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V. Kathpalia, H. O. Lloyd, H. Dababneh, M. J. Rondeau, T. Raevski, D. Coleman, R. H. Silverman; Enhancement of High Frequency Ultrasound Imaging of the Uvea Using Intravenous Contrast Agents. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1192.
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The iris, ciliary body and choroid, constituting the uvea, are characterized by a rich microvasculature. Ultrasound has long had a role in visualization and measurement of blood-flow in major vessels, including those supplying the eye, but conventional Doppler instruments are unable to visualize the slow-flow in the tortuous microvasculature of the uvea. Our aim is to develop techniques based on the use of high-frequency ultrasound and ultrasound contrast agents for visualization and characterization of flow in the uvea.
Experiments were performed using a prototype ultrasound system supporting transducers ranging in center frequency from 10 to 40 MHz. The instrument controlled a 3-axis (xyz) linear motion system for scanning. Radiofrequency data were acquired with 12-bit precision at sample rates of up to 400 MHz. We performed in-vitro experiments measuring backscatter from two microbubble contrast agents: Optison (gas-filled albumen microsphere)and Definity (gas-filled lipid microsphere). We performed in-vivo experiments in which contrast agents were injected into the ear vein of a Dutchbelt rabbit while scans of the eye were acquired.
Both agents exhibited increased backscatter in vitro. The backscatter spectrum largely mimicked the shape of the emitted pulse spectrum, rather than enhancement at discrete frequencies. In vivo, Optison was readily visualized within the iris and ciliary processes when scanned at 35 MHz. Definity, however, did not yield significant enhancement in vivo.
The use of digital signal processing allows visualization of blood flow within the uvea when scanned with high-frequency ultrasound. Contrast agents provide a significant increase in signal-strength and help differentiate flow from ‘clutter’, i.e., stationary tissue structures. We are currently investigating the use of tone-bursts as a means for increasing the enhancement provided by the contrast agents.
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