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G. I. Sun, R. V. P. Chan, M. J. Rondeau, H. O. Lloyd, R. H. Silverman, D. J. Coleman; Swept-Scan Flow Measurements of the Microvasculature of the Retina and Choroid in Age Related Macular Degeneration and Other Chorioretinal Disease States. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4242.
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Currently, there are no commercially available systems for examining slow flow in the complex and tortuous vascular beds of the retina and choroid in humans. Very high frequency ultrasound Doppler of the eye’s anterior circulation has been demonstrated. In this study we evaluate a novel technique using a swept-mode color flow system that is sensitive to flow of < 1mm/s in vessels on the order of 100 µm in size, in patients with various stages of age related macular degeneration (AMD) and other chorioretinal disease states. The swept mode technique involves use of a short, broadband ultrasound pulse, so that axial resolution is maintained, as is not the case in Doppler methods. By closely spacing successive vectors with respect to the beamwidth, speckle associated with blood flow can be visualized and measured.
High frequency ultrasound of the macula was performed in 11 patients with age related macular degeneration, central serous retinopathy (CSR), and other chorioretinal disorders. Patients were scanned in supine position using the water-bath technique and avoiding the crystalline lens. An 8-bit linear scanning system with 250 MHz digitization of radiofrequency echo data was used. The swept scan parameters were transducer center frequency: 22MHz, PRF: 750 Hz, distance between vectors: 2 µm. Image clutter was reduced using various techniques such as FIR filters, eigenfilters, and wavelet-based filters. Several velocity estimation techniques were also employed. The fractional area of the choroid in which perfusion was evident was determined.
Flow velocities of up to 30 mm/s were observed in the choroid, with average velocity approximately 20 mm/s. The fractional area of the choroid with visible perfusion ranged up to 40% and averaged approximately 20%. Reduced flow was seen in late stage wet AMD patients and in patients with CSR. In patients with wet AMD there was a trend in which there were localized areas of increased flow but overall had reduced flow as compared to patients with dry AMD. We also observed localized areas of increased flow around drusen in both wet and dry AMD patients.
Swept-mode color flow is a promising technique for evaluating flow in vascular beds of patients with vision threatening disease and may provide a means of assessing drug-induced changes in flow and a means of evaluation of the pathophysiology of these disease states. With recent advances in ultrasound imaging and image processing techniques, improved detail and spatial resolution will be possible.
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