Abstract: Optical coherence tomography (OCT) angiography is a new non-invasive imaging modality which is providing clinicians with an alternative to traditional dye-based angiography. The images are obtained using the concept of motion contrast and provide a quicker safer way to image the retinal and choroidal circulation. Not only are there practical aspects to support its integration but new insights are being made into the path; hysiology of various retinal choroidal diseases due to its ability to provide a 3-dimensional view of the vasculature which can be segmented in many ways to focus in on the circulation of a given anatomic region of the retina. We are currently in the phase of integration of this new technology into our practices.
Background: The guiding principle of functional brain mapping is that the cortex exhibits a spatial pattern of response reflecting its underlying functional organization. We know that large-scale patterns are common across individuals—everyone roughly has the same visual areas for example, but we do not know about small patterns, like the distribution of ocular dominance and orientation columns. Studies investigating the temporal aspect of brain-to-brain similarity have shown that a large portion of the brain is temporally synchronized across subjects (Hasson et al., 2004), but spatial pattern similarity has been scarcely studied, let alone at a fine scale. In the current study, we investigated fine-scale spatial pattern similarity between subjects during movie viewing and generated a map of prototypical patterns spanning the visual system. Characteristics of the map, such as spatial pattern size and distribution, reveal properties of the underlying structure and organisation of the visual cortex. These results will guide future brain mapping studies in decoding the informative spatial patterns of the visual cortex and increasing the resolution of current brain maps.
Methods: We had 56 subjects watch two movie clips from “Under the Sea 3D:IMAX” during an fMRI scan. Each clip was 5 minutes in length and was presented in 2D and 3D, in random order. We calculated the intersubject correlation of the spatial pattern inside predefined searchlights of diameter 3, 5, 7, 9 and 11 mm, covering the entire brain. A single threshold permutations test was used to test for significance: we generated 1,000 permutations made from scrambling the spatial patterns inside each searchlight of every subject, pooled these permutations together to generate a large distribution and used the 95th percentile to threshold the actual measurements. We compared these spatial pattern correlations to convexity variance between subjects to determine whether spatial pattern correlation could be explained by differing degrees of alignment across the cortex. We also compared spatial pattern correlation during 2D and 3D movie presentation.
Results: We found significant correlations in spatial pattern between subjects in the majority of early visual cortex, as well as higher visual areas. We found that mean spatial pattern similarity in a visual area tended to decrease as we move up the visual hierarchy. Spatial pattern correlation showed significant positive correlation with convexity variance for most visual areas, meaning that as anatomical misalignment increased, patterns became more similar. Spatial pattern correlation therefore cannot be explained by anatomical misalignment. Lastly, spatial pattern correlations tended to be higher for 3D movie presentation compared to 2D.
Conclusions: Our results suggest that many processes in early visual areas and even higher visual areas process visual information the same way in different individuals. Our results expand past studies by exploring spatial patterns instead of temporal patterns and studying at a fine-scale. This is the first study, to our knowledge, exploring fine-scale spatial patterns across the visual system. Our results show that fine-scale structures underlying activation patterns may be highly similar across subjects, pointing to a more ingrained organisation of the visual system than previously believed. This map we termed the “protoSPACE map”, may one day result in the detection of more subtle abnormalities that arise only during realistic vision in situations such as schizophrenia or mild traumatic brain injury, where traditional anatomical MRI scans report no changes.
Background: Pericytes are contractile cells that wrap along the walls of capillaries. In the brain, pericytes play a crucial role in the regulation of capillary diameter and vascular blood flow in response to metabolic demand. During ischemia, it has been suggested that pericytes may constrict capillaries, and that pericytes remain constricted after reperfusion thus resulting in impaired blood flow.
Methods: Here, we used a mouse model of retinal ischemia based on ligation of the central retinal artery to characterize the role of pericytes on capillary constriction. Ischemia was induced in transgenic mice carrying the NG2 promoter driving red fluorescent protein expression to selectively visualize pericytes (line NG2:DsRed).Changes in retinal capillary diameter at 1 hr after ischemia were measured ex vivo in whole-mounted retinas from ischemic and control eyes (n=4–6/group) using a stereological approach. Vessels and pericytes were three-dimensionally reconstructed using IMARIS (Bitplane). Furthermore, we used a novel and minimally invasive two-photon microscopy approach that allowed live imaging of microvasculature changes in the retina.
Results: Our data show a generalized reduction in capillary diameter in ischemic retinas relative to sham-operated controls in all vascular plexus (ischemia: 4.7±0.2 μm, control: 5.2±0.2 μm, student’s t-test, P<0.001). Analysis of the number of capillary constrictions at pericyte locations, visualized in NG2:DsRed mice, demonstrated a substantial increase in ischemic retinas relative to the physiological capillary diameter reductions observed in controls (ischemia: 1,038±277 constrictions at pericyte locations, control: 60±36 constrictions at pericyte locations, student’s t-test, P<0.01). Live imaging using two-photon microscopy confirmed robust capillary constriction at the level of pericytes on retinal capillaries during ischemia (n=6–8/group).
Conclusions: Collectively, our data demonstrate that ischemia promotes rapid pericyte constriction on retinal capillaries causing major microvascular dysfunction in this tissue. To identify the molecular mechanisms underlying the pathological response of pericytes during ischemia, we are currently carrying out experiments in mice and zebrafish to modulate signaling pathways involved in calcium dynamics leading to contractility in these cells.
Background: Decrease of ocular blood flow has been linked to the pathogenesis of ocular diseases such as glaucoma and age-related macular degeneration. Current methods that measure the pulsatile blood flow have major limitations, including the assumption that ocular rigidity is the same in all eyes. Our group has recently developed a new method to measure the pulsatile choroidal volume change by direct visualization of the choroid with OCT imaging and automated segmentation. Our goal in this study is to describe the distribution of PCBF in a healthy Caucasian population.
Methods: Fifty-one subjects were recruited from the Maisonneuve-Rosemont Hospital Ophthalmology Clinic and underwent PCBF measurement in one eye. The distribution of PCBF in healthy eyes was assessed.
Results: The distribution of PCBF among the healthy eyes was found to be 3.94±1.70 μL with this technique.
Conclusions: This study demonstrates the normal range of PCBF values obtained in a healthy Caucasian population. This technique could be used for further investigation of choroid pulsatility and to study glaucoma pathophysiology.
Background: The aim of this project is to develop a new standardized and cost-efficient method to compare optical coherence tomography (OCT) scans to their corresponding paraffin embedded histopathology sections in post-mortem eyes. This correlation will clarify the interpretation of OCT images, and it will also enable direct immunohistochemical characterization of features observed on OCT.
Methods: Study design: donor eyes were obtained from two separate eye banks. In order to minimize post-mortem change like retinal detachment and vitreous opacification, the eyes were fixed in a previously tested fixative solution. Time between death and fixation has been kept under 6 hours. Methods: Using a customized imaging device, nine post-mortem eyes were imaged with a SD-OCT machine. Subsequently, an 8mm trephine was used to isolate a portion of the posterior pole including the macular area and the optic nerve head for histopathological analysis. Paraffin embedded cross sections of the retina were obtained and visually compared to each OCT image (b-scans).
Results: To facilitate the correlation of OCT images to their histopathological sections, three principle aspects were controlled during tissue processing: rotation, tilt and location. Using markings as well as anatomical landmarks, serial histopathological sections in an orientation comparable to OCT b-scans were obtained, thereby facilitating image pairing.
Conclusions: Compared to other well-established methods using resin and electron microscopy, our standardized Methods allowed us to successfully compare OCT b-scans to serial retinal cross sections of a wider macular area at a lower cost. Our novel approach allows us to translate features observed on OCT images into well-established histopathological images, providing the clinician with additional tools to obtain difficult diagnoses with more confidence.
Background: Pericytes are contractile cells that wrap along the walls of capillaries. In the brain, pericytes play a crucial role in the regulation of capillary diameter and vascular blood flow in response to metabolic demand. The contribution of pericytes to microvascular deficits in glaucoma is currently unknown. To address this, we used two-photon excitation microscopy for longitudinal monitoring of retinal pericytes and capillaries in a mouse glaucoma model.
Methods: Ocular hypertension was induced by injection of magnetic microbeads into the anterior chamber of albino mice expressing red fluorescent protein selectively in pericytes (NG2-DsRed). Minimally invasive, multiphoton imaging through the sclera of live NG2-DsRed mice was used to visualize pericytes and capillary diameter at one, two and three weeks after glaucoma induction. In vivo fluctuations in pericyte intracellular calcium were monitored with the calcium indicator Fluo-4. Ex vivo stereological analysis of retinal tissue prior to and after injection of microbeads was used to confirm our in vivo findings.
Results: Live two-photon imaging of NG2-DsRed retinas demonstrated that ocular hypertension induced progressive accumulation of intracellular calcium in pericytes. Calcium uptake correlated directly with the narrowing of capillaries in the superficial, inner, and outer vascular plexuses (capillary diameter: na?ve control =4.7±0.1 μm, glaucoma =4.0±0.1 μm, n=5–6 mice/group, Student’s t-test P<0.05). Frequency distribution analysis showed a substantial increase in the number of small-diameter capillaries (≤3 μm) and a decrease in larger-diameter microvessels (≥5–9 μm) at three weeks after induction of ocular hypertension (n=5–6 mice/group, Student’s t-test P<0.05).
Conclusions: Our data support two main conclusions. First, two-photon excitation microscopy is an effective strategy to monitor longitudinal changes in retinal pericytes and capillaries in live animals at glaucoma onset and progression. Second, ocular hypertension triggers rapid intracellular calcium increase in retinal pericytes leading to substantial capillary constriction. This study identifies retinal pericytes as important mediators of early microvascular dysfunction in glaucoma.
Abstract: Pediatric neuro-ophthalmology is a subspecialty within neuro-ophthalmology. Pediatric neuro-ophthalmic diseases must be considered separate from their adult counterparts, due to the distinctive nature of the examination, clinical presentations, and management choices. This manuscript will highlight four common pediatric neuro-ophthalmic disorders by describing common clinical presentations, recommended management, and highlighting recent developments. Diseases discussed include pediatric idiopathic intracranial hypertension (IIH), pseudopapilledema, optic neuritis (ON) and optic pathway gliomas (OPG). The demographics, diagnosis and management of common pediatric neuro-ophthalmic disease require a working knowledge of the current research presented herein. Special attention should be placed on the differences between pediatric and adult entities such that children can be appropriately diagnosed and treated.
Background: Disruption of the microstructure in corneal stroma can lead to the loss of transparency. The lack of a characterization method for the microstructure prevents such scaffolds to be implemented in tissue transplantation. The non-invasive, three-dimensional (3D) rendering multiphoton microscopy (MPM) poses the potential to solve this problem.
Methods: MPM images and data analyses were performed with three kinds of samples with known and different quality. Isosurfaces (ISOs) were constructed for the evaluation of void volume and collagen distribution.
Results: The differences in the microstructures of these samples were revealed with clear indications and links to their behaviours in rehydration and possible transparency. According to this analysis, the scaffold with the highest void space ratio amongst the three presented the highest successful rates to be thoroughly rehydrated.
Conclusions: Such a method can be developed for assessing the quality of tissue engineered corneas, or donated corneas, and be useful as a powerful research tool in cornea related research.
