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O. Bergamin, M. Piccirelli, P. Boesiger, K. Landau, R. Luechinger; Vitreous Viscoelasticity Revealed by Motion-Encoded MRI. Invest. Ophthalmol. Vis. Sci. 2009;50(13):4817.
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To present a MRI method that determines vitreous deformation and viscoelastic properties in-vivo during eye movement. Prior to this study, the deformation of the vitreous has been simulated ex-vivo as a spherical homogenous fluid rotating around a diameter. Changes in the vitreous viscoelasticity influence the shear stress that may lead to retinal detachment.
Fifteen subjects (6 women, 9 men, age range:22-62y/mean:34y) gazed at a horizontal sinusoidal moving target (2s period, peak velocity 64°/s, amplitude ±20°) for 4min. A microscopy coil at 1.5T (Philips Healthcare, Best, The Netherlands) was placed on one eye to acquire 2D CSPAMM images. The vitreous was overlaid at the 9th of the 15 time phases with a 60x36 mesh centered on the center of rotation and with its outermost polygon on the sclera. The vitreous was tracked using peak-combination HARP with a mesh algorithm. The rotation angle of each concentric polygon forming the mesh was evaluated. The rotation of these concentric polygons in function of time was fitted with the analytic viscoelastic model.
Vitreous deformation over the whole movement range was successfully tracked in 13 volunteers having a monophasic vitreous. For these subjects, vitreous deformation could be fitted by the analytical model. By comparing the phase shift between the innermost and outermost polygons’ rotation, these 13 volunteers split in three groups: phase shift bigger(3), smaller(6) or around(4) 180°. As expected, the group with phase shifts bigger than 180° (corresponding to gel-like vitreous) was the youngest. The difference between the observed deformation of the vitreous and the one expected from ex-vivo measurements of the vitreous viscoelasticity is remarkable, and presumably due to the presence of intact intravitreal membranes. For two subjects, the vitreous deformation was clearly polyphasic: some sectors of the vitreous were gel-like and others liquefied. These sectors were separated by flexible membranes, corresponding to reported intravitreal membrane patterns.
The deformation of the vitreous could be non-invasively determined in-situ, allowing the structure of the intravitreal membranes to remain intact and their effect on the vitreous dynamics to be investigated. The use of HARP and of sequential acquisition allows the determination of vitreous viscoelasticity. The deformation of the vitreous modifies the eye ball inertia. Finally, the determination of vitreous viscoelasticity could help to assess the risk of retinal detachment.
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