Assessing the effects of storage medium on the biomechanical properties of porcine lens with optical coherence elastography

There has been a large amount of research focused on studying the biomechanical properties of the lens ex vivo. However, the storage medium of the lenses may affect the biomechanical evaluation during ex vivo measurements, which has been demonstrated with other tissues such as the cornea. In this work, we utilized a focused micro air-pulse and phase-sensitive optical coherence elastography to quantify the changes in lenticular biomechanical properties when incubated in different media, temperatures, and pHs for up to 24 hours. The results show that the lenses became stiffer when incubated at lower temperatures and higher pHs. Meanwhile, lenses incubated in M-199 were more mechanically stable than lenses incubated in PBS and DMEM

Quantifying changes in lenticular stiffness with optical coherence elastography

Maintaining a normal intraocular pressure (IOP) is important for visual health. Elevated IOPs have been implicated in many diseases, such as glaucoma and uveitis. The effects of an elevated IOP on the delicate tissues of the optic nerve head and retina are well-studied, but there is a lack of information about the effects of high IOPs on the stiffness of the crystalline lens. Changes in lenticular biomechanical properties have been implicated in diseases such as presbyopia and cataract, therefore, measuring lenticular biomechanical properties is crucial to understanding the etiology and progression of the leading causes of vision impairment. Additionally, there has been even less research focused on the effects of storage media on lenticular stiffness. Previous studies have been focused on the “gold standard” of mechanical testing on excised lenses. However, mechanical testing is invasive and destructive, and removal of the lens from the eyeglobe does not allow for properly replicating the lens environment in the eye-globe. Thus, there is a need for noninvasive measurement techniques capable of performing in situ and in vivo elastographic measurements of the lens. Here, we artificially controlled the IOP of whole porcine eye-globes (N=3). Acoustic radiation force induced low amplitude displacements (<10 µm) at the apex of the lenses, which then propagated as an elastic wave. The elastic wave propagation was detected by a phase-sensitive optical coherence elastography (PhS-OCE) system. The results show that the stiffness of the lenses increased when IOP increased from 10 mmHg IOP to 40 mmHg. Additional OCE measurements were made on excised lenses stored in various media (PBS, DMEM, and M-199) at different pHs (4-7) and at different temperatures (4°C, 22°C, and 37°C). The results show that the stiffness of the lenses increased slightly when incubated at 4°C or 22°C, but decreased when the lenses were incubated at 37°C, while lenses incubated in M-199 showed more stability in their stiffness than lenses incubated in PBS and DMEM. Moreover, the lenses stored in M-199 at a pH of 7 showed a decrease in stiffness over 24 hours, while the more acidic M-199 media caused an increase in lenticular stiffness

Quantifying lens elastic properties with optical coherence elastography as a function of intraocular pressure

Normal intraocular pressure (IOP) is crucial for proper maintaining of eye-globe geometry, ocular tissue health, and visual acuity. An elevated IOP is associated with diseases such as glaucoma and uveitis. While the effects of an elevated IOP on the delicate tissues of the optic nerve head and retina are well-studied, the changes in lenticular biomechanical properties as a function of IOP are not as clear. Moreover, changes in lenticular biomechanical properties have been implicated in conditions and diseases such as presbyopia and cataract. However, measuring the biomechanical properties of the lens as it sits inside the eye-globe is a challenge, but it is necessary to correctly understand the interplay between lenticular biomechanical properties and IOP. In this work, we utilized optical coherence elastography (OCE) to measure the biomechanical properties of the porcine lens in situ

Clinical Application of Sonographic Elasticity Imaging for Aging of Deep Venous Thrombosis

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135197/1/jum2003225443.pd

Analysis of the effects of curvature and thickness on elastic wave velocity in cornea-like structures by finite element modeling and optical coherence elastography

Wave models that have been used to extract the biomechanical properties of the cornea from the propagation of an elastic wave are based on an assumption of thin-plate geometry. However, this assumption does not account for the effects of corneal curvature and thickness. This study conducts finite element (FE) simulations on four types of cornea-like structures as well as optical coherence elastography (OCE) experiments on contact lenses and tissue-mimicking phantoms to investigate the effects of curvature and thickness on the group velocity of an elastic wave. The elastic wave velocity as determined by FE simulations and OCE of a spherical shell section decreased from ∼2.8 m/s to ∼2.2 m/s as the radius of curvature increased from 19.1 mm to 47.7 mm and increased from ∼3.0 m/s to ∼4.1 m/s as the thickness of the agar phantom increased from 1.9 mm to 5.6 mm. Both the FE simulation and OCE results confirm that the group velocity of the elastic wave decreases with radius of curvature but increases with thickness. These results demonstrate that the effects of the curvature and thickness must be considered in the further development of accurate wave models for reconstructing biomechanical properties of the cornea

Quantifying the effects of hydration on corneal stiffness with optical coherence elastography

Several methods have been proposed to assess changes in corneal biomechanical properties due to various factors, such as degenerative diseases, intraocular pressure, and therapeutic interventions (e.g. corneal collagen crosslinking). However, the effect of the corneal tissue hydration state on corneal stiffness is not well understood. In this work, we induce low amplitude (< 10 μm) elastic waves with a focused micro air-pulse in fresh in situ rabbit corneas (n = 10) in the whole eye-globe configuration at an artificially controlled intraocular pressure. The waves were then detected with a phase-stabilized swept source optical coherence elastography system. Baseline measurements were taken every 20 minutes for an hour while the corneas were hydrated with 1X PBS. After the measurement at 60 minutes, a 20% dextran solution was topically instilled to dehydrate the corneas. The measurements were repeated every 20 minutes again for an hour. The results showed that the elastic wave velocity decreased as the corneal thickness decreased. Finite element modeling (FEM) was performed using the corneal geometry and elastic wave propagation speed to assess the stiffness of the samples. The results show that the stiffness increased from ~430 kPa during hydration with PBS to ~500 kPa after dehydration with dextran, demonstrating that corneal hydration state, apart from geometry and intraocular pressure, can change the stiffness of the cornea

Model-Based Reconstructive Elasticity Imaging Using Ultrasound

Elasticity imaging is a reconstructive imaging technique where tissue motion in response to mechanical excitation is measured using modern imaging systems, and the estimated displacements are then used to reconstruct the spatial distribution of Young's modulus. Here we present an ultrasound elasticity imaging method that utilizes the model-based technique for Young's modulus reconstruction. Based on the geometry of the imaged object, only one axial component of the strain tensor is used. The numerical implementation of the method is highly efficient because the reconstruction is based on an analytic solution of the forward elastic problem. The model-based approach is illustrated using two potential clinical applications: differentiation of liver hemangioma and staging of deep venous thrombosis. Overall, these studies demonstrate that model-based reconstructive elasticity imaging can be used in applications where the geometry of the object and the surrounding tissue is somewhat known and certain assumptions about the pathology can be made

Evaluation of posterior porcine sclera elasticity in situ as a function of IOP

The biomechanical properties of the sclera could provide key information regarding the progression and etiology of ocular diseases. For example, an elevated intraocular pressure is one of the most common risk factors for glaucoma and can cause pathological deformations in the tissues of the posterior eye, such as the sclera, potentially damaging these vital tissues. Previous work has evaluated scleral biomechanical response to global displacements with techniques such as inflation testing. However, these methods cannot provide localized biomechanical assessments. In this pilot work, we induce low amplitude (< 10 μm) elastic waves using acoustic radiation force in posterior scleral tissue of fresh porcine eyes (n=2) in situ. The wave propagation induced using an ultrasound transducer was detected across an 8 mm region using a phase-sensitive optical coherence elastography system (PhS-OCE). The elastographic measurements were taken at various artificially controlled intraocular pressures (IOP). The IOP was pre-cycled before being set to 10 mmHg for the first measurement. Subsequent measurements were taken at 20 mmHg and 30 mmHg for each sample. The results show an increase in the stiffness of the sclera as a function of IOP. Furthermore, we observed a variation in the elasticity based on direction, suggesting that the sclera has anisotropic biomechanical properties. Our results show that OCE is an effective method for evaluating the mechanical properties of the sclera, and reveals a new area for our future work

Noncontact elastic wave imaging optical coherence elastography for evaluating changes in corneal elasticity due to crosslinking

The mechanical properties of tissues can provide valuable information about tissue integrity and health and can assist in detecting and monitoring the progression of diseases such as keratoconus. Optical coherence elastography (OCE) is a rapidly emerging technique, which can assess localized mechanical contrast in tissues with micrometer spatial resolution. In this work we present a noncontact method of optical coherence elastography to evaluate the changes in the mechanical properties of the cornea after UV-induced collagen cross-linking. A focused air-pulse induced a low amplitude (μm scale) elastic wave, which then propagated radially and was imaged in three dimensions by a phase-stabilized swept source optical coherence tomography (PhSSSOCT) system. The elastic wave velocity was translated to Young’s modulus in agar phantoms of various concentrations. Additionally, the speed of the elastic wave significantly changed in porcine cornea before and after UV-induced corneal collagen cross-linking (CXL). Moreover, different layers of the cornea, such as the anterior stroma, posterior stroma, and inner region, could be discerned from the phase velocities of the elastic wave. Therefore, because of noncontact excitation and imaging, this method may be useful for in vivo detection of ocular diseases such as keratoconus and evaluation of therapeutic interventions such as CXL

Biomechanical assessment of myocardial infarction using optical coherence elastography

Myocardial infarction (MI) leads to cardiomyocyte loss, impaired cardiac function, and heart failure. Molecular genetic analyses of myocardium in mouse models of ischemic heart disease have provided great insight into the mechanisms of heart regeneration, which is promising for novel therapies after MI. Although biomechanical factors are considered an important aspect in cardiomyocyte proliferation, there are limited methods for mechanical assessment of the heart in the mouse MI model. This prevents further understanding the role of tissue biomechanics in cardiac regeneration. Here we report optical coherence elastography (OCE) of the mouse heart after MI. Surgical ligation of the left anterior descending coronary artery was performed to induce an infarction in the heart. Two OCE methods with assessment of the direction-dependent elastic wave propagation and the spatially resolved displacement damping provide complementary analyses of the left ventricle. In comparison with sham, the infarcted heart features a fibrotic scar region with reduced elastic wave velocity, decreased natural frequency, and less mechanical anisotropy at the tissue level at the sixth week post-MI, suggesting lower and more isotropic stiffness. Our results indicate that OCE can be utilized for nondestructive biomechanical characterization of MI in the mouse model, which could serve as a useful tool in the study of heart repair