Safety levels for exposure of cornea and lens to very high-frequency ultrasound

Objective. Very high-frequency (50-MHz) ultrasound Is widely used for Imagi ng the anterior segment of the eye. Our aim was to determine whether exposu res to ultrasound at and above those used in diagnostic imaging systems mig ht cause bioeffects in ocular tissues. Methods. We characterized the output parameters of a polyvinylidene difluoride transducer using a needle hydrop hone. We exposed sites on the cornea or lens of rabbits for up to 30 minute s at a 10-kHz pulse repetition frequency. Tissue obtained immediately or 24 hours after exposure was examined by light microscopy. A numeric model was implemented to calculate expected temperature elevations In the cornea and lens under experimental conditions. Results. No tissue changes were observ ed directly or by slit lamp. Light microscopy showed no abnormalities attri butable to ultrasound exposure. Simulations showed that even long-term expo sures should produce temperature elevations of less than 1 degreesC in both the cornea and lens. Conclusion. With the use of exposure parameters 4 to 5 orders of magnitude greater than encountered in a clinical situation, no tissue changes were observed. This is consistent with the small (0.2 degree sC) temperature rises computed in simulations. The lack of biological effec ts is attributable to the small dimensions of the focal zone, allowing rapi d dissipation of heat, and the low total acoustic power produced by the tra nsducer.