Bubble-based acoustic radiation force for monitoring intraocular lens elasticity

Previously, we demonstrated ultrasonic methods to monitor overall changes in the elastic properties of laser manipulated intraocular lenses. We also presented results using acoustic radiation force on laser-generated bubbles to measure viscoelastic properties of collagen gel phantoms. In the current study, we test the hypothesis that local viscoelastic properties of the intraocular lens can be measured by applying acoustic radiation force to laser-generated bubbles. Presbyopia is an age-related condition resulting from increased stiffness of the lens, reducing its ability to accommodate. A technique to measure local lens viscoelastic properties is needed to better understand the onset of presbyopia and guide potential correction procedures. Laser-induced optical breakdown (LIOB) is used to create bubbles within porcine intraocular lenses. Optical breakdown occurs when sufficiently high threshold fluence is attained at the focus of femtosecond pulsed lasers, inducing plasma formation and bubble generation. The small transient gas bubbles can be used as targets for acoustic radiation force measurements prior to their ultimate collapse. While ultrasonic speckle is extremely limited within the lens, LIOB bubbles provide strong ultrasonic backscatter to measure lens viscoelastic properties. In this investigation, explanted porcine lenses are embedded within a gelatin phantom (5 w/w%) prior to laser treatment. An integrated optical-acoustical system has been constructed enabling simultaneous bubble creation and radiation force experiments. A two-element confocal ultrasonic transducer generates acoustic radiation force with the 1.5 MHz outer element while monitoring the bubble displacement within the lens using the 7.44 MHz inner element. Preliminary experiments have demonstrated the ability to create LIOB bubbles within explanted porcine lenses with lifetimes on the order of a few minutes and at any depth within the lens. Acoustic radiation force experiments with LIOB bubbles in porcine lenses exhibit exponential responses with time constants near 2 ms and maximum displacements on the order of 100 mu m. These results advance the development of an in vivo technique to measure local lens viscoelastic properties.