Highly Nonlinear Solitary Waves (HNSWs) are traditionally used in the field of nondestructive evaluation to inspect a material’s property without causing damage. The research in this paper proposes a new application for HNSWs: predicting changes in Intraocular Pressure (IOP) to ensure optimum treatment and prevent the progression of Glaucoma in an eye. The HNSWs used for assessment were collected from a Polydimethylsiloxane (PDMS) eye model and are initiated and stored with a solitary wave transducer. To collect a full range of HNSWs that represent the biological range of IOPs in humans, the PDMS eye model is pressurized from 12mmHg to 26mmHg with 1mmHg increments and waves are collected at each pressure point. Once a HNSW is collected, it is wirelessly transmitted to a server where it is fed into a convolutional neural network to predict the IOP. This is done by extracting relevant features from the HNSW with a Fast Fourier Transform and constructing a spectrogram which can be fed into the algorithm pixel by pixel. This methodology works due to the association of frequency content in the HNSW and changes of the stiffness in the material. In the case of high IOP, the increased pressure pushes against the artificial PDMS cornea and causes it to become stiffer with a higher Young’s modulus. We evaluated the ability of the algorithm to predict IOP based on the spectrogram.
Collagen crosslinking and degradation have been associated with changes in tissue stiffness. Although femtosecond laser irradiation might induce either collagen crosslinking or collagen degradation, the photomodulation effects are still unclear. Our goal was to characterize effects of photomodulation. We found that collagen crosslinking and degradation were related to the number of treatments, and photomodulation-involved reactants diffuse. The precision of photomodulation was at the micrometer level in both lateral and axial directions. Polarized light microscopy shows no change in collagen architecture in the treatment boundary and without the production of damaging thermoacoustic effects.
We characterized the stiffness and anisotropy of the equatorial sclera and its dependence on intraocular pressure (IOP). Porcine eyes were tested using ultrasound elastography under IOP control. Shear waves were induced using a mechanical shaker, and an off-axis 17.8 MHz transducer used to track the wave propagation in the equatorial and anterior-posterior directions. Wave speed was measured and used to estimate directional Young’s moduli. Anisotropy was defined as the ratio of the equatorial moduli to the anterior-posterior moduli. Sclera was stiffer in the equatorial direction, with anisotropy decreasing with IOP, from, 5.1 at 10 mmHg to 3.7 at 30 mmHg.
Most soft tissues, such as cornea and sclera, exhibit mechanical anisotropy. Obtaining the anisotropy properties of the sclera can be helpful in understanding the mechanical properties of the eyes in the development and progression of glaucoma and myopia. In this work, we have demonstrated a method of quantifying the anisotropic Young’s modulus values for sclera, using a confocal Lamb wave based acoustic radiation force optical coherence elastography (ARF-OCE). We have measured mechanical anisotropy in porcine sclera by measuring the Young’s modulus along both the equator direction and anterior to posterior direction. The results demonstrate the Young’s modus in the equator direction is 15-30% higher than that in the anterior to posterior direction.
Collagen is the most abundant protein and the main load-bearing material in the human body. Collagen architecture and behavior under load are important for understanding their functions in healthy tissues and the pathophysiology of diseased tissues. Structured polarized light imaging (SPLI) has proved to be an effective method to quantify collagen properties of thick collagenous tissues. Compared to without using structured illumination, SPLI provides improved accuracy in quantification and enhanced contrast for visualization of collagen structures by suppressing highly scattered light from the deeper tissue. However, current SPLI requires multiples images, typically 12, which is a challenge for imaging tissue dynamics where acquisition time is highly constrained. In this study, we developed real-time SPLI to address this challenge. The real-time imaging capability was achieved with a polarization-sensitive camera, capable of acquiring images at four polarization states simultaneously at 75 frames/second. Using a rolling image processing technique, we achieved the same frame rate for visualizing tissue dynamics. The performance of real-time SPLI was validated by imaging a rotating chicken tendon. Additionally, we integrated real-time SPLI with a tissue tester and demonstrated that pig scleral tissue, under uniaxial stretching, exhibited nonlinear and region-dependent deformation.
Collagen fiber architecture plays an important role in the mechanical properties of soft tissues. Conventional polarized light microscopy done using linear polarizers and, sometimes, quarter-wave plates is a label-free imaging technique for quantifying collagen fiber architecture, specifically distribution and orientation. However, this technique has several limitations. First, it requires acquiring multiple images with different polarization states, which precludes many timesensitive applications. Second, post-processing, especially image registration, reduces the level of detail discernible. Third, the added optical elements may cause glare under coaxial illumination, thus complicating the use of reflected light microscopy. We have recently demonstrated instant polarized light microscopy (IPOL), that requires only one image and therefore no registration. IPOL utilizes wavelength-dependent polarization to modify the spectrum of the illumination, generating visible colors that depend on fiber orientation and density. Herein we present two further advances on IPOL: we extend it to work with coaxial illumination allowing transmitted and reflected light microscopy, and we integrate it in a dissecting microscope. This permits real-time imaging, limited only by the camera frame rate, making it possible to track dynamic events, such as fast-acting responses to external forces or moving objects. We demonstrate IPOL with a field of view of 11 mm and a long working distance of 65 mm, which simplifies testing of large samples. IPOL provides both fiber distribution and orientation information in a single true-color snapshot, and therefore, it is suitable for time-sensitive applications.
Collagen is a major constituent of the eye and understanding its architecture and biomechanics is critical to preserve and restore vision. We, recently, demonstrated polarized light microscopy (PLM) as a powerful technique for measuring properties of the collagen fibers of the eye, such as spatial distribution and orientation. Our implementation of PLM, however, required sectioning the tissues for imaging using transmitted light. This is problematic because it limits analysis to thin sections. This is not only slow, but precludes study of dynamic events such as pressure-induced deformations, which are central to the role of collagen. We introduce structured polarized light microscopy (SPLM), an imaging technique that combines structured light illumination with PLM to allow imaging and measurement of collagen fiber properties in thick ocular tissues. Using pig and sheep eyes, we show that SPLM rejects diffuse background light effectively in thick tissues, significantly enhancing visualization of optic nerve head (ONH) structures, such as the lamina cribrosa, and improving the accuracy of the collagen fiber orientation measurements. Further, we demonstrate the integration of SPLM with an inflation device to enable direct visualization, deformation tracking, and quantification of collagen fibers in ONHs while under controlled pressure.
Glaucoma is a disease characterized by progressive and irreversible vision loss leading to blindness. This vision loss is believed to be largely determined by the biomechanics of the optic nerve head region. Optic nerve head biomechanics, in turn, is determined by the properties of the constituent collagen. However, it is challenging to visualize and quantify collagen morphology and orientation in situ, and therefore often studies of the region collagen have used histological sections. Here we describe SPLM, a novel imaging technique that combines structured light illumination and polarized light microscopy (PLM) to enable collagen fiber visualization and fiber orientation mapping without requiring tissue sectioning.
We developed a custom automated SPLM imaging system based on an upright microscope and a digital micromirror device (DMD) projector. The high spatial frequency patterns were used to achieve effective background suppression. Enhanced scattering sensitivity with SPLM resulted in images with highly improved visibility of collagen structures, even of tissues covered by pigment. SPLM produced improved fiber orientation maps from superficial layers compared to depth-averaged orientation from regular PLM. SPLM imaging provides valuable information of collagen fiber morphology and orientation in situ thus strengthening the study of ocular collagen fiber biomechanics and glaucoma.
KEYWORDS: In vivo imaging, Optical coherence tomography, Image registration, Eye, 3D metrology, 3D microstructuring, 3D image processing, Tissues, Optic nerve, Blood vessels
Elevated intraocular pressure (IOP) deforms the lamina cribrosa (LC), a structure within the optic nerve head (ONH) in the back of the eye. Evidence suggests that these deformations trigger events that eventually cause irreversible blindness, and have therefore been studied in-vivo using optical coherence tomography (OCT), and ex-vivo using OCT and a diversity of techniques. To the best of our knowledge, there have been no in-situ ex-vivo studies of LC mechanics. Our goal was two-fold: to introduce a technique for measuring 3D LC deformations from OCT, and to determine whether deformations of the LC induced by elevated IOP differ between in-vivo and in-situ ex-vivo conditions. A healthy adult rhesus macaque monkey was anesthetized and IOP was controlled by inserting a 27- gauge needle into the anterior chamber of the eye. Spectral domain OCT was used to obtain volumetric scans of the ONH at normal and elevated IOPs. To improve the visibility of the LC microstructure the scans were first processed using a novel denoising technique. Zero-normalized cross-correlation was used to find paired corresponding locations between images. For each location pair, the components of the 3D strain tensor were determined using non-rigid image registration. A mild IOP elevation from 10 to 15mmHg caused LC effective strains as large as 3%, and about 50% larger in-vivo than in-situ ex-vivo. The deformations were highly heterogeneous, with substantial 3D components, suggesting that accurate measurement of LC microstructure deformation requires high-resolution volumes. This technique will help improve understanding of LC biomechanics and how IOP contributes to glaucoma.
Although it is well documented that abnormal levels of either intraocular (IOP) or intracranial pressure (ICP) can lead to potentially blinding conditions, such as glaucoma and papilledema, little is known about how the pressures actually affect the eye. Even less is known about potential interplay between their effects, namely how the level of one pressure might alter the effects of the other. Our goal was to measure in-vivo the pressure-induced stretch and compression of the lamina cribrosa due to acute changes of IOP and ICP. The lamina cribrosa is a structure within the optic nerve head, in the back of the eye. It is important because it is in the lamina cribrosa that the pressure-induced deformations are believed to initiate damage to neural tissues leading to blindness. An eye of a rhesus macaque monkey was imaged in-vivo with optical coherence tomography while IOP and ICP were controlled through cannulas in the anterior chamber and lateral ventricle, respectively. The image volumes were analyzed with a newly developed digital image correlation technique. The effects of both pressures were highly localized, nonlinear and non-monotonic, with strong interactions. Pressure variations from the baseline normal levels caused substantial stretch and compression of the neural tissues in the posterior pole, sometimes exceeding 20%. Chronic exposure to such high levels of biomechanical insult would likely lead to neural tissue damage and loss of vision. Our results demonstrate the power of digital image correlation technique based on non-invasive imaging technologies to help understand how pressures induce biomechanical insults and lead to vision problems.
The mechanism that maintains a stable blood flow in the brain despite changes in cerebral perfusion pressure (CPP), and therefore guaranties a constant supply of oxygen and nutrients to the neurons, is known as cerebral auto-regulation (CA). In a certain range of CPP, blood flow is mediated by a vasomotor adjustment in vascular resistance through dilation of blood vessels. CA is known to be impaired in diseases like traumatic brain injury, Parkinson’s disease, stroke, hydrocephalus and others. If CA is impaired, blood flow and pressure changes are coupled and thee oxygen supply might be unstable. Lassen’s blood flow auto-regulation curve describes this mechanism, where a plateau of stable blood flow in a specific range of CPP corresponds to intact auto-regulation. Knowing the limits of this plateau and maintaining CPP within these limits can improve patient outcome. Since CPP is influenced by both intracranial pressure and arterial blood pressure, long term changes in either can lead to auto-regulation impairment. Non-invasive methods for monitoring blood flow auto-regulation are therefore needed. We propose too use Near infrared spectroscopy (NIRS) too fill this need. NIRS is an optical technique, which measures microvascular changes in cerebral hemoglobin concentration. We performed experiments on non-human primates during exsanguination to demonstrate that thee limits of blood flow auto-regulation can be accessed with NIRS.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.