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M. J. Rondeau, R. H. Silverman, H. O. Lloyd, D. J. Coleman; Swept-Mode Ultrasound Velocity Measurement of the Ciliary Vasculature. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1220.
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This study assesses the usefulness of Swept-Mode Ultrasound as technique for measuring the slow flow microvasculature of the ciliary body. Swept-mode is a non-Doppler technique that provides a low bias, high accuracy measurement of the slow blood flow seen in vessels down to approximately 35 microns in diameter using a 40 MHz transducer. The technique is, however, susceptible to tissue motion on the order of typical non-voluntary eye movements. We sought to assess if particular acquisition and processing strategies could be used to obtain clinical relevant flow data in humans.
We implemented Swept-Mode data acquisition on our custom high-speed digital scanners. Acquisition parameters such as interline distance, scan angle or number of lines and number of digitized points were varied. Optimal processing parameters were sought. Both narrow and broad-band frequency estimators were evaluated along with a number of different wall-filtering and post-processing noise filtering techniques.
Using a combination of wide angle anatomic (b-scan) localization and short angle swept-mode acquisition with fellow-eye fixation reasonably stable swept-mode scans could be acquired. Special motion rejection algorithms with empirically tuned clutter filtering significantly improved signal-to-noise ratio and detection of flow.
Swept-Mode Ultrasound can be used to measure microvascular flow in the ciliary body in the clinical setting. The technique should allow for better understanding of vascular changes associated with glaucoma as well provide a means for monitoring the ciliary body vascular effects of anti-ocular hypertensive pharmacological therapy, as well as surgical therapy.
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