Spatially mapping the elastic properties of the lens using bubble-based acoustic radiation force

Abstract

Bubble-based acoustic radiation force has been used to measure highly localized tissue viscoelastic properties. In the current study, we use acoustic radiation force applied to laser- induced bubbles to measure the spatial distribution of elastic properties in explanted porcine lenses. A potential in vivo technique to map lens elasticity is crucial to understanding the onset of presbyopia and guiding surgical procedures to restore accommodation. Current techniques to measure lens stiffness are primarily ex vivo procedures requiring lens sectioning to achieve spatial localization. Bubble-based acoustic radiation force can measure the spatial elasticity distribution without disrupting the lens capsule or significantly altering intervening tissue. Microbubbles were generated using laser-induced optical breakdown (LIOB) laterally along the equator of explanted porcine lenses with 1 mm spacing. Optical breakdown occurs when sufficiently high threshold fluence is attained at the focus of femtosecond pulsed lasers, inducing plasma formation and bubble generation. 18 uJ laser pulses from a 800 fs Nd:glass laser generated microbubbles. Optical microscopy provided feedback on bubble size, shape, and lifetime. A two-element confocal ultrasonic transducer applied 6.7 ms acoustic radiation force- tone bursts with the 1.5 MHz outer element while monitoring bubble displacement within the lens using the 7.44 MHz inner element. Bubble displacements found in the lens nucleus were larger than those in the lens cortex, suggesting that the nucleus is stiffer than the cortex. Bubble lifetime, the duration from creation until collapse, also correlated with local lens stiffness. LIOB bubbles not exposed to acoustic radiation force excitation demonstrate much longer lifetimes in the lens nucleus in the lens cortex. This result is consistent with previous findings that longer bubble lifetimes occur in stiffer materials. Both bubble displacements and lifetimes are symmetric about the lens center, as expected, and predict that the porcine lens nucleus is several times stiffer than the lens cortex. Bubble-based acoustic radiation force appears well-suited as a potential in vivo technique to spatially map the elastic properties of the lens.