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T. Manor, N. Patel, R. Margolis, R.H. Silverman, F.L. Lizzi, D.J. Coleman; In vivo and in vitro Cataract Thresholds in the Lens during 7.8 MHz Ultrasound Exposure . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2907.
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Purpose:Regulatory bodies such as the U.S. Food and Drug Adminitration have long–standing standards for diagnostic ultrasound exposure levels. Color flow and continuous wave Doppler instrumentation can readily exceed these thresholds in ocular examinations. Our aim was to study the formation of cataracts in lenses exposed to ultrasound at a frequency of 7.8 MHz, similar to that in some Doppler instruments. Methods:We used a spherically focused ultrasound transducer with an F–ratio of 1.1 and operated at its 5th harmonic, 7.8 MHz. After calibration to determine the relationship between excitation voltage and radiation force, we exposed rabbit eyes in vivo to various power levels. Lenses were observed during exposure and histology obtained subsequently. We also exposed cow lenses in vitro in a temperature regulated waterbath, with implanted thermocouples used to measure temperature levels during exposure and a fibre optic endoscope used to record cataract formation. Results:In vivo, cataracts always formed at a power level of 1900 W/cm2 (mean time = 28 sec), with cataracts forming in about half of cases at 1200 W/cm2(mean time >3 minutes). In vitro, we found a rapid temperature rise (time constant = 7 seconds) in the focal zone. Cataracts appear to have formed at temperatures of 45oC or higher. In vitro cataracts tended to require somewhat higher intensities than in vivo (minimum 1900 W/cm2), but shorter exposure durations. Conclusions:Current standards for ultrasound exposure are more stringent for ophthalmology than any other clinical specialty. The primary concern is thermally mediated cataract formation due to the high attenuation coefficient of the lens and its lack of a blood supply. Our results suggest a wide safety margin of on the order of a factor of 50 with current standards. As reduced exposure levels involve some reduction in diagnostic capability, our findings suggest that less stringent standards would be of benefit for ophthalmic ultrasound, including Doppler.
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