Optical coherence elastography for the measurement of anterior segment biomechanical properties

Optical coherence elastography for the measurement of anterior segment biomechanical properties

The biomechanics of the anterior segment of the eye are known to have an impact on the development and progression of several eye diseases such as keratoconus and may also be involved in eye conditions such as glaucoma and myopia. As such, research into clinical methods to measure such metrics have been at the forefront of eye research with the development of advanced imaging techniques. Several well-established ocular instruments that can measure biomechanical metrics such as corneal hysteresis and corneal resistance, which have been used in clinical settings for many years. However, they do not directly measure the fundamental biomechanical property of elasticity or Young’s modulus of the anterior segment of the eye. As optical coherence tomography is becoming more readily available it is being combined with other techniques to enhance its capabilities. One such approach is the development of optical coherence elastography (OCE), a technique that allows direct measurement of the tissue mechanical properties, such as Young’s modulus. As this method is still in its infancy many clinical optical coherence tomography instruments do not provide the necessary parameters to use standard elastography techniques. Since this could provide novel and important information, this research aims to design an optical coherence elastography method that can measure the elasticity of materials from contact lenses to corneal tissue through hardware and software modifications to a clinically available optical coherence tomography device.
In the first experiment, a novel method of optical coherence elastography was developed. The method used soft contact lenses as a surrogate to biological tissue. They were chosen since contact lenses exhibit similar biomechanical properties to ocular tissues, and because these materials are homogenous and anisotropic. The mechanical properties of the contact lenses were measured by sequential images captured by the optical coherence tomography instrument under a static load applied by a glass plate. Throughout the rigorous testing of the method and the results generated; it was shown that, the elasticity of several types of soft contact lenses were able to be measured accurately and precisely when compared to industry values.
In the second experiment, the methods from experiment 1 were used to measure ex-vivo corneal tissue. A significant improvement from experiment 1 was to apply a pre-load in the first image to remove any surface irregularity and create more consistent boundaries to be measured. Porcine corneal tissue was used in place of human tissue, due to the limited availability of ex-vivo human corneal tissue. This experiment was the first static compression OCE study to investigate Young’s modulus of corneal tissue. The results showed that they were comparable by previously published values of Young’s modulus. The experiment showed that the amount of fluid within the tissue (estimated using the tissue thickness), changed between each trial on the same corneas, and was associated with changes in the measured Young’s modulus. However, the method was shown to produce the same results on average between different corneas, suggesting that similar changes in tissue fluid were occurring between each trial for each of the corneas.
In the third and final experiment, the novel method of OCE was adapted to remove the need to use an electronic force sensor to measure the stress the soft contact lens was subjected to during loading. As stress is still required to determine the elasticity, the method of optical palpation was incorporated into the method. Optical palpation uses a compliant material with known mechanical properties to act as a stress sensor to determine the stress that the material under investigation is subject to during loading. This is done by placing the stress sensor between the material and the compression plate. As such both the sensor and the material are subject to the same stress and thus can be mapped. There are several assumptions with this method. This experiment was conducted using two soft contact lenses on top of each other such that one was the stress sensor, and one was the material under investigation. The results from this experiment showed that with the use of optical palpation and a pre-load, the accuracy of the method for determining the Young’s modulus of soft contact lenses improved.
This program of research has developed a method of OCE using a clinical OCT, incorporating minor hardware modifications and the use of a semi-automatic intensity boundary segmentation. With the addition of optical palpation, the potential for clinical translation is possible. The method was tested on both off the shelf materials of known mechanical properties and animal corneal tissue, with good repeatability and agreement with previous studies. The research provides a platform for the improvement and implementation of the methods into a clinical setting.