Journal of Ultrasound in Medicine, Vol 20, Issue 9 979-986, Copyright © 2001 by American Institute of Ultrasound in Medicine
Safety levels for exposure of cornea and lens to very high-frequency ultrasound
R. H. Silverman, F. L. Lizzi, B. G. Ursea, L. Cozzarelli, J. A. Ketterling, C. X. Deng, R. Folberg and D. J. Coleman
Department of Ophthalmology, Weill Medical College of Cornell University, New York, New York 10021, USA.
OBJECTIVE: Very high-frequency (50-MHz) ultrasound is widely used for
imaging the anterior segment of the eye. Our aim was to determine whether
exposures to ultrasound at and above those used in diagnostic imaging
systems might cause bioeffects in ocular tissues. METHODS: We characterized
the output parameters of a polyvinylidene difluoride transducer using a
needle hydrophone. We exposed sites on the cornea or lens of rabbits for up
to 30 minutes 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 observed directly or by slit lamp. Light microscopy showed no
abnormalities attributable to ultrasound exposure. Simulations showed that
even long-term exposures should produce temperature elevations of less than
1 degree C 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 degrees C) temperature rises computed in simulations.
The lack of biological effects is attributable to the small dimensions of
the focal zone, allowing rapid dissipation of heat, and the low total
acoustic power produced by the transducer.
