(s) in the 450-850 nm wavelength range to simultaneously image any combination of retinal layers. Imaging modalities that could benefit from such a large vergence range are optical coherence tomography (OCT), multi- and hyper-spectral imaging, single- and multi-photon fluorescence. The benefits of the non-planar telescopes in the BAOSO are illustrated by resolving the human foveal photoreceptor mosaic in reflectance using two different superluminescent diodes with 680 and 796 nm peak wavelengths, reaching the eye with a vergence of 0.76 D relative to each other.

Repeatability of Manual Subfoveal Choroidal Thickness Measurements in Healthy Subjects Using the Technique of Enhanced Depth Imaging Optical Coherence Tomography

Abstract: Purpose The aim of this study was to investigate the repeatability of manual measurements of choroidal thickness in healthy subjects imaged on spectral domain optical coherence tomography (OCT) using the enhanced depth imaging (EDI) technique. Methods Fifty consecutive, healthy, young, adult volunteers with no known eye disease were enrolled prospectively. Two good-quality horizontal and vertical line scans through the fovea were obtained for each eye. Using the manual calipers provided by the software of the proprietary device, two experienced OCT readers measured the subfoveal choroidal thickness (SFCT) of the horizontal and vertical line scans for all eyes. The readers were masked to each other's readings. Intraobserver, interobserver, and intrasession coefficients of repeatability (CRs) were calculated. Results Mean (standard deviation [SD]) age of the study subjects was 38 (5) years (range, 30-49 years). Mean (SD) subfoveal choroidal thickness was 332 (90) μm (right eyes) and 332 (91) μm (left eyes). Intraobserver CR was approximately 23 (95% confidence interval [CI], 19-26) μm, whereas interobserver and intrasession CRs were greater at 32 (95% CI, 30-34) and 34 (95% CI, 32-36) μm, respectively. There was no significant difference in SFCT between all pairs of SFCT measurements except for the two intrasession vertical line scans. Conclusion A change of >32 μm was likely to exceed interobserver variability in SFCT. Future studies are required to estimate the repeatability of SFCT measurements in patients with chorioretinal pathology.

Subfoveal Choroidal Thickness in Typical Age-Related Macular Degeneration and Polypoidal Choroidal Vasculopathy

Abstract: Purpose To investigate the subfoveal choroidal thickness in eyes with typical age-related macular degeneration (AMD) and polypoidal choroidal vasculopathy (PCV), using enhanced depth imaging optical coherence tomography.

Intra-arterial catheter for simultaneous microstructural and molecular imaging in vivo

Abstract: The future of imaging is the integration of function and anatomy Hongki Yoo et al have successfully done just that by combining two existing intravascular imaging techniques into a single catheter-based system Their dual-modality intra-arterial catheter uses a combination of optical frequency domain imaging and near-infrared fluorescence imaging to simultaneously provide molecular information in the context of the surrounding three-dimensional microanatomy of the artery wall

Optical Coherence Tomography: History, Current Status, and Laboratory Work

Abstract: Optical coherence tomography (OCT) imaging has become widespread in ophthalmology over the past 15 years, because of its ability to visualize ocular structures at high resolution. This article reviews the history of OCT imaging of the eye, its current status, and the laboratory work that is driving the future of the technology.

In vivo imaging of the microcirculation of the volar forearm using correlation mapping optical coherence tomography (cmOCT).

Abstract: Correlation mapping optical coherence tomography (cmOCT) is a recently proposed technique that extends the capabilities of OCT to enable mapping of vasculature networks. The technique is achieved as a processing step on OCT intensity images that does not require any modification to existing OCT hardware. In this paper we apply the cmOCT processing technique to in vivo human imaging of the volar forearm. We illustrate that cmOCT can produce maps of the microcirculation that clearly follow the accepted anatomical structure. We demonstrate that the technique can extract parameters such as capillary density and vessel diameter. These parameters are key clinical markers for the early changes associated with microvascular diseases. Overall the presented results show that cmOCT is a powerful new tool that generates microcirculation maps in a safe non-invasive, non-contact technique which has clear clinical applications.

Optical coherence tomography in dermatology: technical and clinical aspects

Abstract: Optical coherence tomography (OCT), a fairly new non-invasive optical real-time imaging modality, is an emergent in vivo technique, based on the interference (Michelson interferometry) of infrared radiation and living tissues, that allows high-resolution, 2- or 3-dimensional, cross-sectional visualisation of microstructural morphology of tissues. OCT provides depth-resolved images of tissues with resolution up to a few micrometers and depth up to several millimetres depending on tissue type. The investigations using OCT to assess skin structure in clinical settings started in the past decade and consequently proved that this imaging method is useful in visualizing subsurface structures of normal skin, including the epidermis, dermoepidermal junction, dermis, hair follicles, blood vessels and sweat ducts. An increasing number of papers brought evidence of the utility and the precision of OCT technology, in its different technical variants, in diagnosing and monitoring skin disorders, including malignancies and inflammatory conditions, respectively. The present comprehensive review describes and illustrates technical aspects and clinical applications of OCT methods in dermatology.

Choroidal evaluation using enhanced depth imaging spectral-domain optical coherence tomography in Vogt-Koyanagi-Harada disease.

Abstract: Background:Current imaging modalities used in the evaluation of Vogt-Koyanagi-Harada (VKH) disease include ultrasound, fluorescein angiogram, indocyanine green angiography, and optical coherence tomography (OCT). However, they all fail to give detailed information on the ultrastructural changes of t

In vivo volumetric imaging of human retinal circulation with phase-variance optical coherence tomography

Abstract: We present in vivo volumetric images of human retinal micro-circulation using Fourier-domain optical coherence tomography (Fd-OCT) with the phase-variance based motion contrast method. Currently fundus fluorescein angiography (FA) is the standard technique in clinical settings for visualizing blood circulation of the retina. High contrast imaging of retinal vasculature is achieved by injection of a fluorescein dye into the systemic circulation. We previously reported phase-variance optical coherence tomography (pvOCT) as an alternative and non-invasive technique to image human retinal capillaries. In contrast to FA, pvOCT allows not only noninvasive visualization of a two-dimensional retinal perfusion map but also volumetric morphology of retinal microvasculature with high sensitivity. In this paper we report high-speed acquisition at 125 kHz A-scans with pvOCT to reduce motion artifacts and increase the scanning area when compared with previous reports. Two scanning schemes with different sampling densities and scanning areas are evaluated to find optimal parameters for high acquisition speed in vivo imaging. In order to evaluate this technique, we compare pvOCT capillary imaging at 3x3 mm2 and 1.5x1.5 mm2 with fundus FA for a normal human subject. Additionally, a volumetric view of retinal capillaries and a stitched image acquired with ten 3x3 mm2 pvOCT sub-volumes are presented. Visualization of retinal vasculature with pvOCT has potential for diagnosis of retinal vascular diseases.

Molecular imaging true-colour spectroscopic optical coherence tomography

Abstract: Molecular imaging holds a pivotal role in medicine due to its ability to provide invaluable insight into disease mechanisms at molecular and cellular levels. To this end, various techniques have been developed for molecular imaging, each with its own advantages and disadvantages(1-4). For example, fluorescence imaging achieves micrometre-scale resolution, but has low penetration depths and is mostly limited to exogenous agents. Here, we demonstrate molecular imaging of endogenous and exogenous chromophores using a novel form of spectroscopic optical coherence tomography. Our approach consists of using a wide spectral bandwidth laser source centred in the visible spectrum, thereby allowing facile assessment of haemoglobin oxygen levels, providing contrast from readily available absorbers, and enabling true-colour representation of samples. This approach provides high spectral fidelity while imaging at the micrometre scale in three dimensions. Molecular imaging true-colour spectroscopic optical coherence tomography (METRiCS OCT) has significant implications for many biomedical applications including ophthalmology, early cancer detection, and understanding fundamental disease mechanisms such as hypoxia and angiogenesis.

Total retinal blood flow measurement with ultrahigh speed swept source/Fourier domain OCT.

Abstract: Doppler OCT provides depth-resolved information on flow in biological tissues. In this article, we demonstrate ultrahigh speed swept source/Fourier domain OCT for visualization and quantitative assessment of retinal blood flow. Using swept laser technology, the system operated in the 1050-nm wavelength range at a high axial scan rate of 200 kHz. The rapid imaging speed not only enables volumetric imaging with high axial scan densities, but also enables measurement of high flow velocities in the central retinal vessels. Deep penetration in the optic nerve and lamina cribrosa was achieved by imaging at 1-µm wavelengths. By analyzing en-face images extracted from 3D Doppler data sets, absolute flow in single vessels as well as total retinal blood flow was measured using a simple and robust protocol that does not require measurement of Doppler angles. The results from measurements in healthy eyes suggest that ultrahigh speed swept source/Fourier domain OCT could be a promising technique for volumetric imaging of retinal vasculature and quantitation of retinal blood flow in a wide range of retinal diseases.

Optical coherence elastography: current status and future applications.

Abstract: Optical coherence tomography (OCT) has several advantages over other imaging modalities, such as angiography and ultrasound, due to its inherently high in vivo resolution, which allows for the identification of morphological tissue structures. Optical coherence elastography (OCE) benefits from the superior spatial resolution of OCT and has promising applications, including cancer diagnosis and the detailed characterization of arterial wall biomechanics, both of which are based on the elastic properties of the tissue under investigation. We present OCE principles based on techniques associated with static and dynamic tissue excitation, and their corresponding elastogram image-reconstruction algorithms are reviewed. OCE techniques, including the development of intravascular- or catheter-based OCE, are in their early stages of development but show great promise for surgical oncology or intravascular cardiology applications.

Multimodal photoacoustic and optical coherence tomography scanner using an all optical detection scheme for 3D morphological skin imaging.

Abstract: A noninvasive, multimodal photoacoustic and optical coherence tomography (PAT/OCT) scanner for three-dimensional in vivo (3D) skin imaging is described. The system employs an integrated, all optical detection scheme for both modalities in backward mode utilizing a shared 2D optical scanner with a field-of-view of ~13 × 13 mm(2). The photoacoustic waves were detected using a Fabry Perot polymer film ultrasound sensor placed on the surface of the skin. The sensor is transparent in the spectral range 590-1200 nm. This permits the photoacoustic excitation beam (670-680 nm) and the OCT probe beam (1050 nm) to be transmitted through the sensor head and into the underlying tissue thus providing a backward mode imaging configuration. The respective OCT and PAT axial resolutions were 8 and 20 µm and the lateral resolutions were 18 and 50-100 µm. The system provides greater penetration depth than previous combined PA/OCT devices due to the longer wavelength of the OCT beam (1050 nm rather than 829-870 nm) and by operating in the tomographic rather than the optical resolution mode of photoacoustic imaging. Three-dimensional in vivo images of the vasculature and the surrounding tissue micro-morphology in murine and human skin were acquired. These studies demonstrated the complementary contrast and tissue information provided by each modality for high-resolution 3D imaging of vascular structures to depths of up to 5 mm. Potential applications include characterizing skin conditions such as tumors, vascular lesions, soft tissue damage such as burns and wounds, inflammatory conditions such as dermatitis and other superficial tissue abnormalities.

Segmentation of Intra-Retinal Layers From Optical Coherence Tomography Images Using an Active Contour Approach

Abstract: Optical coherence tomography (OCT) is a noninvasive, depth-resolved imaging modality that has become a prominent ophthalmic diagnostic technique. We present a semi-automated segmentation algorithm to detect intra-retinal layers in OCT images acquired from rodent models of retinal degeneration. We adapt Chan-Vese's energy-minimizing active contours without edges for the OCT images, which suffer from low contrast and are highly corrupted by noise. A multiphase framework with a circular shape prior is adopted in order to model the boundaries of retinal layers and estimate the shape parameters using least squares. We use a contextual scheme to balance the weight of different terms in the energy functional. The results from various synthetic experiments and segmentation results on OCT images of rats are presented, demonstrating the strength of our method to detect the desired retinal layers with sufficient accuracy even in the presence of intensity inhomogeneity resulting from blood vessels. Our algorithm achieved an average Dice similarity coefficient of 0.84 over all segmented retinal layers, and of 0.94 for the combined nerve fiber layer, ganglion cell layer, and inner plexiform layer which are the critical layers for glaucomatous degeneration.

Brain refractive index measured in vivo with high-NA defocus-corrected full-field OCT and consequences for two-photon microscopy

Abstract: Two-photon laser scanning microscopy (2PLSM) is an important tool for in vivo tissue imaging with sub-cellular resolution, but the penetration depth of current systems is potentially limited by sample-induced optical aberrations. To quantify these, we measured the refractive index n' in the somatosensory cortex of 7 rats in vivo using defocus optimization in full-field optical coherence tomography (ff-OCT). We found n' to be independent of imaging depth or rat age. From these measurements, we calculated that two-photon imaging beyond 200 µm into the cortex is limited by spherical aberration, indicating that adaptive optics will improve imaging depth.

Optical Coherence Tomography May Be Used to Predict Visual Acuity in Patients with Macular Edema

Abstract: Purpose. To determine whether the volume of retinal tissue passing between the inner and outer retina in macular edema could be used as an indicator of visual acuity. Methods. Diabetic and uveitic patients with cystoid macular edema (81 subjects, 129 eyes) were recruited. Best corrected logMAR visual acuity and spectral optical coherence tomography (OCT/SLO; OTI, Toronto, ONT, Canada) were performed in all patients. Coronal OCT scans obtained from a cross section of the retina between the plexiform layers were analyzed with a grid of five concentric radii (500, 1000, 1500, 2000, and 2500 μm centered on the fovea). The images were analyzed to determine the amount of retinal tissue present within each ring. A linear regression model was developed to determine the relationship between tissue integrity and logMAR visual acuity. Results. A linear relationship between tissue integrity and VA was demonstrated. The volume of retinal tissue between the plexiform layers in rings 1 and 2 (up to 1000 μm from the foveal center) predicted 80% of visual acuity. By contrast, central macular thickness within the central 1000 μm predicted only 14% of visual acuity. Conclusions. This study showed that the cross-sectional area of retinal tissue between the plexiform layers in cystoid macular edema, as imaged by OCT, is the best indicator of visual function at baseline. Further prospective treatment trials are needed to investigate this parameter as a predictor of visual outcome after intervention.

In vivo three-dimensional optical coherence elastography

Abstract: We present the first three-dimensional (3D) data sets recorded using optical coherence elastography (OCE). Uni-axial strain rate was measured on human skin in vivo using a spectral-domain optical coherence tomography (OCT) system providing >450 times higher line rate than previously reported for in vivo OCE imaging. Mechanical excitation was applied at a frequency of 125 Hz using a ring actuator sample arm with, for the first time in OCE measurements, a controlled static preload. We performed 3D-OCE, processed in 2D and displayed in 3D, on normal and hydrated skin and observed a more elastic response of the stratum corneum in the hydrated case.

Enhanced depth imaging optical coherence tomography.

Abstract: Imaging the choroid with conventional commercial spectral-domain optical coherence tomography (SD-OCT) has been difficult, mainly because of difficulty in signal transmission beyond the retinal pigment epithelium. A recent modification to the standard technique, termed enhanced depth imaging optical coherence tomography (EDI-OCT), was able to image the choroid with reasonable clarity using commercial SD-OCTs. The aim of this article was to review the technique, principle, recent findings, and possible future developments regarding EDI-OCT. A MEDLINE search on all published articles on EDI-OCT was performed up to December 2010. The principle behind EDI-OCT was discussed. Modification to the conventional technique in image acquisition was described and illustrated with figures. EDI-OCT findings in various retinal and choroidal diseases were discussed. Advantages and disadvantages were also discussed. EDI-OCT has proved to be a promising novel technique in imaging the choroid.

Spatial distribution of posterior pole choroidal thickness by spectral domain optical coherence tomography.

Abstract: The choroid, a vascular meshwork between the retina and sclera, plays a major role in providing oxygen and nutrition to the outer layers of the retina.1 In recent years, increased awareness of its role in ocular development and its known association with many diseases of the posterior pole have stimulated a renewed focus on understanding choroidal anatomy and physiology.2 A number of methods, including histology3 and ultrasonography,4 have previously been used to quantify choroidal thickness (CT); however, the overall precision of these approaches is still lacking. Fortunately, the recent introduction of spectral domain optical coherence tomography (SD-OCT) and the description of “enhanced depth imaging” scanning protocols have afforded a new opportunity to improve the accuracy of quantitative choroidal assessment. As a result, changes in CT have now been studied using different OCT technologies over a wide range of ocular pathologies (e.g., glaucoma,5 inherited retinal diseases,6 high myopia,7 central serous chorioretinopathy,8 polypoidal choroidal vasculopathy,9 neovascular age-related macular degeneration [AMD],9 and Vogt Koyanagi Harada disease10). Although the use of OCT has provided many new insights into choroidal morphology, to date there exists a notable disparity between the CT measurements obtained in different studies. For example, several studies on the cross-sectional variation of CT measurements in “normal” subjects have shown CT to be greatest at the fovea, with decreasing thickness more nasally than temporally.9,11,12 Other results suggest that the foveal CT is thinner than the choroid superior to the fovea.13 These discrepancies may arise due to the use of different image acquisition methodologies, a generally limited field of view for scanning, and inconsistencies in choosing the exact locations for repeated CT measurement. Regardless, the exact spatial distribution of CT changes in the macular region of human eyes remains unclear. In addition, most OCT-derived studies have focused on assessment of CT in the macular region alone, examining the correlation between macular CT changes and disease expression. However, unlike the neurosensory retina, a complex and highly organized neural structure with the fovea as its unmistaken center, the choroid is a vascular layer with a potentially very different topography. Determination of choroidal spatial distribution may thus be difficult, and potentially inaccurate, in the context of macular scanning alone. In this report, we aim to address these issues by obtaining larger, two-dimensional (2D) maps of the spatial distribution of the choroid in the posterior pole and identifying its specific patterns in healthy volunteers using SD-OCT.

Pilot study of optical coherence tomography measurement of retinal blood flow in retinal and optic nerve diseases.

Abstract: The development of noninvasive methods such as magnetic resonance angiography1 and functional magnetic resonance imaging (MRI)2 to measure cerebral hemodynamics has greatly enhanced the study of neurologic diseases and functional neuroanatomy. Such methods would also be very helpful in the eye because the leading causes of blindness in the industrialized world—diabetic retinopathy, macular degeneration, and glaucoma—are all related to abnormal retinal3,4 or optic nerve blood flow.5 Unfortunately, MRI resolution is too coarse for quantitative imaging of retinal blood vessels, which have a very fine caliber. Although several techniques are being used for retinal blood flow evaluation, they all have serious limitations. Ultrasound color Doppler imaging has sufficient resolution to measure only the larger retrobulbar vessels.6 It can measure blood velocity but not vessel diameter; therefore, volumetric blood flow cannot be determined. Several types of laser Doppler techniques are able to measure flow in individual retinal vessels7–9 or capillary beds.10 Although it is possible to measure total retinal blood flow by adding measurements from individual vessels, this requires many measurements over a long session.8 These specialized instruments are generally available only in major research centers because they are expensive. Fluorescein and indocyanine green angiographies are widely used to visualize retinal and choroidal circulations. However, they do not provide quantitative measurements of blood flow and require the intravenous injection of dyes that have potential side effects.11 Optical coherence tomography (OCT)12 is commonly used in the diagnosis and management of retinal diseases.13–16 It has the requisite resolution to image retinal blood vessels.17 Because it is a coherent detection technique, OCT can detect the Doppler frequency shift of back-scattered light, which provides information on blood flow velocity.18,19 With the development of high-speed Fourier-domain OCT,20–22 it has become possible to capture the pulsatile dynamics of blood flow.23,24 Using Doppler Fourier-domain OCT, we developed a double circular scanning pattern (Fig. 1) that measures flow in all the blood vessels around the optic nerve head four to six times per second.25 Total retinal blood flow could be calculated with the data sampled within 2 seconds. We have demonstrated that flow measurements in normal subjects26 and in a patients with diabetic retinopathy27 can be reproducibly obtained. In this study, we used this new technique in a systematic investigation of blood flow abnormalities in retinal and optic nerve diseases. Figure 1. (a) Fundus photograph showing the double circular pattern of the OCT beam scanning retinal blood vessels emerging from the optic disc. (b) The relative position of a blood vessel in the two OCT cross-sections is used to calculate the Doppler angle θ ...

Evaluation of macular thickness and peripapillary retinal nerve fiber layer thickness for detection of early glaucoma using spectral domain optical coherence tomography.

Abstract: PURPOSE To evaluate the diagnostic ability of macular parameters and peripapillary retinal nerve fiber layer (RNFL) parameters for early glaucoma using spectral domain optical coherence tomography (SD-OCT). MATERIALS AND METHODS One eye from 32 early glaucoma patients (including preperimetric glaucoma) and 32 normal participants underwent macular scans and peripapillary RNFL scans with SD-OCT 3 times on the same day. The discrimination power of each parameter to detect early glaucoma was determined by areas under receiver operating characteristics curve (AROC) and sensitivity at fixed specificity. Correlation of OCT data with visual field defects was evaluated by linear regression analysis. Reproducibility was also evaluated. RESULTS Significant differences between early glaucoma and normal participants were found for all parameters except fovea in macular scans and in the superior and inferior quadrants, at 12, 3, 6, 7, 11 o'clock, and average RNFL thickness in RNFL scans. The best parameters based on AROC and sensitivity at a specificity of >90% were temporal outer macula thickness (AROC, 0.79; sensitivity, 63%) in macular parameters and inferior quadrant (AROC, 0.82; sensitivity, 53%) in RNFL parameters. The highest correlation with mean deviation was found in inferior inner macular volume (r=0.50, P<0.001). The mean intraclass correlation coefficient was 0.96 in macular scans and 0.84 in RNFL scans. Test-retest variability ranged from 2.3 to 10.1 μm in macular thickness, 0 to 0.06 mm in macular volume, and 5.8 to 18.9 μm in RNFL thickness. CONCLUSION For the diagnosis of early glaucoma by SD-OCT, macular parameters had high discriminating power and high reproducibility comparable with peripapillary RNFL parameters.