2007_abstracts - ABSTRACTS ULTRASONIC IMAGING AND TISSUE...

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ABSTRACTS 1. High-Frequency Imaging 1.1 High-reso lution paramet ric imaging of the vitreous, retina and choroid, Ron ald H. Silverman, 1, 2 Mark J. Ron deau, 1 R. V. Paul Chan 1 and D. Jack son Coleman, 1 1 Weill Med i cal College of Cor nell Uni versity, New York, NY 10021 and 2 Riv er side Re search In sti tute, New York, NY 10038, s [email protected] (in vited over view). The retina is physically and nutritionally supported by the underlying choroid, a vascular tissue that is part of the uveal tract of the eye. The retina and choroid are each approximately one-quarter millimeter in thickness. The retinal photoreceptors are one of the most metabol- ically active sites in the human body and thus require a rich microvasculature. The photoreceptors and outer layers of the retina are oxygenated by diffusion from the underly- ing choroid and the inner retina by the retinal vessels. Diseases of the retina and choroid such as small melanomas, diabetic retinopathy (DR) and age-related macular degeneration (ARMD) may be associated with vascular abnormalities including ischemia, neovascular- ization, vessel leakage and hemorrhage. This, and vitreoretinal traction, may result in blood and cellular debris entering the vitreous, with potential for subsequent organization of vitreous membranes. Ultrasound (US) and optical coherence tomography (OCT) are useful and complemen- tary techniques for evaluation of the retina, choroid and vitreous. OCT provides higher reso- lution (<10 m m) than US, but is limited to imaging only the posterior pole (through the pupil) under clear media conditions. It also has limited penetration in depth beyond the retinal pig- ment epithelium. US can be used to image the central and peripheral retina, choroid and or- bital tissues, and is unaffected by optical opacities. The traditional 10 MHz US used in ophthal mology, however, provides inade quate resolution for eval uation of the retina and choroid because of their thinness in respect to wavelength (150 m m). Detection of early changes associated with ARMD, such as retinochoroidal perfusion, re- organization of microvasculature, anatomic changes in the macula (tears, holes, epiretinal membranes) and vitreous debris, would be clinically useful, especially in light of new and effective treatment modalities. The central challenge for US in characterizing these tissues is their thinness and depth. (The axial length of the eye is about 24 mm.) We have been ex- ploring a number of approaches based on postprocessing of radiofrequency data to improve US detection of these changes. Scanning is typically performed using a reverse arc (sector) scan geometry, avoiding the plane of the crystalline lens, using a focused single-element 20 MHz transducer. Postprocessing methods include spectral parameter imaging, nonlinear 20/40 MHz tissue harmonic imaging, wavelet analysis and swept-mode color-flow imag- ing. We have applied these methods in combination with OCT to visualize the vitreous, retina and choroid and tissue changes associated with disease, including retinochoroidal
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