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M. A. R. Siddiqui, A. C. Fairhead, H. R. Atta; Ultrasound Velocity in Heavier Ocular Tamponade Media: Implications for Biometry. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2236.
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Heavy Silicone Oil (HSO) is shown to be an effective tamponade in proliferative vitreoretinopathies and inferior retinal detachments. Intravitreal HSO causes lens opacification. HSO removal and cataract surgery may be performed at the same time if a precise biometry can be obtained. Ultrasound (US) provides a reliable means to measure the axial length of eyes, particularly in dense cataracts and in conditions where the architecture of retina may have been altered. The velocity of sound varies with the type of silicone oil (SO) and is also dependant on temperature.The aim of this study was to evaluate the velocity of US in various ocular tamponade agents and to study the influence of varying temperature of these agents on US velocity.
Five tamponade agents ( SO 1000, SO 5000, Oxane Hd, Densiron 68, and F-Decalin) were studied in vitro. The temperature of each agent was changed under controlled conditions and the velocity of sound was calculated by the echo time measured from a reflecting target fixed at 1cm from an A-scan probe. The probe emitted 0.5µs pulses, with a centre frequency of 5MHz.
The velocity of sound at 37ºC (body temperature) for SO 1000, SO 5000, Oxane Hd, Densiron 68, and F-Decalin was calculated as 994, 975, 940, 931 and 653 ms-1 respectively. With 95% confidence, the true speed is believed to lie within ± 5ms-1 of these values. . The speeds varied with temperature by between -2.2 and -3.6ms-1C-1, depending on the particular oil. They are not expected to vary with ultrasound frequency.
Variability in the velocity of US should be taken into account when performing biometry. Biometry machines should be adjusted for various tamponade media when calculating intra-ocular lens power.
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