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M.S. Patel, O. Gal, D.J. Coleman, H.O. Lloyd, J. Cannata, K.K. Shung, R.H. Silverman; 70 MHz Ultrasound Imaging of the Anterior Segment . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3299.
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© ARVO (1962-2015); The Authors (2016-present)
Resolution is a function of ultrasound frequency and pulse–length. Transducers in the 35–50 MHz range have been used for over a decade for anterior segment imaging. In this study we describe findings involving use of a broadband 70MHz transducer for evaluation of the cornea and anterior segment.
We compared a 35 MHz lithium niobate transducer (6 mm aperture by 12 mm focal length) with a 70 MHz lithium niobate transducer (2.5 mm aperture by 5 mm focal length). Transducer characteristics were first determined by acquiring the reflection from an optical flat placed in the focal plane. Immersion scanning of the cornea, iris and ciliary body of a Dutchbelt rabbit in vivo and a human subject six years post–LASIK was performed. Radiofrequency (RF) data were digitized at a sample rate of 400 MHz or higher. Spectral parameter (midband–fit) imaging was used to improve signal to noise ratio and to reduce speckle.
The 70 MHz transducer allowed clear separation of echoes from the anterior epithelium and the epithelial/stromal interface, a thickness of approximately 50 microns. The 35 MHz transducer resolved these interfaces, but far less clearly. Also, at 70 MHz, the amplitude of the epithelial/stromal interface was relatively larger compared to that of the anterior epithelium. The RF trace of the 70 MHz corneal scan showed two distinct echo complexes at the anterior surface; the first representing the anterior epithelial surface and the second representing the epithelial–stromal interface. In comparison, the RF trace of the 35 MHz corneal scan showed only partial separation of these interfaces. The LASIK interface was faintly seen with both the 70 MHz and the 35 MHz transducer. 70 MHz data also provided greater backscatter from the stroma. 70 MHz images of the angle, iris and ciliary body of a human subject showed improved resolution. Scans with focus on the ciliary body obtained in the human subject in a resting state and at various degrees of accommodative effort showed ciliary muscle conformation.
Because attenuation by tissue and the coupling medium increases exponentially with frequency, there are practical limitations on how high we can go in frequency (and hence resolution) for clinical studies. This study demonstrates that a 70 MHz transducer with a 5 mm focal length can provide images of both cornea and ciliary body in rabbits and human subjects in vivo. The improved resolution obtained with this system will enable more detailed studies of the cornea and ciliary body in health and disease.
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