1. Articles from Michael D. Twa

    1-24 of 29 1 2 »
    1. Spatial Assessment of Heterogeneous Tissue Natural Frequency Using Micro-Force Optical Coherence Elastography

      Spatial Assessment of Heterogeneous Tissue Natural Frequency Using Micro-Force Optical Coherence Elastography

      Analysis of corneal tissue natural frequency was recently proposed as a biomarker for corneal biomechanics and has been performed using high-resolution optical coherence tomography (OCT)-based elastography (OCE). However, it remains unknown whether natural frequency analysis can resolve local variations in tissue structure. We measured heterogeneous samples to evaluate the correspondence between natural frequency distributions and regional structural variations. Sub-micrometer sample oscillations were induced point-wise by microliter air pulses (60–85 Pa, 3 ms) and detected correspondingly at each point using a 1,300 nm spectral domain common path OCT system with 0.44 nm phase detection sensitivity. The resulting ...

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    2. In Vivo Human Corneal Shear-wave Optical Coherence Elastography

      In Vivo Human Corneal Shear-wave Optical Coherence Elastography

      SIGNIFICANCE A novel imaging technology, dynamic optical coherence elastography (OCE), was adapted for clinical noninvasive measurements of corneal biomechanics. PURPOSE Determining corneal biomechanical properties is a long-standing challenge. Elasticity imaging methods have recently been developed and applied for clinical evaluation of soft tissues in cancer detection, atherosclerotic plaque evaluation, surgical guidance, and more. Here, we describe the use of dynamic OCE to characterize mechanical wave propagation in the human cornea in vivo , thus providing a method for clinical determination of corneal biomechanical properties. METHODS High-resolution phase-sensitive optical coherence tomography imaging was combined with microliter air-pulse tissue stimulation to perform dynamic ...

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    3. Confocal air-coupled ultrasonic optical coherence elastography probe for quantitative biomechanics

      Confocal air-coupled ultrasonic optical coherence elastography probe for quantitative biomechanics

      We present an air-coupled ultrasonic radiation force probe co-focused with a phase-sensitive optical coherence tomography (OCT) system for quantitative wave-based elastography. A custom-made 1 MHz spherically focused piezoelectric transducer with a concentric 10 mm wide circular opening allowed for confocal micro-excitation of waves and phase-sensitive OCT imaging.

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    4. Dynamic Optical Coherence Elastography of the Anterior Eye: Understanding the Biomechanics of the Limbus

      Dynamic Optical Coherence Elastography of the Anterior Eye: Understanding the Biomechanics of the Limbus

      Purpose : Currently, the biomechanical properties of the corneo-scleral limbus when the eye-globe deforms are largely unknown. The purpose of this study is to evaluate changes in elasticity of the cornea, sclera, and limbus when subjected to different intraocular pressures (IOP) using wave-based optical coherence elastography (OCE). Special attention was given to the elasticity changes of the limbal region with respect to the elasticity variations in the neighboring corneal and scleral regions. Methods : Continuous harmonic elastic waves (800 Hz) were mechanically induced in the sclera near the corneo-sclera limbus of in situ porcine eye-globes ( n = 8). Wave propagation was imaged using ...

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    5. Clinical Corneal Optical Coherence Elastography Measurement Precision: Effect of Heartbeat and Respiration

      Clinical Corneal Optical Coherence Elastography Measurement Precision: Effect of Heartbeat and Respiration

      Purpose : Normal physiological movements (e.g., respiration and heartbeat) induce eye motions during clinical measurements of human corneal biomechanical properties using optical coherence elastography (OCE). We quantified the effects of respiratory and cardiac-induced eye motions on clinical corneal OCE measurement precision and repeatability. Methods : Corneal OCE was performed using low-force, micro-air-pulse tissue stimulation and high-resolution phase-sensitive optical coherence tomography (OCT) imaging. Axial surface displacements of the corneal apex were measured (M-mode) at a 70-kHz sampling rate and three different stimulation pressures (20–60 Pa). Simultaneously, the axial corneal position was tracked with structural OCT imaging, while the heartrate and respiration ...

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    6. Theory and design of Schwarzschild scan objective for Optical Coherence Tomography

      Theory and design of Schwarzschild scan objective for Optical Coherence Tomography

      Optical coherence elastography (OCE) is one form of multi-channel imaging that combines high-resolution optical coherence tomography (OCT) imaging with mechanical tissue stimulation. This combination of structural and functional imaging can require additional space to integrate imaging capabilities with additional functional elements (e.g., optical, mechanical, or acoustic modulators) either at or near the imaging axis. We address this challenge by designing a novel scan lens based on a modified Schwarzchild objective lens, comprised of a pair of concentric mirrors with potential space to incorporate additional functional elements and minimal compromise to the available scan field. This scan objective design allows ...

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    7. Modified wavelength scanning interferometry for simultaneous tomography and topography of the cornea with Fourier domain optical coherence tomography

      Modified wavelength scanning interferometry for simultaneous tomography and topography of the cornea with Fourier domain optical coherence tomography

      Visual acuity is dependent on corneal shape and size. A minor variation in surface geometry can cause a deformation of corneal geometry, which affects its optical performance. In this work we demonstrate an algorithm for the simultaneous measurement of corneal tomography and topography with a traditional point-scanning Fourier domain optical coherence tomography (FD-OCT) system. A modified wavelength scanning interferometry (mWSI) algorithm enabled topographical evaluation of the surface with nanometer-scale resolution, which is superior to the micrometer-scale resolution of traditional OCT structural imaging. We validated the technique with an optically flat mirror, standard roughness gauges, and atomic force microscopy (AFM). The ...

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

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

      Purpose To quantify the effects of the hydration state on the Young's modulus of the cornea . Setting Biomedical Optics Laboratory, University of Houston, Houston, Texas, USA. Design Experimental study. Methods Noncontact, dynamic optical coherence elastography (OCE) measurements were taken of in situ rabbit corneas in the whole eye–globe configuration (n = 10) and at an artificially controlled intraocular pressure of 15 mm Hg. Baseline OCE measurements were taken by topically hydrating the corneas with saline for 1 hour. The corneas were then dehydrated topically with a 20% dextran solution for another hour, and the OCE measurements were repeated. A ...

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    9. Effects of Thickness on Corneal Biomechanical Properties Using Optical Coherence Elastography

      Effects of Thickness on Corneal Biomechanical Properties Using Optical Coherence Elastography

      SIGNIFICANCE Measured corneal biomechanical properties are driven by intraocular pressure, tissue thickness, and inherent material properties. We demonstrate tissue thickness as an important factor in the measurement of corneal biomechanics that can confound short-term effects due to UV riboflavin cross-linking (CXL) treatment. PURPOSE We isolate the effects of tissue thickness on the measured corneal biomechanical properties using optical coherence elastography by experimentally altering the tissue hydration state and stiffness. METHODS Dynamic optical coherence elastography was performed using phase-sensitive optical coherence tomography imaging to quantify the tissue deformation dynamics resulting from a spatially discrete, low-force air pulse (150-μm spot size ...

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    10. Common-path phase-sensitive optical coherence tomography provides enhanced phase stability and detection sensitivity for dynamic elastography

      Common-path phase-sensitive optical coherence tomography provides enhanced phase stability and detection sensitivity for dynamic elastography

      Phase-sensitive optical coherence elastography (PhS-OCE) is an emerging optical technique to quantify soft-tissue biomechanical properties. We implemented a common-path OCT design to enhance displacement sensitivity and optical phase stability for dynamic elastography imaging. The background phase stability was greater in common-path PhS-OCE (0.24 ± 0.07nm) than conventional PhS-OCE (1.60 ± 0.11μm). The coefficient of variation for surface displacement measurements using conventional PhS-OCE averaged 11% versus 2% for common-path PhS-OCE. Young’s modulus estimates showed good precision (95% CIs) for tissue phantoms: 24.96 ± 2.18kPa (1% agar), 49.69 ± 4.87kPa (1.5% agar), and 116.08 ...

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    11. Quantifying the effects of UV-A/riboflavin crosslinking on the elastic anisotropy and hysteresis of the porcine cornea by noncontact optical coherence elastography

      Quantifying the effects of UV-A/riboflavin crosslinking on the elastic anisotropy and hysteresis of the porcine cornea by noncontact optical coherence elastography

      The collagen fibril orientation of the cornea can provide critical information about cornea tissue health because diseases such as keratoconus and therapeutic interventions such as UV-A/riboflavin corneal collagen crosslinking (CXL) can alter the ultrastructural arrangement of collagen fibrils. Here, we quantify the elastic anisotropy and hysteresis of in situ porcine corneas as a function of intraocular pressure (IOP) with noncontact optical coherence elastography. Moreover, the effects of UV-A riboflavin corneal collagen crosslinking on the elastic anisotropy and hysteresis were evaluated. The propagation of an air-pulse induced elastic wave was imaged at stepped meridional angles by a home built phasestabilized ...

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    12. Applanation optical coherence elastography: noncontact measurement of intraocular pressure, corneal biomechanical properties, and corneal geometry with a single instrument

      Applanation optical coherence elastography: noncontact measurement of intraocular pressure, corneal biomechanical properties, and corneal geometry with a single instrument

      Current clinical tools provide critical information about ocular health such as intraocular pressure (IOP). However, they lack the ability to quantify tissue material properties, which are potent markers for ocular tissue health and integrity. We describe a single instrument to measure the eye-globe IOP, quantify corneal biomechanical properties, and measure corneal geometry with a technique termed applanation optical coherence elastography (Appl-OCE). An ultrafast OCT system enabled visualization of corneal dynamics during noncontact applanation tonometry and direct measurement of micro air-pulse induced elastic wave propagation. Our preliminary results show that the proposed Appl-OCE system can be used to quantify IOP, corneal ...

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    13. How Will Optometry Respond to Technological Evolution?

      How Will Optometry Respond to Technological Evolution?

      I recently visited the Computer History Museum in Mountain View, California, in the heart of Silicon Valley. This museum chronicles the evolution of computing technology from slide rules to self-driving cars. Their exhibits showcase fascinating stories of what the power of the human mind can achieve. The tangible manifestations of these achievements on display played important parts in ending wars (the ENIGMA cipher machine or the Manhattan Project), walking on the moon, computer animation, the world-wide web, and today’s smartphones. These achievements required advances in engineering, math, sciences, art, language, and culture. However, the real stories behind the innovations ...

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    14. Assessing the effects of riboflavin/UV-A crosslinking on porcine corneal mechanical anisotropy with optical coherence elastography

      Assessing the effects of riboflavin/UV-A crosslinking on porcine corneal mechanical anisotropy with optical coherence elastography

      In this work we utilize optical coherence elastography (OCE) to assess the effects of UV-A/riboflavin corneal collagen crosslinking (CXL) on the mechanical anisotropy of in situ porcine corneas at various intraocular pressures (IOP). There was a distinct meridian of increased Young’s modulus in all samples, and the mechanical anisotropy increased as a function of IOP and also after CXL. The presented noncontact OCE technique was able to quantify the Young’s modulus and elastic anisotropy of the cornea and their changes as a function of IOP and CXL, opening new avenues of research for evaluating the effects of ...

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    15. Optical coherence elastography assessment of corneal viscoelasticity with a modified Rayleigh-Lamb wave model

      Optical coherence elastography assessment of corneal viscoelasticity with a modified Rayleigh-Lamb wave model

      The biomechanical properties of the cornea play a critical role in forming vision. Diseases such as keratoconus can structurally degenerate the cornea causing a pathological loss in visual acuity. UV-A/riboflavin corneal collagen crosslinking (CXL) is a clinically available treatment to stiffen the cornea and restore its healthy shape and function. However, current CXL techniques do not account for pre-existing biomechanical properties of the cornea nor the effects of the CXL treatment itself. In addition to the inherent corneal structure, the intraocular pressure (IOP) can also dramatically affect the measured biomechanical properties of the cornea. In this work, we present ...

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    16. Validation of Macular Choroidal Thickness Measurements from Automated SD-OCT Image Segmentation

      Validation of Macular Choroidal Thickness Measurements from Automated SD-OCT Image Segmentation

      Purpose: Spectral domain optical coherence tomography (SD-OCT) imaging permits in vivo visualization of the choroid with micron-level resolution over wide areas and is of interest for studies of ocular growth and myopia control. We evaluated the speed, repeatability, and accuracy of a new image segmentation method to quantify choroid thickness compared to manual segmentation. Methods: Two macular volumetric scans (25 x 30[degrees]) were taken from 30 eyes of 30 young adult subjects in two sessions, 1 hour apart. A single rater manually delineated choroid thickness as the distance between Bruch's membrane and sclera across three B-scans (foveal, inferior ...

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      Mentions: Duke University
    17. Investigating Elastic Anisotropy of the Porcine Cornea as a Function of Intraocular Pressure With Optical Coherence Elastography

      Investigating Elastic Anisotropy of the Porcine Cornea as a Function of Intraocular Pressure With Optical Coherence Elastography

      PURPOSE: To evaluate the elastic anisotropy of porcine corneas at different intraocular pressures (IOPs) using a noncontact optical coherence elastography (OCE) technique. METHODS: A focused air-pulse induced low amplitude (≤ 10 µ m) elastic waves in fresh porcine corneas (n = 7) in situ in the whole eye globe configuration. A home-built phase-stabilized swept source optical coherence elastography (PhS-SSOCE) system imaged the elastic wave propagation at different stepped radial directions. A closed-loop feedback system was used to artificially control the IOP and the OCE measurements were repeated as the IOP was incrementally increased from 15 to 30 mm Hg in 5-mm Hg increments ...

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    18. Evaluating the Effects of Riboflavin/UV-A and Rose-Bengal/Green Light Cross-Linking of the Rabbit Cornea by Noncontact Optical Coherence Elastography

      Evaluating the Effects of Riboflavin/UV-A and Rose-Bengal/Green Light Cross-Linking of the Rabbit Cornea by Noncontact Optical Coherence Elastography

      Purpose : The purpose of this study was to use noncontact optical coherence elastography (OCE) to evaluate and compare changes in biomechanical properties that occurred in rabbit cornea in situ after corneal collagen cross-linking by either of two techniques: ultraviolet-A (UV-A)/riboflavin or rose-Bengal/green light. Methods : Low-amplitude (≤10 μm) elastic waves were induced in mature rabbit corneas by a focused air pulse. Elastic wave propagation was imaged by a phase-stabilized swept source OCE (PhS-SSOCE) system. Corneas were then cross-linked by either of two methods: UV-A/riboflavin (UV-CXL) or rose-Bengal/green light (RGX). Phase velocities of the elastic waves were fitted ...

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    19. A comparison study of Riboflavin/UV-A and Rose-Bengal/Green light cross-linking of the rabbit corneas using optical coherence elastography

      A comparison study of Riboflavin/UV-A and Rose-Bengal/Green light cross-linking of the rabbit corneas using optical coherence elastography

      The biomechanical properties of the cornea are critical factors which determine its health and subsequent visual acuity. Keratoconus is a structural degeneration of the cornea which can diminish vision quality. Riboflavin/UV-A corneal collagen cross-linking (UV-CXL) is an emerging treatment that increases the stiffness of the cornea and improves its ability to resist further degeneration. While UV-CXL has shown great promise for effective therapy of the keratoconus, there are concerns associated with the UV irradiation, such as keratocyte cytotoxicity. Rose-bengal/green light corneal collagen cross-linking (RGX) has been proposed as an alternative to UV-CXL. Because of the high absorbance of ...

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    20. Spatial mapping of the biomechanical properties of rabbit cornea after cross-linking using optical coherence elastography

      Spatial mapping of the biomechanical properties of rabbit cornea after cross-linking using optical coherence elastography

      Keratoconus, a structural degeneration of the cornea, is often treated with UV-induced collagen cross-linking (CXL) to increase tissue resistance to further deformation and degeneration. Optimal treatment would be customized to the individual and consider pre-existing biomechanical properties as well as the effects induced by CXL. This requires the capability to noninvasively measure corneal mechanical properties. In this study, we demonstrate the use of phase-stabilized swept source optical coherence elastography (PhS-SSOCE) to assess the relaxation rate of a deformation which was induced by a focused air-pulse in tissue-mimicking gelatin phantoms of various concentration and partially cross-linked rabbit corneas. The temporal relaxation ...

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    21. Quantitative assessment of corneal viscoelasticity using optical coherence elastography and a modified Rayleigh–Lamb equation

      Quantitative assessment of corneal viscoelasticity using optical coherence elastography and a modified Rayleigh–Lamb equation

      We demonstrate the use of a modified Rayleigh–Lamb frequency equation in conjunction with noncontact optical coherence elastography to quantify the viscoelastic properties of the cornea. Phase velocities of air-pulse-induced elastic waves were extracted by spectral analysis and used for calculating the Young’s moduli of the samples using the Rayleigh–Lamb frequency equation (RLFE). Validation experiments were performed on 2% agar phantoms ( n = 3 ) and then applied to porcine corneas ( n = 3 ) in situ . The Young’s moduli of the porcine corneas were estimated to be ∼ 60     kPa with a shear viscosity ∼ 0.33     Pa ⋅ s . The results demonstrate ...

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    22. Differentiating untreated and cross-linked porcine corneas of the same measured stiffness with optical coherence elastography

      Differentiating untreated and cross-linked porcine corneas of the same measured stiffness with optical coherence elastography

      Structurally degenerative diseases, such as keratoconus, can significantly alter the stiffness of the cornea, directly affecting the quality of vision. Ultraviolet-induced collagen cross-linking (CXL) effectively increases corneal stiffness and is applied clinically to treat keratoconus. However, measured corneal stiffness is also influenced by intraocular pressure (IOP). Therefore, experimentally measured changes in corneal stiffness may be attributable to the effects of CXL, changes in IOP, or both. We present a noninvasive measurement method using phase-stabilized swept-source optical coherence elastography to distinguish between CXL and IOP effects on measured corneal stiffness. This method compared the displacement amplitude attenuation of a focused air-pulse-induced ...

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    23. Spatial characterization of corneal biomechanical properties with optical coherence elastography after UV cross-linking

      Spatial characterization of corneal biomechanical properties with optical coherence elastography after UV cross-linking

      Corneal collagen cross-linking (CXL) is a clinical treatment for keratoconus that structurally reinforces degenerating ocular tissue, thereby limiting disease progression. Clinical outcomes would benefit from noninvasive methods to assess tissue material properties in affected individuals. Regional variations in tissue properties were quantified before and after CXL in rabbit eyes using optical coherence elastography (OCE) imaging. Low-amplitude (<1µm) elastic waves were generated using micro air-pulse stimulation and the resulting wave amplitude and speed were measured using phase-stabilized swept-source OCE. OCE imaging following CXL treatment demonstrates increased corneal stiffness through faster elastic wave propagation speeds and lower wave amplitudes.

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    24. Dynamic optical coherence tomography measurements of elastic wave propagation in tissue-mimicking phantoms and mouse cornea in vivo

      Dynamic optical coherence tomography measurements of elastic wave propagation in tissue-mimicking phantoms and mouse cornea in vivo

      We demonstrate the use of phase-stabilized swept-source optical coherence tomography to assess the propagation of low-amplitude (micron-level) waves induced by a focused air-pulse system in tissue-mimicking phantoms, a contact lens, a silicone eye model, and the mouse cornea in vivo. The results show that the wave velocity can be quantified from the analysis of wave propagation, thereby enabling the estimation of the sample elasticity using the model of surface wave propagation for the tissue-mimicking phantoms. This noninvasive, noncontact measurement technique involves low-force methods of tissue excitation that can be potentially used to assess the biomechanical properties of ocular and other ...

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    1-24 of 29 1 2 »
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    Corneal Topography: The Evolving Technology Noncontact measurement of elasticity for the detection of soft-tissue tumors using phase-sensitive optical coherence tomography combined with a focused air-puff system Quantitative Evaluation of Factors Influencing the Repeatability of SD-OCT Thickness Measurements in the Ra Dynamic OCT measurements of corneal biomechanical properties after UV cross-linking in the rabbit Development of a Rat Schematic Eye from in vivo Biometry and the Correction of Lateral Magnification in SD-OCT Imaging Dynamic optical coherence tomography measurements of elastic wave propagation in tissue-mimicking phantoms and mouse cornea in vivo Spatial characterization of corneal biomechanical properties with optical coherence elastography after UV cross-linking Quantitative assessment of corneal viscoelasticity using optical coherence elastography and a modified Rayleigh–Lamb equation Spatial mapping of the biomechanical properties of rabbit cornea after cross-linking using optical coherence elastography Assessing the effects of riboflavin/UV-A crosslinking on porcine corneal mechanical anisotropy with optical coherence elastography Optical coherence tomography findings in patients with transfusion-dependent β-thalassemia Higher-order regression three-dimensional motion-compensation method for real-time optical coherence tomography volumetric imaging of the cornea