Oxygen saturation measurements of the retinal vasculature in treated asymmetrical primary open-angle glaucoma using hyperspectral imaging

Toward real-time margin assessment in breast-conserving surgery with hyperspectral imaging

Margin assessment in breast-conserving surgery (BSC) remains a critical challenge, with 20-25% of cases resulting in inadequate tumor resection, increasing the risk of local recurrence and the need for additional treatment. In this study, we evaluate the diagnostic performance of hyperspectral imaging (HSI) as a non-invasive technique for assessing resection margins in ex vivo lumpectomy specimens. A dataset of over 200 lumpectomy specimens was collected using two hyperspectral cameras, and a classification algorithm was developed to distinguish between healthy and tumor tissue within margins of 0 and 2 mm. The proposed approach achieved its highest diagnostic performance at a 0 mm margin, with a sensitivity of 92%, specificity of 78%, accuracy of 83%, Matthews correlation coefficient of 68%, and an area under the curve of 89%. The entire resection surface could be imaged and evaluated within 10 minutes, providing a rapid and non-invasive alternative to conventional margin assessment techniques. These findings represent a significant advancement toward real-time intraoperative margin assessment, highlighting the potential of HSI to enhance surgical precision and reduce re-excision rates in BCS.

Hyperspectral retinal imaging biomarkers of ocular and systemic diseases

Hyperspectral imaging is a frontier in the field of medical imaging technology. It enables the simultaneous collection of spectroscopic and spatial data. Structural and physiological information encoded in these data can be used to identify and localise typically elusive biomarkers. Studies of retinal hyperspectral imaging have provided novel insights into disease pathophysiology and new ways of non-invasive diagnosis and monitoring of retinal and systemic diseases. This review provides a concise overview of recent advances in retinal hyperspectral imaging.

Macular Oxygen Saturation in Glaucoma Using Retinal Oximetry of Visible Light Optical Coherence Tomography: A Pilot Study

Purpose

A cross-sectional pilot study to compare macular oxygen saturation (sO2) and associated clinical measurements between normal and glaucoma subjects and to evaluate whether macular sO2 can be a diagnostic metric for early-stage glaucoma.

Methods

Forty-eight eyes of 35 subjects from three groups were included: normal subjects (16 eyes, 10 subjects), suspect/pre-perimetric glaucoma (GS/PPG) subjects (17 eyes, 12 subjects), and perimetric glaucoma (PG) subjects (15 eyes, 13 subjects). We performed retinal oximetry of visible light optical coherence tomography (VIS-OCT) in macular vessels, with 512 × 256 sampling points over a 5 × 5 mm2 area. Zeiss Cirrus OCT scans and a 24-2 visual field test (VFT) were conducted. Statistical analysis was conducted.

Results

Significant differences were observed among the three groups for all VIS-OCT, Zeiss OCT, and VFT variables. As glaucoma severity increased, macular AsO2 (arterial sO2) and A-V sO2 (arteriovenous sO2 difference) decreased, whereas macular VsO2 (venous sO2) increased. Macular AsO2 and A-V sO2 were found to be statistically correlated with ganglion cell layer + inner plexiform layer (GCL+IPL) and circumpapillary retinal nerve fiber layer in all eyes, as well as in PG eyes. Within the PG group, a dominant correlation between AsO2 and ganglion cell layer + inner plexiform layer was observed in the more damaged lower hemifield.

Conclusions

Glaucoma subjects showed altered macular sO2, indicating reduced oxygen consumption. The sO2 measured by VIS-OCT could be a potential metric for early glaucoma diagnosis.

Translational Relevance

This study shows macular sO2 measurements via VIS-OCT could bridge advanced imaging technology and clinical glaucoma detection.

Relationship Between Retinal Oxygen Saturation and the Severity of Visual Field Damage in Glaucoma

PRCIS

Increased oxygen saturation (StO 2 ) was significantly associated with the severity of visual field (VF) damage in patients with glaucoma.

OBJECTIVE

To investigate the association between retinal StO 2 percentage and the severity of VF loss in glaucoma.

METHODS

A total of 198 eyes from 131 patients with glaucoma were included in this cross-sectional study. Participants underwent imaging using ocular oximetry (Zilia) and 24-2 Swedish Interactive Threshold Algorithm standard VF (Carl Zeiss-Meditec). StO 2 (%) was measured at 2 locations of the peripapillary optic nerve head (superotemporal, and inferotemporal). Measurements were reported as the mean of at least 5 measurements in each location. Associations between the severity of VF loss, reported as mean deviation, and StO 2 (%) were calculated.

RESULTS

A total of 198 eyes of 131 patients (mean (95% CI) age, 71.1 (68.9,73.3) years, 68 females (51.9%), and 63 males (48.1%) were analyzed. In univariable analysis, higher StO 2 -0.06 (-0.12, 0.00) was associated with severity in all hemifields ( P = 0.047). Multivariate regression analysis showed that each 1% increase in StO 2 was associated with -0.06 (-0.12, -0.00) dB loss in mean deviation in all hemifields ( P = 0.043). In multivariate regression analysis in the superior hemifields, higher StO 2 -0.07 (-0.16, 0.01) tended to be associated with superior hemifield severity ( P = 0.09).

CONCLUSIONS

Retinal oximetry enabled the continuous quantitative measurement of retinal StO 2. Increased StO 2 was significantly associated with the severity of VF damage in patients with glaucoma.

Separating Surface Reflectance from Volume Reflectance in Medical Hyperspectral Imaging

Hyperspectral imaging has shown great promise for diagnostic applications, particularly in cancer surgery. However, non-bulk tissue-related spectral variations complicate the data analysis. Common techniques, such as standard normal variate normalization, often lead to a loss of amplitude and scattering information. This study investigates a novel approach to address these spectral variations in hyperspectral images of optical phantoms and excised human breast tissue. Our method separates surface and volume reflectance, hypothesizing that spectral variability arises from significant variations in surface reflectance across pixels. An illumination setup was developed to measure samples with a hyperspectral camera from different axial positions but with identical zenith angles. This configuration, combined with a novel data analysis approach, allows for the estimation and separation of surface reflectance for each direction and volume reflectance across all directions. Validated with optical phantoms, our method achieved an 83% reduction in spectral variability. Its functionality was further demonstrated in excised human breast tissue. Our method effectively addresses variations caused by surface reflectance or glare while conserving surface reflectance information, which may enhance sample analysis and evaluation. It benefits samples with unknown refractive index spectra and can be easily adapted and applied across a wide range of fields where hyperspectral imaging is used.

Glaucoma Detection through a Novel Hyperspectral Imaging Band Selection and Vision Transformer Integration

Conventional diagnostic methods for glaucoma primarily rely on non-dynamic fundus images and often analyze features such as the optic cup-to-disc ratio and abnormalities in specific retinal locations like the macula and fovea. However, hyperspectral imaging techniques focus on detecting alterations in oxygen saturation within retinal vessels, offering a potentially more comprehensive approach to diagnosis. This study explores the diagnostic potential of hyperspectral imaging for glaucoma by introducing a novel hyperspectral imaging conversion technique. Digital fundus images are transformed into hyperspectral representations, allowing for a detailed analysis of spectral variations. Spectral regions exhibiting differences are identified through spectral analysis, and images are reconstructed from these specific regions. The Vision Transformer (ViT) algorithm is then employed for classification and comparison across selected spectral bands. Fundus images are used to identify differences in lesions, utilizing a dataset of 1291 images. This study evaluates the classification performance of models using various spectral bands, revealing that the 610-780 nm band outperforms others with an accuracy, precision, recall, F1-score, and AUC-ROC all approximately at 0.9007, indicating its superior effectiveness for the task. The RGB model also shows strong performance, while other bands exhibit lower recall and overall metrics. This research highlights the disparities between machine learning algorithms and traditional clinical approaches in fundus image analysis. The findings suggest that hyperspectral imaging, coupled with advanced computational techniques such as the ViT algorithm, could significantly enhance glaucoma diagnosis. This understanding offers insights into the potential transformation of glaucoma diagnostics through the integration of hyperspectral imaging and innovative computational methodologies.

Macular Oxygen Saturation in Glaucoma Using Retinal Oximetry of Visible Light Optical Coherence Tomography

Purpose

Oxygen saturation (sO2) plays a critical role in retinal pathophysiology, especially at the macula, which undergoes significant energy consumption. While macular damage has been suggested to be involved in early-stage glaucoma, there has been no report to date on non-invasive macular sO2 in glaucoma. Therefore, we conducted this study to compare macular sO2 associated with other clinical measurements between normal and glaucoma subjects and evaluate whether there are significant differences.

Method

This is a cross-sectional study. We used visible light optical coherence tomography (VIS-OCT) for retinal oximetry in perifoveal vessels. The subjects from groups of normal, suspect/pre-perimetric glaucoma (GS/PPG) and perimetric glaucoma (PG) were scanned using VIS-OCT in the macular region with a sampling density of 512×256 in an area of 5×5 mm2. 48 eyes (16 normal, 17 GS/PPG and 15 PG) were included for the analysis. For each eye, we measured the sO2 of arterioles (AsO2), venules (VsO2), and calculated the difference between arterioles and venules (A-V sO2=AsO2-VsO2), oxygen extraction (OE=(AsO2-VsO2)/AsO2 ×100%). Additionally, we included Zeiss Cirrus OCT scans and 24-2 visual field test (VFT) for clinical benchmark. One-way ANOVA was used to compare the differences among the three groups. Spearman correlation tests were used for correlation sO2 markers to standard metrics including the thickness of ganglion cell layer and inner plexiform layer (GCL+IPL), circumpapillary retinal nerve fiber layer (cpRNFL) and mean deviation (MD) in VFT.

Result

Significant differences were found among three groups for all VIS-OCT, Zeiss OCT, and VFT variables. Macular AsO2, A-V sO2, OE decreased, and VsO2 increased along with severity. Macular AsO2 and A-V sO2 were statistically correlated with GCL+IPL and cpRNFL in all eyes, as well as only PG eyes. Within PG eyes, the correlation between AsO2 and GCL+IPL is dominant in more damaged lower hemifield.

Conclusion

The GS/PPG and PG subjects had significantly higher macular VsO2, lower A-V sO2 and OE indicating less oxygen consumption. The sO2 measured by retinal oximetry of VIS-OCT can be a potential metric for the early diagnosis of glaucoma.

Targeted spectroscopy in the eye fundus

Significance

The assessment of biomarkers in the eye is rapidly gaining traction for the screening, diagnosis, and monitoring of ocular and neurological diseases. Targeted ocular spectroscopy is a technology that enables concurrent imaging of the eye fundus and analysis of high-quality spectra from a targeted region within the imaged area. This provides structural, compositional, and functional information of specific regions of the eye fundus from a non-invasive approach to ocular biomarker detection.

Aim

The aim of our study was to demonstrate the multimodal functionality and validation of targeted ocular spectroscopy. This was done in vitro, using a reference target and a model eye, and in vivo.

Approach

Images and spectra from different regions of a reference target and a model eye were acquired and analyzed to validate the system. Targeted ocular fluorescence spectroscopy was also demonstrated with the same model. Subsequently, in vivo imaging and diffuse reflectance spectra were acquired to assess blood oxygen saturation in the optic nerve head and the parafovea of healthy subjects.

Results

Tests conducted with the reference target showed accurate spectral analysis within specific areas of the imaging space. In the model eye, distinct spectral signatures were observed for the optic disc, blood vessels, the retina, and the macula, consistent with the variations in tissue composition and functions between these regions. An ocular oximetry algorithm was applied to in vivo spectra from the optic nerve head and parafovea of healthy patients, showing significant differences in blood oxygen saturation. Finally, targeted fluorescence spectral analysis was performed in vitro.

Conclusions

Diffuse reflectance and fluorescence spectroscopy in specific regions of the eye fundus open the door to a whole new range of monitoring and diagnostic capabilities, from assessment of oxygenation in glaucoma and diabetic retinopathy to photo-oxidation and photodegradation in age-related macular degeneration.

Sensitivity of visible range multi-wavelength algorithms for retinal tissue oximetry to acquisition parameters

This study examined the sensitivity of broadband spectroscopy algorithms for retinal tissue oximetry to spectral acquisition parameters. Monte Carlo simulations were conducted on a 4-layer retinal model to assess the impact of various parameters. The optimal spectral range for accurate measurements was determined to be 530 nm to 585 nm. Decreased spectral resolution below 4 nm significantly reduced accuracy. Using an acquisition area larger than the blood vessel resulted in an underestimation of oxygen saturation, especially for high values. A threshold was observed where increased light intensity had no significant impact on measurement variability. The study highlights the importance of informed parameter selection for accurately assessing retinal microcapillary oxygenation and studying local hemodynamics.

Multispectral Imaging for Vein Localization and Contrast Enhancement

Intravenous (IV) catheterization is a common procedure. Still, there is a 26% chance of the first attempt catheterization failure due to the changing visibility of veins because of the patient's skin tone and body fat content. Ultrasound assistive devices help locate deeper veins but are not practical in emergencies, and transillumination assistive devices have a low field of view. Commercial near-infrared (NIR) imaging devices are effective in vein localization but are expensive and are not used in low-cost clinical settings. To overcome this, NIR Multispectral Imaging (MSI) was used to find the optimal wavelength that provides the enhanced visualization of veins for all skin types and Body Mass Index (BMI). The band with the highest vein-to-skin contrast ratio was selected and contrast enhancement was done using our proposed method. The primary blocks of the proposed method are Gamma correction, Contrast Limited Adaptive Histogram Equalization (CLAHE), Adaptive Thresholding, and image Fusion. The optimal spectral range was found to be 814-876 nm and our method increased the contrast by 0.41, 0.375, and 0.39 for fair, brown, and dark brown skin types, respectively, with different BMI.Clinical relevance- From the study, we can develop a potentially low-cost vein localization assistive device for training medical and nursing students and use it in emergencies for venous access to improve confidence in IV catheterization.

Monte-Carlo simulation and tissue-phantom model for validation of ocular oximetry

Ocular oximetry, in which blood oxygen saturation is evaluated in retinal tissues, is a promising technique for the prevention, diagnosis and management of many diseases and conditions. However, the development of new tools for evaluating oxygen saturation in the eye fundus has often been limited by the lack of reference tools or techniques for such measurements. In this study, we describe a two-step validation method. The impact of scattering, blood volume fraction and lens yellowing on the oximetry model is investigated using a tissue phantom, while a Monte Carlo model of the light propagation in the eye fundus is used to study the effect of the fundus layered-structure. With this method, we were able to assess the performance of an ocular oximetry technique in the presence of confounding factors and to quantify the impact of the choroidal circulation on the accuracy of the measurements. The presented strategy will be useful to anyone involved in studies based on the eye fundus diffuse reflectance.

Topical Review: Studies of Ocular Function and Disease Using Hyperspectral Imaging

SIGNIFICANCE

Advances in imaging technology over the last two decades have produced significant innovations in medical imaging. Hyperspectral imaging (HSI) is one of these innovations, enabling powerful new imaging tools for clinical use and greater understanding of tissue optical properties and mechanisms underlying eye disease.Hyperspectral imaging is an important and rapidly growing area in medical imaging, making possible the concurrent collection of spectroscopic and spatial information that is usually obtained from separate optical recordings. In this review, we describe several mainstream techniques used in HSI, along with noteworthy advances in optical technology that enabled modern HSI techniques. Presented also are recent applications of HSI for basic and applied eye research, which include a novel method for assessing dry eye syndrome, clinical slit-lamp examination of corneal injury, measurement of blood oxygen saturation in retinal disease, molecular changes in macular degeneration, and detection of early stages of Alzheimer disease. The review also highlights work resulting from integration of HSI with other imaging tools such as optical coherence tomography and autofluorescence microscopy and discusses the adaptation of HSI for clinical work where eye motion is present. Here, we present the background and main findings from each of these reports along with specific references for additional details.

Trans-scleral Illumination - The Future of Retinal Imaging?

We report how illumination of the retina through the sclera enables the recording of high-contrast reflectance and fluorescence images of the retina, free of the cornea and lens glare and autofluoresence that degrades images recorded using traditional illumination through the pupil.

Advances in Retinal Oximetry

Similar to other organs, the retina relies on tightly regulated perfusion and oxygenation. Previous studies have demonstrated that retinal blood flow is affected in a variety of eye and systemic diseases, including diabetic retinopathy, age-related macular degeneration, and glaucoma. Although measurement of peripheral oxygen saturation has become a standard clinical measurement through the development of pulse oximetry, developing a noninvasive technique to measure retinal oxygen saturation has proven challenging, and retinal oximetry technology currently remains inadequate for reliable clinical use. Here, we review current strategies and approaches, as well as several newer technologies in development, and discuss the future of retinal oximetry.

A review of the medical hyperspectral imaging systems and unmixing algorithms' in biological tissues

Hyperspectral fluorescence imaging (HFI) is a well-known technique in the medical research field and is considered a non-invasive tool for tissue diagnosis. This review article gives a brief introduction to acquisition methods, including the image preprocessing methods, feature selection and extraction methods, data classification techniques and medical image analysis along with recent relevant references. The process of fusion of unsupervised unmixing techniques with other classification methods, like the combination of support vector machine with an artificial neural network, the latest snapshot Hyperspectral imaging (HSI) and vortex analysis techniques are also outlined. Finally, the recent applications of hyperspectral images in cellular differentiation of various types of cancer are discussed.

Retinal oximetry in glaucoma: investigations and findings reviewed

Abnormalities of the retinal blood supply have been widely implicated in primary open-angle glaucoma (POAG). Impaired blood supply to the retina and optic nerve head (ONH) may be a primary pathophysiologic mechanism contributing to POAG ('vascular hypothesis'). However, the decreased metabolic activity of atrophic tissue is itself known to induce both vascular changes and decreased blood flow due to reduced oxygen demand. Therefore, primary nonvascular factors could potentially induce glaucomatous atrophy, with subsequent secondary vascular pathology ('mechanical hypothesis'). Retinal oximetry holds great promise in the investigation of glaucoma pathogenesis, as it can provide useful data on retinal metabolic oxygen demand, especially when combined with measurements of retinal blood flow. This review surveys the research on retinal metabolism in POAG using spectroscopic retinal oximetry. The use of mathematical models in combination with oximetric data to investigate the role of retinal metabolism and oxygen supply in POAG is also discussed.

The role of neuroinflammation in the pathogenesis of glaucoma neurodegeneration

The chapter is a review enclosed in the volume "Glaucoma: A pancitopatia of the retina and beyond." No cure exists for glaucoma. Knowledge on the molecular and cellular alterations underlying glaucoma neurodegeneration (GL-ND) includes innovative and path-breaking research on neuroinflammation and neuroprotection. A series of events involving immune response (IR), oxidative stress and gene expression are occurring during the glaucoma course. Uveitic glaucoma (UG) is a prevalent acute/chronic complication, in the setting of chronic anterior chamber inflammation. Managing the disease requires a team approach to guarantee better results for eyes and vision. Advances in biomedicine/biotechnology are driving a tremendous revolution in ophthalmology and ophthalmic research. New diagnostic and imaging modalities, constantly refined, enable outstanding criteria for delimiting glaucomatous neurodegeneration. Moreover, biotherapies that may modulate or inhibit the IR must be considered among the first-line for glaucoma neuroprotection. This review offers the readers useful and practical information on the latest updates in this regard.

Hyperspectral Imaging and the Retina: Worth the Wave?

Purpose

Hyperspectral imaging is gaining attention in the biomedical field because it generates additional spectral information to study physiological and clinical processes. Several technologies have been described; however an independent, systematic literature overview is lacking, especially in the field of ophthalmology. This investigation is the first to systematically overview scientific literature specifically regarding retinal hyperspectral imaging.

Methods

A systematic literature review was conducted, in accordance with PRISMA Statement 2009 criteria, in four bibliographic databases: Medline, Embase, Cochrane Database of Systematic Reviews, and Web of Science.

Results

Fifty-six articles were found that meet the review criteria. A range of techniques was reported: Fourier analysis, liquid crystal tunable filters, tunable laser sources, dual-slit monochromators, dispersive prisms and gratings, computed tomography, fiber optics, and Fabry-Perrot cavity filter covered complementary metal oxide semiconductor. We present a narrative synthesis and summary tables of findings of the included articles, because methodologic heterogeneity and diverse research topics prevented a meta-analysis being conducted.

Conclusions

Application in ophthalmology is still in its infancy. Most previous experiments have been performed in the field of retinal oximetry, providing valuable information in the diagnosis and monitoring of various ocular diseases. To date, none of these applications have graduated to clinical practice owing to the lack of sufficiently large validation studies.

Translational Relevance

Given the promising results that smaller studies show for hyperspectral imaging (e.g., in Alzheimer's disease), advanced research in larger validation studies is warranted to determine its true clinical potential.

A background correction method to compensate illumination variation in hyperspectral imaging

Hyperspectral imaging (HSI) can measure both spatial (morphological) and spectral (biochemical) information from biological tissues. While HSI appears promising for biomedical applications, interpretation of hyperspectral images can be challenging when data is acquired in complex biological environments. Variations in surface topology or optical power distribution at the sample, encountered for example during endoscopy, can lead to errors in post-processing of the HSI data, compromising disease diagnostic capabilities. Here, we propose a background correction method to compensate for such variations, which estimates the optical properties of illumination at the target based on the normalised spectral profile of the light source and the measured HSI intensity values at a fixed wavelength where the absorption characteristics of the sample are relatively low (in this case, 800 nm). We demonstrate the feasibility of the proposed method by imaging blood samples, tissue-mimicking phantoms, and ex vivo chicken tissue. Moreover, using synthetic HSI data composed from experimentally measured spectra, we show the proposed method would improve statistical analysis of HSI data. The proposed method could help the implementation of HSI techniques in practical clinical applications, where controlling the illumination pattern and power is difficult.

Convolutional Neural Networks for Spectroscopic Analysis in Retinal Oximetry

Retinal oximetry is a non-invasive technique to investigate the hemodynamics, vasculature and health of the eye. Current techniques for retinal oximetry have been plagued by quantitatively inconsistent measurements and this has greatly limited their adoption in clinical environments. To become clinically relevant oximetry measurements must become reliable and reproducible across studies and locations. To this end, we have developed a convolutional neural network algorithm for multi-wavelength oximetry, showing a greatly improved calculation performance in comparison to previously reported techniques. The algorithm is calibration free, performs sensing of the four main hemoglobin conformations with no prior knowledge of their characteristic absorption spectra and, due to the convolution-based calculation, is invariable to spectral shifting. We show, herein, the dramatic performance improvements in using this algorithm to deduce effective oxygenation (SO₂), as well as the added functionality to accurately measure fractional oxygenation (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\bf{SO}}}_{{\bf{2}}}^{{\boldsymbol{f}}{\boldsymbol{r}}}$$\end{document}SO2fr). Furthermore, this report compares, for the first time, the relative performance of several previously reported multi-wavelength oximetry algorithms in the face of controlled spectral variations. The improved ability of the algorithm to accurately and independently measure hemoglobin concentrations offers a high potential tool for disease diagnosis and monitoring when applied to retinal spectroscopy.

A clinically translatable hyperspectral endoscopy (HySE) system for imaging the gastrointestinal tract

Hyperspectral imaging (HSI) enables visualisation of morphological and biochemical information, which could improve disease diagnostic accuracy. Unfortunately, the wide range of image distortions that arise during flexible endoscopy in the clinic have made integration of HSI challenging. To address this challenge, we demonstrate a hyperspectral endoscope (HySE) that simultaneously records intrinsically co-registered hyperspectral and standard-of-care white light images, which allows image distortions to be compensated computationally and an accurate hyperspectral data cube to be reconstructed as the endoscope moves in the lumen. Evaluation of HySE performance shows excellent spatial, spectral and temporal resolution and high colour fidelity. Application of HySE enables: quantification of blood oxygenation levels in tissue mimicking phantoms; differentiation of spectral profiles from normal and pathological ex vivo human tissues; and recording of hyperspectral data under freehand motion within an intact ex vivo pig oesophagus model. HySE therefore shows potential for enabling HSI in clinical endoscopy.

Ocular Manifestations of Alzheimer's and Other Neurodegenerative Diseases: The Prospect of the Eye as a Tool for the Early Diagnosis of Alzheimer's Disease

Dementia, including Alzheimer's disease (AD), is a major disorder, leading to several ocular manifestations amongst the elderly population. These visual disorders may be due to retinal nerve degenerative changes, including nerve fibre layer thinning, degeneration of retinal ganglion cells, and changes to vascular parameters. There is no cure for Alzheimer's, but medicines can slow down the development of many of the classic symptoms, such as loss of memory and communication skills, mood swings, and depression. The disease diagnosis is difficult, and it is only possible through PET scans of the brain, detecting evidence of the accumulation of amyloid and tau. PET is expensive and invasive, requiring the injection of radioactive tracers, which bind with these proteins and glow during scanning. Recently, scientists developed promising eye-scan techniques that may detect Alzheimer's disease at its earliest stage, before major symptoms appear, leading to improved management of the disease symptoms. In this review, we are discussing the visual abnormalities of Alzheimer's and other neurodegenerative diseases, focused on ocular functional-visual-structural biomarkers, retinal pathology, and potential novel diagnostic tools.

Regulation of oxygen saturation in retinal blood vessels in response to dynamic exercise

PURPOSE

To evaluate the impact of dynamic exercise on retinal vessel oxygen saturation in healthy individuals.

METHODS

Twenty-six healthy participants underwent moderate dynamic exercise (modified Master's two-step exercise). In all subjects, intraocular pressures (IOP), systolic and diastolic blood pressures (SBP and DBP), retinal vessel calibres and retinal arterial and venous oxygen saturation were measured at baseline, immediately following exercise and 15 min postexercise.

RESULTS

Moderate dynamic exercise increased SBP and DBP immediately postexercise (SBP: 116 (±13) mmHg to 150 (±21) mmHg; p < 0.001 and DBP: 69 (±10) mmHg to 74 (±10); p < 0.001), while IOP decreased by an average of 2 mmHg (baseline: 13 (±3) mmHg)) immediately postexercise (11 (±2) mmHg). Oxygen saturation in retinal arteries remained unchanged (baseline = 93 ± 8%; immediately postexercise = 94 ± 9% and 15 min postexercise = 96 ± 8%; p = 0.069), but increased in retinal veins immediately postexercise and did not return to baseline values within 15 min postexercise (baseline = 54 ± 12%; immediately postexercise = 56 ± 15%; 15 min postexercise=57 ± 12%; p = 0.036).

CONCLUSION

There is a mild increase in retinal venous oxygen saturation and there is a trend towards an increase in arterial saturation in otherwise healthy individuals following dynamic exercise.

Hydrogen sulfide supplement attenuates the apoptosis of retinal ganglion cells in experimental glaucoma

Glaucoma is a group of neurodegenerative eye diseases characterized by progressive impairment of visual function due to loss of retinal ganglion cells (RGC). As hydrogen sulfide (H₂S) was reported to play a role in the process of glaucomatous neuropathy and improve RGC survival in experimental glaucoma, the authors aimed to investigate the anti-apoptosis effect of H₂S supplement in a rat glaucoma model, and further tried to explore the involved factors in the neuroprotection. A chronic ocular hypertension (COH) rat model induced by intracameral injection of cross-linking hydrogel was employed to simulate glaucoma and 288 rats were subjected to experimental procedures in the present study. After 4 weeks of sodium hydrosulfide (NaHS) administration following COH induction, the apoptosis of RGC isolated from experimented rats as well as apoptosis of neurons in ganglion cell layer (GCL), intrinsic apoptotic pathway activity, mitochondrial function, glial activation, NF-κB pathway activity, NADPH oxidase activity, autophagy activity and TNF-α level in retina were evaluated. The results showed that H₂S supplement effectively attenuated the apoptosis of RGC in experimental glaucoma, and the neuroprotection by H₂S might correlate with preservation of mitochondrial function, attenuation of oxidative stress, suppression of glial activation, inhibition of inflammatory pathways and downregulation of autophagy. Our study indicated that H₂S supplement resulted in significant neuroprotection through attenuation of RGC apoptosis which might be linked with multiple factors in experimental glaucoma. The new therapeutic strategy might potentially contribute to benefit glaucoma treatment, which is worth further concerns.

Glaucoma related retinal oximetry: a technology update

There are two long-standing theories about the pathogenesis of glaucoma – barotrauma and the effect of vascular hypoxia. Currently, it is still unknown whether diminished blood flow is the cause or result of glaucomatous atrophy of ganglion cells and the optic nerve. Though many other imaging techniques used to directly assess ocular blood flow have been well studied, they are limited by their inability to directly assess metabolism in the ocular tissues or measure the oxygen carrying capacity in the vessels. Retinal oximetry is a relatively novel, noninvasive imaging technique that reliably measures oxygen saturation levels in the retinal vessels, offering surrogate markers for the metabolic demands of the eye. The clinical significance of these measurements has not been well established. Thus, this review gives an overview of ocular imaging and current retinal oximetry techniques, while contextualizing the important oximetry studies that have investigated the vascular theory behind glaucoma.

The Future of Imaging in Detecting Glaucoma Progression

Ocular imaging has been heavily incorporated into glaucoma management and provides important information that aids in the detection of disease progression. Longitudinal studies have shown that the circumpapillary retinal nerve fiber layer is an important parameter for glaucoma progression detection, whereas other studies have demonstrated that macular parameters, such as the ganglion cell inner plexiform layer and optic nerve head parameters, also are useful for progression detection. The introduction of novel technologies with faster scan speeds, wider scanning fields, higher resolution, and improved tissue penetration has enabled the precise quantification of additional key ocular structures, such as the individual retinal layers, optic nerve head, choroid, and lamina cribrosa. Furthermore, extracting functional information from scans such as blood flow rate and oxygen consumption provides new perspectives on the disease and its progression. These novel methods promise improved detection of glaucoma progression and better insight into the mechanisms of progression that will lead to better targeted treatment options to prevent visual damage and blindness.

Hyperspectral Image Enhancement and Mixture Deep-Learning Classification of Corneal Epithelium Injuries

In our preliminary study, the reflectance signatures obtained from hyperspectral imaging (HSI) of normal and abnormal corneal epithelium tissues of porcine show similar morphology with subtle differences. Here we present image enhancement algorithms that can be used to improve the interpretability of data into clinically relevant information to facilitate diagnostics. A total of 25 corneal epithelium images without the application of eye staining were used. Three image feature extraction approaches were applied for image classification: (i) image feature classification from histogram using a support vector machine with a Gaussian radial basis function (SVM-GRBF); (ii) physical image feature classification using deep-learning Convolutional Neural Networks (CNNs) only; and (iii) the combined classification of CNNs and SVM-Linear. The performance results indicate that our chosen image features from the histogram and length-scale parameter were able to classify with up to 100% accuracy; particularly, at CNNs and CNNs-SVM, by employing 80% of the data sample for training and 20% for testing. Thus, in the assessment of corneal epithelium injuries, HSI has high potential as a method that could surpass current technologies regarding speed, objectivity, and reliability.

Multispectral oximetry of murine tendon microvasculature with inflammation

We report a novel multispectral imaging technique for localised measurement of vascular oxygen saturation (SO₂) in vivo. Annular back-illumination is generated using a Schwarzchild-design reflective objective. Analysis of multispectral data is performed using a calibration-free oximetry algorithm. This technique is applied to oximetry in mice to measure SO₂ in microvasculature supplying inflamed tendon tissue in the hind leg. Average SO₂ for controls was 94.8 ± 7.0 % (N = 6), and 84.0 ± 13.5 % for mice with inflamed tendon tissue (N = 6). We believe this to be the first localised measurement of hypoxia in tendon microvasculature due to inflammation. Quantification of localised SO₂ is important for the study of inflammatory diseases such as rheumatoid arthritis, where hypoxia is thought to play a role in pathogenesis.

Retinal oximetry in humans using visible-light optical coherence tomography [Invited]

We measured hemoglobin oxygen saturation (sO₂) in the retinal circulation in healthy humans using visible-light optical coherence tomography (vis-OCT). The measurements showed clear oxygenation differences between central retinal arteries and veins close to the optic nerve head. Spatial variations at different vascular branching levels were also revealed. In addition, we presented theoretical and experimental results to establish that noises in OCT intensity followed Rice distribution. We used this knowledge to retrieve unbiased estimation of true OCT intensity to improve the accuracy of vis-OCT oximetry, which had inherently lower signal-to-nose ratio from human eyes due to safety and comfort limitations. We demonstrated that the new statistical-fitting sampling strategy could reduce the estimation error in sO₂ by three percentage points (pp). The presented work aims to provide a foundation for using vis-OCT to achieve accurate retinal oximetry in clinical settings.

In vivo oximetry of human bulbar conjunctival and episcleral microvasculature using snapshot multispectral imaging

Multispectral imaging (MSI) is a well-established technique for non-invasive oximetry of retinal blood vessels, which has contributed to the understanding of a variety of retinal conditions, including glaucoma, diabetes, vessel occlusion, and retinal auto-regulation. We report the first study to use snapshot multi-spectral imaging (SMSI) for oximetry of the bulbar conjunctival and episcleral microvasculature in the anterior segment of the eye. We report the oxygen dynamics of the bulbar conjunctival and episcleral microvasculature at normoxia and at acute mild hypoxia conditions. A retinal-fundus camera fitted with a custom Image-Replicating Imaging Spectrometer was used to image the bulbar conjunctival and episcleral microvasculature in ten healthy human subjects at normoxia (21% Fraction of Inspired Oxygen [FiO2]) and acute mild hypoxia (15% FiO2) conditions. Eyelid closure was used to control oxygen diffusion between ambient air and the sclera surface. Four subjects were imaged for 30 seconds immediately following eyelid opening. Vessel diameter and Optical Density Ratio (ODR: a direct proxy for oxygen saturation) of vessels was computed automatically. Oximetry capability was validated using a simple phantom that mimicked the scleral vasculature. Acute mild hypoxia resulted in a decrease in blood oxygen saturation (SO2) (i.e. an increase in ODR) when compared with normoxia in both bulbar conjunctival (p < 0.001) and episcleral vessels (p = 0.03). Average episcleral diameter increased from 78.9 ± 8.7 μm (mean ± standard deviation) at normoxia to 97.6 ± 14.3 μm at hypoxia (p = 0.02). Diameters of bulbar conjunctival vessels showed no significant change from 80.1 ± 7.6 μm at normoxia to 80.6 ± 7.0 μm at hypoxia (p = 0.89). When exposed to ambient air, hypoxic bulbar conjunctival vessels rapidly reoxygenated due to oxygen diffusion from ambient air. Reoxygenation occured in an exponential manner, and SO2 reached normoxia baseline levels. The average ½ time to full reoxygenation was 3.4 ± 1.4 s. As a consequence of oxygen diffusion, bulbar conjunctival vessels will be highly oxygenated (i.e. close to 100% SO2) when exposed to ambient air. Episcleral vessels were not observed to undergo any significant oxygen diffusion, instead behaving similarly to pulse oximetry measurements. This is the first study to the image oxygen dynamics of bulbar conjunctival and episcleral microvasculature, and consequently, the first study to directly observe the rapid reoxygenation of hypoxic bulbar conjunctival vessels when exposed to ambient air. Oximetry of bulbar conjunctival vessels could potentially provide insight into conditions where oxygen dynamics of the microvasculature are not fully understood, such as diabetes, sickle-cell diseases, and dry-eye syndrome. Oximetry in the bulbar conjunctival and episcleral microvasculature could be complimentary or alternative to retinal oximetry.

Hypoxia and Dark Adaptation in Diabetic Retinopathy: Interactions, Consequences, and Therapy

In diabetes, retinal blood flow is compromised, and retinal hypoxia is likely to be further intensified during periods of darkness. During dark adaptation, rod photoreceptors in the outer retina are maximally depolarized and continuously release large amounts of the neurotransmitter glutamate-an energetically demanding process that requires the highest oxygen consumption per unit volume of any tissue of the body. In complete darkness, even more oxygen is consumed by the outer retina, producing a steep fall in the retinal oxygen tension curve which reaches a nadir at the depth of the mitochondrial-rich rod inner segments. In contrast to the normal retina, the diabetic retina cannot meet the added metabolic load imposed by the dark-adapted rod photoreceptors; this exacerbates retinal hypoxia and stimulates the overproduction of vascular endothelial growth factor (VEGF). The use of nocturnal illumination to prevent dark adaptation, specifically reducing the rod photoreceptor dark current, should ameliorate diabetic retinopathy.

Assessment of oxygen saturation in retinal vessels of normal subjects and diabetic patients with and without retinopathy using Flow Oximetry System

PURPOSE

To assess oxygen saturation (StO2) in retinal vessels of normal subjects and diabetic patients with and without retinopathy using the modified version of the Flow Oximetry System (FOS) and a novel assessment software.

METHODS

The FOS and novel assessment software were used to determine StO2 levels in arteries and veins located between 1 and 2 mm from the margin of the optic disc and in the macular area.

RESULTS

Eighteen normal subjects, 15 diabetics without diabetic retinopathy (DM no DR), and 11 with non-proliferative diabetic retinopathy (NPDR) were included in final analysis. The mean [± standard deviation (SD)] StO2 in retinal arteries was 96.9%±3.8% in normal subjects; 97.4%±3.7% in DM no DR; and 98.4%±2.0% in NPDR. The mean venous StO2 was 57.5%±6.8% in normal subjects; 57.4%±7.5% in DM no DR; and 51.8%±6.8% in NPDR. The mean arterial and venous StO2 across the three groups were not statistically different (P=0.498 and P=0.071, respectively). The arterio-venous differences between the three study groups, however, were found to be statistically significant (P=0.015). Pairwise comparisons have demonstrated significant differences when comparing the A-V difference in the NPDR group to either normal subjects (P=0.02) or diabetic patients without DR (P=0.04).

CONCLUSIONS

The arterio-venous difference was greater, and statistically significant, in patients with NPDR when compared to normal subjects and to patients with diabetes and no retinopathy. The mean venous StO2 was lower, but not statistically significant, in NPDR compared with diabetics without retinopathy and with normal subjects.

Rapid prototyping of biomimetic vascular phantoms for hyperspectral reflectance imaging

The emerging technique of rapid prototyping with three-dimensional (3-D) printers provides a simple yet revolutionary method for fabricating objects with arbitrary geometry. The use of 3-D printing for generating morphologically biomimetic tissue phantoms based on medical images represents a potentially major advance over existing phantom approaches. Toward the goal of image-defined phantoms, we converted a segmented fundus image of the human retina into a matrix format and edited it to achieve a geometry suitable for printing. Phantoms with vessel-simulating channels were then printed using a photoreactive resin providing biologically relevant turbidity, as determined by spectrophotometry. The morphology of printed vessels was validated by x-ray microcomputed tomography. Channels were filled with hemoglobin (Hb) solutions undergoing desaturation, and phantoms were imaged with a near-infrared hyperspectral reflectance imaging system. Additionally, a phantom was printed incorporating two disjoint vascular networks at different depths, each filled with Hb solutions at different saturation levels. Light propagation effects noted during these measurements—including the influence of vessel density and depth on Hb concentration and saturation estimates, and the effect of wavelength on vessel visualization depth—were evaluated. Overall, our findings indicated that 3-D-printed biomimetic phantoms hold significant potential as realistic and practical tools for elucidating light–tissue interactions and characterizing biophotonic system performance.