Abstract
Abstract: :
Purpose: The impact of biomechanical properties of corneas on intraocular pressure measurement is not well understood. These properties may be altered by disease or refractive surgery. In vivo determination of cornea biomechanics has been technologically challenging. We propose a non–invasive ultrasound method that integrates a mathematical model for quantitative determination of biomechanical properties of ocular tissue. Our goal in this study is to conduct feasibility tests on the preliminary model and system for charactering cornea biomechanics using high–frequency ultrasound. Methods: A mathematical model of elastic wave propagation was constructed to simulate ultrasound wave propagation in an eye immersed in water bath. Mechanically, the system was composed of two continuous subspaces (water bath and aqueous humor) and a thin layer (cornea) in between. The reflection coefficient from the thin cornea layer was mathematically solved by enforcing the boundary conditions at the interfaces between tissue and substrates. The biomechanical properties of cornea were then varied to study their effects on the reflection spectrum, which was obtained from the reflection coefficients at a range of frequencies. Preliminary experimental verification was performed on phantom samples made from contact lenses that were embedded in 2% agarose gel, or directly suspended in water. A broadband ultrasound transducer (20 MHz, V316, Panametrics) was used. The reflected signals were recorded by a 2G/8–bit digitizer, and displayed by LabView interface on computer. Results: We simulated reflection spectra from cornea thin layers and varied the magnitudes of Lame’s constants (λ and µ), as well as density and thickness to observe their effects on the spectral changes. Our computer simulation showed that each corneal variable affected the spectral characteristics (i.e., positions and magnitudes of local minima and maxima) if varied. Reflection spectra from the phantom samples were obtained. Conclusions: Our model of ultrasound propagation has demonstrated that the characteristics of the reflection spectra were dependent on the magnitudes of mechanical properties of cornea. Our initial experimental study suggested that there were detectable ultrasonic reflections from the phantom samples, and the reflection spectra had similar characteristics as theoretical prediction. We concluded that the ultrasound model and system were feasible for in vivo non–invasive determination of cornea biomechanical properties. Further theoretical analysis may be needed to optimize parameter sensitivity and de–coupling.
Keywords: cornea: basic science • intraocular pressure • refractive surgery