Ultrasound Characterization on Cornea Phantoms

Abstract

Abstract: : Purpose: Biomechanical properties of corneas may be a significant confounding factor in intraocular pressure measurement by tonometry methods. These properties may also be indicative of ocular diseases or altered by refractive surgery. Previously we reported a preliminary model and system for in vivo determination of corneal biomechanical properties. In this study, we tested the sensitivity of this approach in differentiating the mechanical properties of cornea phantom samples made from different materials. Methods: Contact lenses were used as cornea phantoms and measured by our ultrasonic system. The position of the contact lenses was controlled by XYZ linear stages and two goniometers to ensure that the ultrasonic beam was centered at the apex. Ultrasound waves were applied by a broadband ultrasound transducer (20 MHz, V316, Panametrics), driven by a pulser/receiver (5900PR, Panametrics). All reflected signals were recorded by a 500 MHz/8–bit digitizer (Acqiris, DP105), and displayed and processed by a PC. A reference signal was recorded by replacing the contact lens by a piece of flat plexiglass. Normalized power spectra were calculated from the measured reflections using Fast Fourier Transformation. A data processing software was constructed to generate estimation on each of the four physical properties of the contact lens: density, thickness, Lame’s constant and shear modulus µ, by minimizing the discrepancy between the experimental spectra and the theoretical predications based on the wave propagation model we previously developed. Results: The normalized spectra from different contact lenses had different minima and maxima that were indicative of their distinct physical properties. It appeared that the maxima on the spectra from the harder lenses had higher magnitudes in comparison to those from the softer lenses. The reconstructed properties for the lenses were different for different categories of lenses; particularly, the harder lenses had higher values of reconstructed shear modulus. Conclusions: Our studies on cornea phantoms suggested that the ultrasonic system was sensitive in detecting the differences in the physical properties of the cornea–like structures. Further theoretical analysis is needed to optimize the sensitivity and accuracy of this method.

Keywords: cornea: basic science • contact lens