Visible Optical Coherence Tomography based Multimodal Imaging for Quantification of Retinal Lipofuscin

Retinal degeneration is the leading cause of irreversible low vision and blindness in the world, that describes conditions characterized by progressive loss of photoreceptors. Retinal Pigment Epithelium (RPE) is located under photoreceptors’ outer segments and plays an important role in the maintenance of photoreceptors by completing the visual cycle and phagocytosis of shed photoreceptor outer segments. Lipofuscin, a byproduct of the visual cycle, is a nondegradable compound that accumulates in the RPE cells and eventually damages the RPE cells and inevitably causes photoreceptor degeneration. Lipofuscin is the major cause of fundus fluorescence that can be detected by Fundus Autofluorescent (FAF) imaging systems. Reliable and quantified FAF values are necessary for lipofuscin quantification which can be a significant tool in the diagnosis of retinal degenerative disease in early stages and provide a better opportunity for treatment before the loss of vision stage. However, FAF is attenuated by the ocular media prior to the RPE, including cornea, lens, vitreous body, retinal layers in front of the RPE, and the melanin granules within the RPE cells that migrate to the apical region upon light exposure. This attenuation varies among people and for an individual over time and cannot be measured directly, thus hurdles measurement of the true FAF values. Further, differences in acquisition systems such as illumination power and detector sensitivity, directly affect the measured FAF. This issue has been addressed by implementing a reference target in the FAF imaging system. Normalizing the FAF signal to that of the target eliminates the dependency on the acquisition parameters. However, the issue of pre-RPE and RPE melanin attenuation remains unresolved. Further, the fluorescence characteristics of the commercially available fluorescent reference are quite different than retinal lipofuscin that challenges the quantification of the absolute amount of lipofuscin in the RPE. In this dissertation, we propose a new multimodal imaging system based on visible-light optical coherence tomography (VIS-OCT) that provides a three-dimensional image. The technology simultaneously acquires VIS-OCT and FAF with a single broadband visible light source. Since both images are originated from the same group of photons and travel through the same ocular media at the same time, the attenuation factor is similar in both modalities. Therefore, by normalizing FAF by VIS-OCT of the RPE layer, the attenuation of the pre_RPE media can be eliminated. Further, we implemented two reference targets to quantify VIS-OCT and FAF and eliminate the dependency on acquisition parameters. These references were later substituted by a single customized reference that consists of the major lipofuscin fluorophore, called A2E. The quantitative imaging independent of system fluctuation, and attenuation of pre-RPE and RPE melanin was successfully tested on retinal simulating phantoms, in vivo on the animal retina, and human subjects. The in vivo quantification in small animals linearly correlates with A2E content measured by mass spectrometry. Quantitative imaging of human retinas is consistent with the linear accumulation of lipofuscin with age. The VIS-OCT-FAF has the potential for clinical diagnosis

The Preferred Retinal Locus in Macular Degeneration: Relating Structure and Function

Purpose: Central field loss (CFL) that ensues from macular degeneration can impact many activities of daily living, including reading, in both younger (as in Stargardt disease, STGD) and older (age-related macular degeneration, AMD) subjects. Subjects with CFL typically choose a non-central retinal location, called the preferred retinal locus (PRL) for fixation. This dissertation aims to understand and relate functional and structural changes within the PRL. Methods: Preliminary studies determined the effectiveness of the MP-1 microperimeter (a) to compensate for excessively unstable fixational eye movements (FEMs), such as occur in subjects with CFL, and (b) to accurately register the retinal test locations on baseline and subsequent automated follow-up testing. Subsequently, the following functional measures were obtained for 29 subjects with bilateral CFL: (a) reading performance using hand-held MNRead charts and LCD-displayed MP1 Read charts, (b) contrast-detection thresholds using the Freiburg acuity test, (c) fixation stability on 3-letter words, measured as bivariate contour ellipse areas (BCEAs) with the MP-1, (d) sensitivity in the central visual field determined with a standard 10-2 threshold grid, and (e) fine-grained sensitivity within the word-fixation PRL for supra-threshold 13x13 arc min spots. Spectral-domain optical coherence tomography (SD-OCT) was used to assess structural characteristics of the PRL, specifically, thickness ratios for the retinal pigment epithelium - Bruch’s membrane complex (RPE-BM), the photoreceptor and outer nuclear layer (PL), and the total retina layers (TRL) between PRL locations where test spots were and were not consistently detected. Finally, 8 younger (50 years) naive subjects with normal vision read high and low contrast sentences presented one word at a time at the fovea and 5 and 10° in the inferior field. Random 13x13 arc min blocks corresponding to 0-78% of the text area were set to the background luminance to simulate retinal micro-scotomas (MSs) and a staircase algorithm estimated the threshold reading rate. Results: The MP-1 compensated ~90% of the experimentally induced increase in FEMs and the average registration error was ~8 arc min. The maximum reading speed of subjects with CFL correlated poorly with contrast thresholds, BCEA, PRL eccentricity, median sensitivity around the PRL and all retinal thickness ratios. Twenty-two of 29 subjects with CFL (AMD: 8/10 subjects; STGD: 10/12 subjects) exhibited one or more MSs, defined as local regions of insensitivity for supra-threshold targets within the PRL. Although the average percentage of MSs was similar in the cohorts with AMD (25.4%) and STGD (20.3%), reading speed was significantly faster in STGD than AMD subjects. Average thickness ratios for RPE-BM, PL and TRL were 0.97, 0.84 and 0.86 respectively in the AMD cohort and 0.97, 0.77 and 0.89 respectively in the STGD cohort. Only TRL in subjects with AMD differed significantly from 1. In normally-sighted subjects, log reading rate decreased significantly with decreasing contrast and increasing age, eccentricity, and density of element-deletions. For a given eccentricity and contrast, a higher density of element-deletions maximally affected the older subjects. Conclusion: The compensation of the MP-1 for excessive FEMs and the registration between retinal test locations during baseline and follow-up testing are sufficient to assess functional changes within local retinal regions in subjects with CFL. MSs exist within the PRL of a high proportion of subjects with CFL, but are not strongly associated with structural changes determined using SD-OCT. Based on a simulation in normally-sighted subjects, we expect impact of MSs on reading to be greater for older than younger subjects with CFL.Optometry, College o

Multimodal imaging in age-related macular degeneration

Age-related macular degeneration (AMD) is a leading cause of blindness and affects approximately one in seven Australians aged 50 years and above. Currently, this complex condition is not easily and uniformly assessed. The signs of AMD differ between eyes and also occur in other macular disorders. This hinders accurate diagnosis and classification, which is fundamental to optimal patient care. Ocular imaging and visual function assessment have the potential to minimise the devastating consequences of disease through early detection. However, multiple devices are now commercially available and the impact of these technologies in clinical practice may not be straightforward. For instance, their usefulness may depend on accessibility and the operator’s knowledge and clinical skills. The impact on patient management, as well as alternative models of eye-care delivery, requires clarification. This thesis aims to explore the current and potential utility of imaging technologies (optical coherence tomography, infrared imaging, monochromatic retinal photography and fundus autofluorescence) in the assessment and management of AMD and other diseases of retinal pigment epithelium dysfunction. The findings show that optometrists self-describe high levels of practice competency and make ready use of imaging in everyday practice. However, they also unwittingly demonstrated low awareness of the evidence base in AMD. Furthermore, when their interpretation of images was tested using a series of case vignettes, their diagnostic accuracy as a group improved by only five per cent (from 61 per cent to 66 per cent); their tendency to refer increased by four per cent. These factors might be improved through education. A series of open-access, chair-side reference charts were consequently devised to help optometrists use imaging technologies more effectively in clinical practice. The additive contribution of multimodal structural and functional testing was particularly emphasised. Finally, a novel model of intermediate-tier eye-care in Australia was shown to substantially reduce the number of false positive cases or cases without a specific diagnosis. Interestingly, this model was acclaimed by reviewers as “scoring highly for originality and of international relevance”. Most excitingly, the thesis concludes with future directions regarding collaborative care and multimodal imaging, where detection of disease might be facilitated via a computational approach

Astigmatism and Pseudoaccommodation in Pseudophakic Eyes

noAdvanced IOLs with circumferential zones of different power provide pseudoaccommodation. We investigated the potential for power variation with meridian, namely astigmatism, to provide pseudo-accommodation. With appropriate power and axis orientations, acceptable pseudo-accommodation can be achieved

Multimodal Assessment of Structure and Function in Inherited Retinal Degenerations

Inherited retinal degenerations (IRDs) affect approximately 1:4,000 people worldwide and are currently the leading cause of vision loss of people between the ages of 15-45. The mechanisms of degeneration in many IRDs remain unclear. It is vital to establish relationships between retinal structural and functional measures in order to better understand the relationship between the genetic mutations and the phenotypes they cause in these diseases. For example, retinitis pigmentosa (RP), the most prevalent IRD, and choroideremia (CHM), an X-linked degeneration that affects choroidal, RPE and photoreceptor cells, both progress over time due to rod, then cone, photoreceptor loss. Greater understanding of disease progression may clarify the underlying mechanisms of retinal degenerations. Furthermore, the development of novel outcome measures could accelerate clinical trials of treatments designed to treat these relentless, sight-threatening diseases. The experiments and research described in this document use multimodal state-of-the-art, novel, high-resolution techniques in addition to standard technologies used in the clinical setting to study the relationship between structure and function of the retina including rod and cone photoreceptors, retinal pigment epithelium, choriocapillaris and other retinal layers affected in the eyes of patients with retinal degeneration

Analysis of variation in retinal vascular assessment

Changes in retinal vascular parameters have been shown to be associated with systemic vascular diseases. The current assessment of retinal vascular parameters is based on a solo captured image and computer assisted measurement. The solo image assessment ignores the short term, dynamic change of the retinal vessel and its impact on the measurement. Variation in retinal vessel diameter during the cardiac cycle has been debated in the past, while other retinal vascular parameters have never been verified if affected by the cardiac cycle. There is a lack of comprehensive study on the various sources of variation. This thesis has comprehensively studied the variations from the various sources: (i) human cardiac cycle; (ii) multiple graders; (iii) different software; (iv) repeated photographs; (v) region of interest; (vi) the summary method and (vii) measurement protocol. The results showed there was significant change of retinal individual vessel diameters during the cardiac cycle while this change became non-significant after the individual vessel diameters were summarised using a summary method. Other retinal vascular parameters, such as tortuosity, branching angle, LDR and fractal dimension, had little to no variation over the cardiac cycle. Significant variations were found between graders and different measurement software. This thesis has shown that variation due to the cardiac cycle can be minimised using the ECG synchronised retinal photographs. The work has also suggested that the significant variations between different graders and the measurement software should be considered in all future studies when comparing their results. Number of strategies such as minimum length of measured vessel that reduce the variability in the measurement have also been identified, and these should be considered when developing new methodologies. To summarise, this thesis has identified variations and their sources in retinal vascular assessment that will contribute to reduce the variability of vessel measurements leading to improved clinical assessment and identified techniques to mitigate some of these

Accuracy of fundus autofluorescence imaging for the diagnosis and monitoring of retinal conditions: a systematic review

BackgroundNatural fluorescence in the eye may be increased or decreased by diseases that affect theretina. Imaging methods based on confocal scanning laser ophthalmoscopy (cSLO) can detect this ‘fundusautofluorescence’ (FAF) by illuminating the retina using a specific light ‘excitation wavelength’. FAFimaging could assist the diagnosis or monitoring of retinal conditions. However, the accuracy of themethod for diagnosis or monitoring is unclear.ObjectiveTo conduct a systematic review to determine the accuracy of FAF imaging using cSLO for thediagnosis or monitoring of retinal conditions, including monitoring of response to therapy.Data sourcesElectronic bibliographic databases; scrutiny of reference lists of included studies andrelevant systematic reviews; and searches of internet pages of relevant organisations, meetings and trialregistries. Databases included MEDLINE, EMBASE, The Cochrane Library, Web of Science and the Mediondatabase of diagnostic accuracy studies. Searches covered 1990 to November 2014 and were limited tothe English language.Review methodsReferences were screened for relevance using prespecified inclusion criteria to capturea broad range of retinal conditions. Two reviewers assessed titles and abstracts independently. Full-textversions of relevant records were retrieved and screened by one reviewer and checked by a second. Datawere extracted and critically appraised using the Quality Assessment of Diagnostic Accuracy Studies criteria(QUADAS) for assessing risk of bias in test accuracy studies by one reviewer and checked by a second.At all stages any reviewer disagreement was resolved through discussion or arbitration by a third reviewer.ResultsEight primary research studies have investigated the diagnostic accuracy of FAF imaging in retinalconditions: choroidal neovascularisation (one study), reticular pseudodrusen (three studies), cystoid macularoedema (two studies) and diabetic macular oedema (two studies). Sensitivity of FAF imaging using anexcitation wavelength of 488 nm was generally high (range 81–100%), but was lower (55% and 32%) intwo studies using longer excitation wavelengths (514 nm and 790 nm, respectively). Specificity rangedfrom 34% to 100%. However, owing to limitations of the data, none of the studies provide conclusiveevidence of the diagnostic accuracy of FAF imaging.LimitationsNo studies on the accuracy of FAF imaging for monitoring the progression of retinalconditions or response to therapy were identified. Owing to study heterogeneity, pooling of diagnosticoutcomes in meta-analysis was not conducted. All included studies had high risk of bias. In most studiesthe patient spectrum was not reflective of those who would present in clinical practice and no studiesadequately reported how FAF images were interpreted.ConclusionsAlthough already in use in clinical practice, it is unclear whether or not FAF imaging isaccurate, and whether or not it is applied and interpreted consistently for the diagnosis and/or monitoringof retinal conditions. Well-designed prospective primary research studies, which conform to the paradigmof diagnostic test accuracy assessment, are required to investigate the accuracy of FAF imaging in diagnosisand monitoring of inherited retinal dystrophies, early age-related macular degeneration, geographicatrophy and central serous chorioretinopathy.Study registrationThis study is registered as PROSPERO CRD42014014997.FundingThe National Institute for Health Research Health Technology Assessment programme

Abnormal Retinal Reflectivity To Short-Wavelength Light In Type 2 Idiopathic Macular Telangiectasia

PURPOSE: Macular telangiectasia Type 2 (MacTel) is a bilateral, progressive, potentially blinding retinal disease characterized by vascular and neurodegenerative signs, including an increased parafoveal reflectivity to blue light. Our aim was to investigate the relationship of this sign with other signs of macular telangiectasia Type 2 in multiple imaging modalities. METHODS: Participants were selected from the MacTel Type 2 study, based on a confirmed diagnosis and the availability of images. The extent of signs in blue-light reflectance, fluorescein angiographic, optical coherence tomographic, and single- and dual-wavelength autofluorescence images were analyzed. RESULTS: A well-defined abnormality of the perifovea is demonstrated by dual-wavelength autofluorescence and blue-light reflectance in early disease. The agreement in area size of the abnormalities in dual-wavelength autofluorescence and in blue-light reflectance images was excellent: for right eyes: ρ = 0.917 (P < 0.0001, 95% confidence interval 0.855-0.954, n = 46) and for left eyes: ρ = 0.952 (P < 0.0001, 95% confidence interval 0.916-0.973, n = 49). Other changes are less extensive initially and expand later to occupy that area and do not extend beyond it. CONCLUSION: Our findings indicate that abnormal metabolic handling of luteal pigment and physical changes giving rise to increased reflectance are widespread in the macula throughout the natural history of the disease, precede other changes, and are relevant to early diagnosis

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