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.
Optical coherence tomography (OCT) is a non-invasive, label-free imaging technique with high resolution. Due to the relatively low scan rate of OCT and involuntary bulk motion of tissues, the OCT image will be distorted by the motion artifacts. The motion artifacts can be reduced by hardware and software methods. In hardware methods, additional hardware is used to track the motion of the object, and extra scans may be required during data acquisition. The software methods can simplify the device and the data acquisition. However, the motion correction based on the cross-correlation analysis is time-consuming. In this study, we proposed a fast motion correction method for OCT images based on image feature matching. First, the motion-related mismatch in the slow scan direction was compensated by the image feature matching between the adjacent B-scans based on the oriented FAST and rotated BRIEF (ORB) approach. Then the axial motion in A-scans was corrected by the boundary detection of the tissue structure and the non-rigid transformation between the corresponding A-lines in the adjacent B-scans. The fast motion correction method was validated by the OCT imaging of a rat ear. The results show that the method can effectively correct motion artifacts of OCT images with a fast processing speed.
Optical coherence tomography (OCT) enables high-resolution, label-free two-dimensional cross-sectional and three-dimensional volumetric imaging of biological tissues. Combining OCT imaging with external force excitation, optical coherence elastography (OCE) provides noninvasive elasticity quantification of samples for the pathological analysis of tissues and early diagnosis of diseases. However, the OCE system with a fixed OCT sample arm cannot be used for elasticity measurements of tissues located in a narrow space, such as an oral cavity and an ear canal, because the OCT beam and external force cannot easily reach the tissues. In this study, we developed a handheld OCE method for the elasticity measurements based on elastic wave imaging. The handheld probe integrated an air pulse excitation unit and a microelectromechanical system-based scan imaging unit. A short air pulse induced the elastic wave in a sample. Then the OCT data was captured by an M-B scan protocol, and the tissue vibration was analyzed by Doppler phase shifts. After elastic wave visualization, the elastic wave velocity was calculated for the elasticity quantification of the sample. The results show that the handheld OCE method can induce and image the elastic wave and, thus, quantify the elastic modulus with high flexibility for the tissue in a narrow, deep space.
The elastic properties can be an indicator of pathological changes of biological tissue. Acoustic radiation force optical coherence elastography (ARF-OCE) allows remote, non-invasive assessment of the elastic properties of tissue. In this study, we proposed an acousto-optic coupling ARF-OCE method for the elasticity measurements. The acousto-optic coupling unit employs a rectangular prism with a close refractive index but significantly different acoustic impedance compared to water. Therefore, the surface of the rectangular prism immersed in water can reflect the ultrasound beam while transmitting the optical coherence tomography (OCT) detection beam. We demonstrated the acousto-optic coupling ARFOCE method using agar phantoms. The results show that the ARF-OCE method can induce elastic vibrations in the direction parallel to the OCT beam, resulting in higher detection sensitivity and a larger scanning range.
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