Optical coherence elastography (OCE) is an imaging technique for measuring elastic properties based on optical coherence tomography (OCT). Benefitting from the high-resolution imaging and sensitive motion detection of OCT, OCE has been used to assess the elasticity distribution of ocular tissue, skin tissue, etc. The biological tissues consist of multiple layers with various biomechanical properties and, thus, show directionally dependent elasticity. In previous OCE measurements, the tissues were regarded as isotropic materials, and the elasticity was measured in one direction, which may cause the bias of elasticity assessment. In this study, we developed the OCE method for directionally dependent elasticity assessment. Acoustic radiation force (ARF) generated by a ring ultrasonic transducer was applied to induce vibration in an axial direction and an elastic wave propagating in the lateral direction. The OCT beam, parallel to the ARF, scanned the sample through the central hole of the ring ultrasonic transducer. Then, phase-resolved OCT analysis was used to detect the vibration and visualize wave propagation. The vibration amplitude depends on the axial elasticity, and the lateral elasticity determines the elastic wave velocity. Therefore, the measurements of the vibration amplitude and the wave velocity using ARF-OCE can assess the elasticity of the sample in orthogonal directions. The results from the phantom measurements show that the OCE method can reliably evaluate the directionally dependent elasticity for the anisotropic samples.
Pathological changes in ocular tissues can cause differences in tissue elasticity. Therefore, tissue elasticity is a valuable indicator of early ocular diseases. To measure the elasticity of ocular tissues, we proposed a whole-eye optical coherence elastography (OCE) method. To image the whole eye, an electrically tunable lens (ETL) was integrated into the system to adjust the focal plane. For inducing elastic waves in an entire eye, the acoustic radiation force (ARF) from one transducer stimulated the anterior segment from the side of the eye, and another transducer excited the retina from the front of the eye. The elastic waves in the porcine cornea and retina were visualized by Doppler phase shift analysis. Then, we measured the elastic wave propagation velocities and calculated Young's moduli of the tissues. In the ex-vivo experiments of porcine eyes, the elastic moduli of the cornea and retina were quantified, respectively. The experimental results demonstrate the feasibility and potential clinical value of the proposed method for assessing tissue elasticity in ocular structures.
Optical coherence elastography (OCE) can quantify the tissue elasticity by measuring the velocities of elastic wave propagation in the tissue. Due to the high sensitivity and micron-level resolution, OCE is especially suitable for biomechanical property measurements of the ocular tissues. Usually, the external excited elastic wave is visualized by optical coherence tomography (OCT). However, the imaging depth of the OCE system is limited by the OCT system and the excitation depth of external force. In this study, we proposed a method extending the OCE imaging depth with an electrically tunable lens (ETL). The method was validated by detecting the propagation of elastic waves in the corneas and retinas of porcine eyes using an acoustic radiation force-based OCE system. Firstly, an acoustic simulation was taken for the ring ultrasound transducer. Secondly, a mathematical model of the ETL was established for dynamic control of the imaging depth. Thirdly, the optical simulation of the sample arm was performed to analyze the critical optical parameters and evaluating the imaging quality of the system. Also, the optimal working depth of the OCT system was discussed. Lastly, an OCE system with a ring ultrasound transducer and an ETL was built. The experimental results on ex vivo porcine eyes showed the imaging depth of the system was 22 mm. This method can extend the depth of elasticity detection and, thus, provides a powerful tool for non-invasive, high-resolution biomechanical analysis of the ocular tissues.
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