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


CONCLUSIONS
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.
Silverman
R H
RH
Department of Ophthalmology, Weill Medical College of Cornell University, New York, New York 10021, USA.
Lizzi
F L
FL
Ursea
B G
BG
Cozzarelli
L
L
Ketterling
J A
JA
Deng
C X
CX
Folberg
R
R
Coleman
D J
DJ
D000818:Animals; D003198:Computer Simulation; D003315:Cornea / Q000000981:diagnostic imaging* / Q000473:pathology; D006801:Humans; D007904:Lens Cortex, Crystalline / Q000000981:diagnostic imaging* / Q000473:pathology; D023421:Models, Animal; D009799:Ocular Physiological Phenomena; D011817:Rabbits; D014463:Ultrasonography / Q000009:adverse effects / Q000379:methods / Q000592:standards*