Characterization of the viscoelastic properties of in vitro crystalline lens samples using ultrasound elastography

Abstract

With aging, the stiffening of the crystalline lens [K. R. Heys et al., Mol. Vision 10, 956 (2004); R. F. Fisher, J. Physiol. 212(1), 147–180 (1971)] can hinder accommodation and reduce near-vision in more than 75% of individuals above 40 year old [T. R. Fricke et al., Ophthalmology 125(10), 1492–1499 (2018)], an impairment known as presbyopia. Mapping lens elasticity using shear wave elastography holds significant promise for monitoring potential treatments for presbyopia. However, because of the transparency of the lens to ultrasound, the tracking of waves can be performed only on its boundaries. The goal of this study is to characterize the viscoelastic properties of in vitro crystalline lens samples with a curvilinear harmonic method based on noise correlation algorithms. This procedure consists of precise measurements of the dispersion of surface waves across a large frequency range (0.1–3.5 kHz), thus allowing for clear identification of the wave properties needed to correctly estimate the elasticity. The proposed method was applied to gelatin phantoms and excised porcine lens samples. This enabled the observation of two regions in the dispersion curves: a sharp decrease in dispersion at low frequencies (<1 kHz), which was partly due to guided waves, and a smoother slope at high frequencies (>1 kHz), which was attributed to viscoelastic dispersion. In contrast to previous studies, shear elasticity and viscosity moduli were computed at higher frequencies with a Kelvin–Voigt model. If our approach confirms the shear viscosity of lenses, then the shear elastic moduli of lenses are almost an order of magnitude greater than the results of previous studies.