Young's modulus estimation of bovine lens ex-vivo using a laser-induced microbubble under impulsive acoustic radiation force

Dept. of Mech. Eng., Univ. of Texas at Austin, 204 E. Dean Keeton St., Austin, TX 78712, .
The Journal of the Acoustical Society of America (Impact Factor: 1.5). 10/2011; 130(4):2423. DOI: 10.1121/1.3654707
Source: PubMed


According to the most widely accepted theory of presbyopia, the age-related loss of accommodation is attributed to Young's modulus changes in the lens. Previously, we have developed an approach to measure the mechanical properties of viscoelastic medium using microbubbles and acoustic radiation force. In this study, we tested this technique to assess the mechanical properties of bovine lenses ex-vivo. An impulsive acoustic radiation force was applied to laser-induced microbubbles created with nanosecond laser pulses at different locations in the lens. An acoustic radiation force with typical duration less than 150 μs was generated by a 3.5 MHz transducer. For accurate spatio-temporal measurements of the microbubble's displacement, a custom-made ultrasound system consisting of two 25 MHz transducers was built. The first transducer emitted a train of pulses and the other transducer received the train of echoes reflected from the microbubble. The developed system was operating at 400 kHz pulse repetition frequency. The results show good agreement between experimental measurements and the theoretical model of microbubble dynamics. Evaluation of the spatial distribution of elasticity demonstrates that the Young's modulus of the nucleus is higher than that of the cortex for bovine eye lenses of mature animals. [Work supported by NIH Grant EY018081.].

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    ABSTRACT: Using a microbubble-based acoustic radiation force approach, spatial variations of Young's modulus and shear viscosity of the porcine vitreous humors in two groups-young pigs (6 months old) and mature pigs (2 to 3 years old)-were measured in situ. The measurements in these groups (4 specimens in each group) were performed in several positions along an anterior-to-posterior direction. At each position, microbubbles were generated by focusing a nanosecond pulsed laser beam and the displacement of each microbubble in response to an impulsive acoustic radiation force was measured every 10 μs using a custom-made high-pulse-repetition-frequency ultrasound system. Based on measured dynamics of the microbubble, Young's modulus and shear viscosity at various locations of the vitreous were reconstructed. Young's moduli of the young and mature porcine vitreous at anterior region were the highest, whereas the central region had the lowest values, indicating the clear spatial variations in the vitreous humor elasticity in both groups.
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