Cavitation threshold in the porcine crystalline lens.

Abstract

Objective: To evaluate ultrasonic cavitation thresholds in young (6 months) and old (2-6 years) porcine crystalline lenses using a 2 MHz transducer.

Methods: For cavitation threshold estimation, five to nine samples were exposed to bursts of different peak negative pressures from 20.6 to 36.1 MPa with duty cycles (DCs) of 0.025%, 0.05%, 0.1% and subsequently 0.000006% at maximum output. Cavitation signals were recorded and their energies were calculated. The cavitation events percentage was estimated through the detection of energy values outside 5 standard deviations from the mean value.

Results: Cavitation thresholds were estimated at -31.7 MPa for a 0.1% DC but severe cataracts were observed. In order to determine a therapeutic application, a more suitable ultrasound scheme using an ultra-low DC (0.6e^-5%, 12 cycles at 1 Hz PRF) was used and a cavitation threshold was determined at -36.1 MPa. In that case, no cataract was observed on a macroscopic scale. Nevertheless, important variations in cavitation thresholds between samples and the two tested populations were observed, most likely due to variations in viscoelastic properties, water content distribution, temperature increases and age disparity.

Conclusion: The results show a trend toward higher cavitation probability while increasing the negative pressure at that focus. However, no significant differences in cavitation events percentage could be observed due to the limited sample size. Ultra-low DC results show the feasibility of cavitation nucleation while reducing heat deposition. The safety aspect needs to be explored further.