Optical coherence elastography (OCE), the elastography extension of optical coherence tomography (OCT), has been proposed to quantify the biomechanical properties of ocular tissues (e.g., cornea and sclera) for early detection of different diseases, such as keratoconus and cataract. In wave-based OCE, various tissue stimulation methods have been demonstrated to induce waves in ocular tissues. Acoustic radiation force (ARF) is commonly used as a non-contact excitation source with tightly controlled stimulation parameters for various tissues, including the cornea and crystalline lens. However, ARFβs reliance on tightly focusing acoustic pressure within the tissue raises concerns about potential tissue damage. The aim of this study was to assess the safety of ARF-OCE on freshly enucleated ex vivo porcine eyes and investigate the ability of safe acoustic pressures to produce detectable displacements for OCE. In this study, the maximum value for ophthalmic acoustic pressure set by the Food and Drug Administration (FDA) was set as the 100% threshold in our assessment, and it was determined using a needle hydrophone. OCT and confocal microscopy were used to assess the integrity of the porcine crystalline lens before and after ARF-based OCE experiments. The maximum ARF intensity allowed by the FDA produced detectable wave propagation on the crystalline lens without damaging the lens.
Phenylthiourea (PTU) is often used to block pigmentation and make zebrafish completely transparent for easy optical imaging. PTU inhibits melanogenesis by inhibiting tyrosinase. Although the PTU is commonly used, it does have some side effects. PTU at a concentration of 0.2 mM (0.003%) significantly reduces the zebrafish eye size due to the inhibition of thyroid hormone production. Furthermore, low levels of thyroid hormones in zebrafish increase the stiffness of the intervertebral joints, altering their swimming behavior. The aim of this study was to assess the structural modifications and biomechanical properties of 5-day post-fertilization (dpf) zebrafish eyes after being exposed to PTU using optical coherence tomography and reverberant optical coherence elastography, respectively. Wild-type zebrafish (n=3), treated with PTU (0.2 mM), were compared with non-treated zebrafish (n=3). The results show a significant reduction (p=0.02) in the mean eye diameter of the fishes treated with PTU (312.66 ± 8.71 μm) versus the non-treated group (340.18 ± 4.38 μm). On the other hand, the non-treated group showed a significantly slower (p=0.02) shear wave speed (0.97 ± 0.12 m/s) compared with the PTU-treated group (2.65 ± 0.51 m/s), indicating that PTU induces a biomechanical change in the stiffness of the developing eye. PTU is a potent inhibitor of the pigmentation of zebrafish; however, it can also severely affect its biomechanical properties, specifically eye development, reducing eye diameter and increasing its stiffness.
SignificanceQuantifying the biomechanical properties of the whole eye globe can provide a comprehensive understanding of the interactions among interconnected ocular components during dynamic physiological processes. By doing so, clinicians and researchers can gain valuable insights into the mechanisms underlying ocular diseases, such as glaucoma, and design interventions tailored to each patientβs unique needs.AimThe aim of this study was to evaluate the feasibility and effectiveness of a multifocal acoustic radiation force (ARF) based reverberant optical coherence elastography (RevOCE) technique for quantifying shear wave speeds in different ocular components simultaneously.ApproachWe implemented a multifocal ARF technique to generate reverberant shear wave fields, which were then detected using phase-sensitive optical coherence tomography. A 3D-printed acoustic lens array was employed to manipulate a collimated ARF beam generated by an ultrasound transducer, producing multiple focused ARF beams on mouse eye globes ex vivo. RevOCE measurements were conducted using an excitation pulse train consisting of 10 cycles at 3Β kHz, followed by data processing to produce a volumetric map of the shear wave speed.ResultsThe results show that the system can successfully generate reverberant shear wave fields in the eye globe, allowing for simultaneous estimation of shear wave speeds in various ocular components, including cornea, iris, lens, sclera, and retina. A comparative analysis revealed notable differences in wave speeds between different parts of the eye, for example, between the apical region of the cornea and the pupillary zone of the iris (p = 0.003). Moreover, the study also revealed regional variations in the biomechanical properties of ocular components as evidenced by greater wave speeds near the apex of the cornea compared to its periphery.ConclusionsThe study demonstrated the effectiveness of RevOCE based on a non-invasive multifocal ARF for assessing the biomechanical properties of the whole eyeball. The findings indicate the potential to provide a comprehensive understanding of the mechanical behavior of the whole eye, which could lead to improved diagnosis and treatment of ocular diseases.
Reverberant optical coherence elastography (RevOCE) relies on multiple external excitation sources, such as mechanical actuators or vibrators, to produce random waves propagating in arbitrary directions. In this work, a preliminary study on acoustic beam manipulation to produce multi-focal acoustic radiation force is introduced using a planar ultrasound transducer and a 3D-printed acoustic lens array for RevOCE. An unfocused acoustic beam generated by an ultrasound transducer with 1 MHz central frequency and 38-mm element diameter was coupled to seven uniformly distributed acoustic focusing lenses. The spatial distribution of the acoustic field at the focal plane was measured with a needle hydrophone. The effectiveness of the system in generating reverberant shear wave fields was assessed by performing RevOCE imaging of tissue-mimicking gelatin phantoms. The multi-focus acoustic lens-transducer system was coupled with a phase-sensitive optical coherence tomography (PhS-OCT) system. The RevOCE measurement was conducted by sending ten cycles of a tone burst at 2 kHz. The measured acoustic pressure field showed that the array of concave spherical acoustic lenses spatially distributed the acoustic energy into multiple focal spots in the desired focal plane. Furthermore, RevOCE imaging in tissue-mimicking phantom indicated the effectiveness of the acoustic lens-transducer system in inducing reverberant shear wave fields for probing mechanical properties.
Measuring the mechanical properties of the cornea can help understand the structure and physiology of the eye, early detection of disease, and evaluation of therapy outcomes. In this work, we investigate the effect of collagen XII deficiency on the stiffness of the murine cornea using a multimodal approach for biomechanical analysis. Wave-based optical coherence elastography (OCE), heartbeat OCE, and Brillouin microscopy were all utilized to assess the mechanical properties of wild-type and collagen XII deficient ex vivo murine corneas as a function of IOP. All three techniques show that collagen XII deficiency leads to a dramatic decrease in corneal stiffness. Future work will investigate how these measurement techniques can be translated for in vivo assessment of corneal elasticity to understand the contribution of various proteins to corneal structural and mechanical integrity.
The fundamental physiological function of the iris is to control the amount of light entering the eye, which requires the coordinated constriction or dilation of the pupil, affected by two antagonistic muscles, namely the sphincter and radial muscles. Disorders of the iris, including these muscles, may lead to ocular pathologies, such as primary angle-closure glaucoma. Here, we assessed the regional biomechanical properties of the iris using phase-sensitive optical coherence elastography (PhS-OCE) to quantify the shear wave speed arising from perturbations generated using an acoustic radiation force (ARF) transducer of resonant frequency 3.5 MHz and focal length 19 mm. We determined regional shear wave speeds by tracking elastic wave propagation in ex vivo porcine irides. Results showed that the mean shear wave speed in the pupillary zone (~2.1 m/s) was consistently greater than in the ciliary zone (~1.87 m/s). These findings indicate that the mechanical properties of the iris exhibit regional heterogeneity, which may be related to the microstructure of the iris (muscle locations/extent) and intrinsic elastic properties.
Embryo development is driven by several substantial events that cause structural modifications during growth. The progressive changes in the embryo during this important period cause simultaneous alterations of its biomechanical properties. Understanding the structural modifications and changes in stiffness during embryo development is important for comprehending its growth and potentially detecting congenital diseases. The aim of this study is to map the biomechanical properties of embryos in 3D during development. Murine embryos at gestational day (GD) 11 were imaged using 3D Reverberant optical coherence elastography (Rev-OCE). The embryos were placed on a glass window, which was vibrated by an actuator at 1 kHz. In addition to providing the vibration, the glass window also enabled imaging in the common path configuration, which eliminated environmental noise and improved the displacement sensitivity of the system to sub-nanometer levels. The results showed the structural changes and the differences in stiffness in the embryo. The stiffness of the embryos from GD 11 showed stiffer areas along the developing spinal cord. Combining high-resolution OCT with elastography allowed us to understand the structural and biomechanical changes in the embryo during its development. Thus, this study provides important insights into embryo mechanical properties, which could serve as a potential biomarker for deficiencies in embryo development. Our future work is focused on imaging embryos at different stages as well as studying mutant models of congenital diseases, such as neural tube defects.
The healthy development of embryos depends on several critical biomechanical processes, such as neurulation and the formation of the cardiovascular system. Thus, understanding the structural modifications and changes in stiffness during development is important for understanding the etiology of various congenital diseases, such as anencephaly or spina bifida. In this work, we demonstrate the ability of reverberant optical coherence elastography (Rev-OCE) to map the biomechanical properties of various small animal embryos in high resolution in 3D completely noninvasively and without the need for any exogenous contrast agents. Rev-OCE measurements were performed in both murine and zebrafish embryos to showcase its capability to map the stiffness of commonly used small animal models of disease. The murine embryos were dissected from CD1 mice at gestational day 11, and the zebrafish embryos were isolated at 7 days post fertilization. Rev-OCE imaging was performed using a phase-sensitive optical coherence tomography (PhS-OCT) system, where the samples were placed on a glass window that was attached to a piezoelectric bender. The bender vibrated and generated randomly oriented shear waves in the samples, which were detected by the PhS-OCT system. In addition to holding the samples, the glass window enabled common path imaging for sub-nanometer levels of displacement sensitivity. The results show a clear spatial distribution of stiffness in the embryos. For example, the spinal region of the murine embryos was stiffer than other tissues, and in the zebrafish embryos, the head and swim bladder were stiffer. Embryonic elasticity could provide valuable insight into the critical embryonic developmental process and etiology of various congenital defects.
The crystalline lens is enclosed in a membrane, which presses upon the lens molding it into the required shape to enable dynamic focusing of light. Yet, the effect of the capsule membrane characteristics on the lens biomechanical properties has not been fully investigated. In this study, the lens viscoelasticity was assessed using phase-sensitive spectral-domain optical coherence tomography (PhS-OCT) coupled with acoustic radiation force (ARF) excitation before and after the capsular bag was dissected away. The ARF excitation was focused on the anterior pole of the lens, and two orthogonal MB mode scans were performed for each lens sample before and after removal of the lens capsule. The resulting elastic wave group velocity, π, in the lens with capsule intact (π = 2.60 ± 0.21 π/π ) was found to be significantly higher (p < .001) than after the capsule was removed (π = 1.14 ± 0.15 π/π ). Similarly, the viscoelastic assessment using surface wave dispersion model showed that both the Youngβs modulus and shear viscosity of the encapsulated lens (πΈ = 8.14 ± 1.10 πππ, π = 0.89 ±0.093 ππ β π ) was significantly higher than that of the decapsulated lens (πΈ = 3.10± 0.43 πππ, π = 0.28 ±0.021 ππ β π ). These findings, together with the geometric differences between encapsulated and decapsulated lens as quantified from structural OCT images, indicate that the capsule is a key lenticular component that determines the stiffness and structural integrity of the lens.
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