Motion Correction in Optical Coherence Tomography for Multi-modality Retinal Image Registration

Optical coherence tomography (OCT) is a recently developed non-invasive imaging modality, which is often used in ophthalmology. Because of the sequential scanning in form of A-scans, OCT suffers from the inevitable eye movement. This often leads to mis-alignment especially among consecutive B-scans, which affects the analysis and processing of the data such as the registration of the OCT en face image to color fundus image. In this paper, we propose a novel method to correct the mis-alignment among consecutive B-scans to improve the accuracy in multi-modality retinal image registration. In the method, we propose to compute decorrelation from overlapping B-scans and to detect the eye movement. Then, the B-scans with eye movement will be re-aligned to its precedent scans while the rest of B-scans without eye movement are untouched. Our experiments results show that the proposed method improves the accuracy and success rate in the registration to color fundus images

Deep learning network to correct axial and coronal eye motion in 3D OCT retinal imaging

Optical Coherence Tomography (OCT) is one of the most important retinal imaging technique. However, involuntary motion artifacts still pose a major challenge in OCT imaging that compromises the quality of downstream analysis, such as retinal layer segmentation and OCT Angiography. We propose deep learning based neural networks to correct axial and coronal motion artifacts in OCT based on a single volumetric scan. The proposed method consists of two fully-convolutional neural networks that predict Z and X dimensional displacement maps sequentially in two stages. The experimental result shows that the proposed method can effectively correct motion artifacts and achieve smaller error than other methods. Specifically, the method can recover the overall curvature of the retina, and can be generalized well to various diseases and resolutions

Review on retrospective procedures to correct retinal motion artefacts in OCT imaging

Motion artefacts from involuntary changes in eye fixation remain a major imaging issue in optical coherence tomography (OCT). This paper reviews the state-of-the-art of retrospective procedures to correct retinal motion and axial eye motion artefacts in OCT imaging. Following an overview of motion induced artefacts and correction strategies, a chronological survey of retrospective approaches since the introduction of OCT until the current days is presented. Pre-processing, registration, and validation techniques are described. The review finishes by discussing the limitations of the current techniques and the challenges to be tackled in future developments

Automated Fovea Detection in Spectral Domain Optical Coherence Tomography Scans of Exudative Macular Disease

In macular spectral domain optical coherence tomography (SD-OCT) volumes, detection of the foveal center is required for accurate and reproducible follow-up studies, structure function correlation, and measurement grid positioning. However, disease can cause severe obscuring or deformation of the fovea, thus presenting a major challenge in automated detection. We propose a fully automated fovea detection algorithm to extract the fovea position in SD-OCT volumes of eyes with exudative maculopathy. The fovea is classified into 3 main appearances to both specify the detection algorithm used and reduce computational complexity. Based on foveal type classification, the fovea position is computed based on retinal nerve fiber layer thickness. Mean absolute distance between system and clinical expert annotated fovea positions from a dataset comprised of 240 SD-OCT volumes was 162.3 µm in cystoid macular edema and 262 µm in nAMD. The presented method has cross-vendor functionality, while demonstrating accurate and reliable performance close to typical expert interobserver agreement. The automatically detected fovea positions may be used as landmarks for intra- and cross-patient registration and to create a joint reference frame for extraction of spatiotemporal features in “big data.” Furthermore, reliable analyses of retinal thickness, as well as retinal structure function correlation, may be facilitated

Ultra-widefield optical coherence tomography of the human retina

Optical coherence tomography (OCT) has become a standard of care in ophthalmology since it was first demonstrated over 20 years ago. Images acquired using commercial OCT systems have been reported to have a maximum imaging length of only 9 mm, which is equivalent to an internal-field of view of 45◦. This narrow-field limitation has been acceptable because of the high level of clinical data available in the macular region and the relative simplicity of imaging this area of the retina. Commercially-available scanning laser ophthalmoloscope (SLO) systems such as the Optos 200Tx are capable of providing a fundus image with an internal-field of view of 200◦. These systems have shown that significant early disease markers can be found earlier by investigating the retinal periphery. It is therefore clinically desirable to merge the 3D measurement of OCT with the ultra widefield capability of the Optos SLO to allow clinicians to investigate the underlying morphology and progression of disease in the retinal periphery. To meet this clinical need, an ultra-widefield SD:OCT prototype system has been developed using the Optos ellipsoidal-mirror architecture. Modifications were made to the standard SD:OCT system to compensate for off-axis defocus, varying optical path difference, the changing corneal birefringence and the limitations inherent for the Optos ellipsoidal mirrors. The optical performance of the ultra-widefield SD:OCT system was verified using a novel wide-field phantom eye (WPE). The WPE was designed to measure; the transverse and axial point-spread function, field of view, imaging range, sensitivity roll-off, dispersion and measurement accuracy of the ultra-widefield SD:OCT system in both the posterior and peripheral segments of the retina. From these modifications, commercially-viable, ultra-widefield, SD:OCT has been demonstrated and verified using the WPE. In addition, we report the use of the WPE to compare both the imaging performance and measurement accuracy of the following ophthalmic instruments: the Optos 200Tx, Heidelberg Spectralis, Zeiss FF4, Optovue iVue, Zeiss Cirrus and Optos OCT/SLO. The WPE was successful in extracting image performance metrics for imaging feature sizes above 20 μm; however, targets fabricated using 3D-printing will require either a further advancement of the technology or hybridising with higher-precision structures to measure axial and transverse resolution

Identification of Surrogate Anatomic Identifiers of Disease Progression in Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is the leading cause of vision loss in patients over 50 in the developed world. The visual impairment is due to either choroidal neovascularisation (wet AMD) or geographic atrophy (GA). Drusen is the hallmark of AMD but the presence of drusen does not inform progression to wet AMD. Although the disease is mostly bilateral, the rate of progression of disease in both eyes may not be simultaneous. If one eye is affected by wet AMD, the risk of progression of the fellow eye to wet AMD increases by 10% every year. However, there are no markers that inform the time of conversion to wet AMD. For this reason, there is an unmet need to identify biomarkers that can fully predict the progression to wet AMD in order to allow early intervention before permanent damage. My thesis aimed to assess whether changes in imaging characteristics can more precisely explain conversion. I studied various cohorts including (a) normal aging eyes (b) eyes with early/ intermediate AMD and (c) fellow eyes of unilateral wet AMD to study the conversion to wet AMD. Firstly, I evaluated longitudinally volume changes in inner and outer retinal layers of 71 eyes with early/intermediate AMD using optical coherence tomography (OCT). Our results showed that inner and outer retina layer volumes may differentiate AMD eyes from healthy eyes. When comparing those who progressed to wet AMD at year 2 to those who did not, we found that baseline volume of GCIPL may differentiate between the 2 groups. As it is an inner retinal change, I hypothesized that heritability of the retinal layers may influence the rate of retinal layer changes and that may in turn help understand the changes seen in aging and AMD. I worked with the TWIN Study database, in which OCT was done in eyes of twins of different age groups and OCT data were available on 364 eyes of 184 (92 pair) twins. I evaluated whether heritability was responsible for ageing changes of the retinal layers. I found that total retinal volume and inner retinal layer volumes may be affected by genetic factors

Structural and functional macular inter-ocular asymmetry in patients with high myopia

BACKGROUND: Retinal evaluation performed in the optometry clinic allows optometrists to track the prognosis of retinal disease and to assess anatomical and functional changes related to age and pathological and non-pathological conditions. OBJECTIVES: The purpose of this research was to investigate the normal range of inter-ocular asymmetry in retinal and choroidal thickness and in retinal sensitivity high myopia without ocular fundus manifestations and to determine the relationship between inter-ocular asymmetry and refractive error. METHODS: Forty-three patients (35 07 ± 13.31 years) with high myopia, and 45 healthy participants (39.9 ± 14.1 years) were administered an ocular coherence tomography and a microperimetry examination to determine retinal and choroidal thickness and retinal sensitivity at the foveal region and at 1, 2 and 3 mm, nasally, temporally, superiorly and inferiorly. Absolute inter-ocular differences were calculated to determine the normal range of asymmetry, in 95% confidence intervals. RESULTS: The choroid was thinner in the myopic group at all explored locations (all p < 0.05), with larger absolute inter­ ocular differences in most of the choroidal locations under evaluation (all p < 0.05). Similarly, retinal sensitivity was reduced in the myopic group, although statistically significant differences were only encountered at the subfoveal location (p = 0.001). Retinar sensitivity asymmetry was found to increase with refractive error. CONCLUSION: The expanded range of retinal and choroidal thickness, as well as retinal sensitivity asymmetry found in high myopia in the absence of disease is of relevance when exploring these patients for early signs of ocular pathology.Fundamento y objetivos: La evaluación de la retina que se realiza en la clínica optométrica permite a los optometristas evaluar el estado de la retina y explorar los cambios funcionales y anatómicos asociados a la edad y a condiciones patológicas y no patológicas. El objetivo de la presente tesis era investigar el rango normal de asimetría inter-ocular en espesor retiniano y coroideo y en sensibilidad retiniana en pacientes con miopía alta sin manifestaciones oculares y determinar la posible asociación entre asimetría y error refractivo. Métodos: Cuarenta y tres pacientes (35.07 ± 13.31 años) con miopía alta y 45 participantes sanos (39.9 ± 14.1 años) siguieron un examen con tomografía de coherencia óptica y microperimetría para determinar el espesor de la retina y la coroides y la sensibilidad en retina en la zona foveal y a 1, 2 y 3 mm, en los cuadrantes nasal, temporal, superior e inferior. Se calcularon las diferencias inter-oculares absolutas para determinar el rango normal de asimetría, y los intervalos de confianza al 95%. Resultados: La coroides presento mayor delgadez en el grupo de miopes en todas las zonas sometidas a examen (todas las p < 0.05), con mayores diferencias absolutas en términos de asimetría en la mayoría de las zonas exploradas (p < 0.05). Igualmente, la sensibilidad retiniana se encontró reducida en el grupo miope, si bien sólo se hallaron diferencias estadísticas en la zona subfoveal (p = 0.001). La asimetría en sensibilidad retiniana aumentaba con el error refractivo. Conclusiones: El rango extendido en asimetría en espesor de retina y coroides y en sensibilidad retiniana hallado en miopía alta sin afectación ocular es muy relevante en el momento de explorar estos pacientes para la detección de los primeros signos de patología ocular.Postprint (published version

Wide-field anterior ocular surface morphometrics

The current understanding of anterior eye shape in humans is limited due to available technology and its accessibility. Accurate curvature metrics of specific areas of the peripheral cornea, corneo-limbal junction and anterior sclera have remained obscured by the limits of the palpebral aperture, since the upper and lower eyelids cover most of the vertical aspect. This thesis starts by comparing the ‘gold standard’ keratometry measurements to commonly used topographic systems. Keratometric analogues were found to be significantly different and in addition provided spurious vertical anterior ocular surface (AOS) profiles. These findings revealed a need to establish an accurate model. Magnetic resonance imaging (MRI) potentially offers the best opportunity to image the entire AOS structure. However, preliminary studies in this thesis demonstrated that the use of a 3-Tesla MRI scanner was unable to obtain sufficiently resolute data to meet requirements. As an alternative, ocular impression taking techniques were adopted during the remainder of this work to acquire the AOS data. Eye casts from impression moulds were scanned using active laser triangulation and virtual 3-dimensional surfaces rendered. Further investigations defined the most suitable material for impression taking and the amount of deformation of the AOS caused by the procedure. The ocular impression casting and scanning process was examined for accuracy and reliability. This protocol was used to sample a population of normal white European eyes in order to establish a database and define wide-field AOS variability. Volumetric and 2-dimensional topographic profiles were extracted from the digital 3-dimensional representation obtained, allowing for the analysis of point-to-point curvature differences. For the first time, the entire AOS shape has been defined with known accuracy. In addition, effects of myopic refractive error and gender are presented. This data is of potential importance to ophthalmic surgeons, ocularists, contact lens practitioners, vision scientists and researchers, in the form of a digital archive of normal white European wide-field AOS topography as a reference source.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Wide-field anterior ocular surface morphometrics

The current understanding of anterior eye shape in humans is limited due to available technology and its accessibility. Accurate curvature metrics of specific areas of the peripheral cornea, corneo-limbal junction and anterior sclera have remained obscured by the limits of the palpebral aperture, since the upper and lower eyelids cover most of the vertical aspect. This thesis starts by comparing the ‘gold standard’ keratometry measurements to commonly used topographic systems. Keratometric analogues were found to be significantly different and in addition provided spurious vertical anterior ocular surface (AOS) profiles. These findings revealed a need to establish an accurate model. Magnetic resonance imaging (MRI) potentially offers the best opportunity to image the entire AOS structure. However, preliminary studies in this thesis demonstrated that the use of a 3-Tesla MRI scanner was unable to obtain sufficiently resolute data to meet requirements. As an alternative, ocular impression taking techniques were adopted during the remainder of this work to acquire the AOS data. Eye casts from impression moulds were scanned using active laser triangulation and virtual 3-dimensional surfaces rendered. Further investigations defined the most suitable material for impression taking and the amount of deformation of the AOS caused by the procedure. The ocular impression casting and scanning process was examined for accuracy and reliability. This protocol was used to sample a population of normal white European eyes in order to establish a database and define wide-field AOS variability. Volumetric and 2-dimensional topographic profiles were extracted from the digital 3-dimensional representation obtained, allowing for the analysis of point-to-point curvature differences. For the first time, the entire AOS shape has been defined with known accuracy. In addition, effects of myopic refractive error and gender are presented. This data is of potential importance to ophthalmic surgeons, ocularists, contact lens practitioners, vision scientists and researchers, in the form of a digital archive of normal white European wide-field AOS topography as a reference source.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Wide-field anterior ocular surface morphometrics

The current understanding of anterior eye shape in humans is limited due to available technology and its accessibility. Accurate curvature metrics of specific areas of the peripheral cornea, corneo-limbal junction and anterior sclera have remained obscured by the limits of the palpebral aperture, since the upper and lower eyelids cover most of the vertical aspect. This thesis starts by comparing the ‘gold standard’ keratometry measurements to commonly used topographic systems. Keratometric analogues were found to be significantly different and in addition provided spurious vertical anterior ocular surface (AOS) profiles. These findings revealed a need to establish an accurate model. Magnetic resonance imaging (MRI) potentially offers the best opportunity to image the entire AOS structure. However, preliminary studies in this thesis demonstrated that the use of a 3-Tesla MRI scanner was unable to obtain sufficiently resolute data to meet requirements. As an alternative, ocular impression taking techniques were adopted during the remainder of this work to acquire the AOS data. Eye casts from impression moulds were scanned using active laser triangulation and virtual 3-dimensional surfaces rendered. Further investigations defined the most suitable material for impression taking and the amount of deformation of the AOS caused by the procedure. The ocular impression casting and scanning process was examined for accuracy and reliability. This protocol was used to sample a population of normal white European eyes in order to establish a database and define wide-field AOS variability. Volumetric and 2-dimensional topographic profiles were extracted from the digital 3-dimensional representation obtained, allowing for the analysis of point-to-point curvature differences. For the first time, the entire AOS shape has been defined with known accuracy. In addition, effects of myopic refractive error and gender are presented. This data is of potential importance to ophthalmic surgeons, ocularists, contact lens practitioners, vision scientists and researchers, in the form of a digital archive of normal white European wide-field AOS topography as a reference source