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H. O. Lloyd, R. H. Silverman, D. Z. Reinstein, D. J. Coleman; Acoustic Radiation Force Impulse (ARFI) Imaging for Characterization of Corneal Viscoelastic Properties. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1752.
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The cornea maintains its shape as an equilibrium condition balancing its elastic properties against the evenly distributed force of aqueous fluid pressure. Pressure exerted against the corneal surface will result in corneal flattening and opportunity to observe the stress/strain association as the tissue deforms and recovers. External force can be generated by an air pressure pulse, as in the Ocular Response Analyzer (ORA) or by acoustic radiation. Displacement data allows measurement of oscillatory frequency (related to stiffness) and the time-constant of the damping function (related to viscosity). Our aim was to determine the potential of acoustic radiation force impulse (ARFI) imaging for characterizing corneal viscoelastic properties.
We measured surface displacement following exposure of ex vivo pig corneas to acoustic radiation force using a two-element annular transducer, with an 18-MHz outer ring and a 36-MHz inner element, sharing a common focus. We focused the probe on the surface of the cornea and recorded M-scans with the 36-MHz element at a pulse repetition frequency of 1-KHz. After 100-msec, we excited the outer element with an 18-MHz toneburst, varying amplitude and duration between trials. We then recorded M-scans for an additional 0.5-sec and measured surface displacement as a function of time. Acoustic power was measured using a calibrated needle hydrophone.
Displacements occurring following the application of acoustic radiation force exhibited a damped oscillatory response with a resonance frequency of 20-Hz and damping coefficient of 40-msec. We calculated the mechanical index (MI), the derated spatial peak pulse average (ISPPA.3) and spatial peak temporal average (ISPTA.3) intensities for a 5-msec toneburst repeated at a 1-Hz pulse repetition frequency. Results were MI = 0.062, ISPPA.3 = 1.76 mW/cm2 and ISPTA.3 = 9.0 W/cm2), which are within FDA standards for ophthalmic safety for diagnostic ultrasound.
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