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M. Piccirelli, R. Luechinger, M. Soellinger, A. Rutz, P. Boesiger, O. Bergamin; New MRI Methods for Dynamic Imaging of the Movement of the Extraocular Muscles and the Orbital Fat . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5065.
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© ARVO (1962-2015); The Authors (2016-present)
For clinical use, ocular misalignment is mainly investigated by fixation and subjective tasks and additionally evaluated with static MRI or CT. In more complex cases however, a better understanding of the dynamics of the eye movement is needed. New scanning methods provide dynamic, high–resolution anatomical and motion encoded (CSPAMM) MRI of the orbit.
For reproducible and accurate eye alignment, subjects gazed at a horizontal sinusoidally moving target (peak velocity 63°/sec, amplitude ±20°), presented with a typical fMRI setup. A microscopy coil (47mm diameter) at 1.5T was used to acquire dynamic 3D–TFE images (15 slices, 256x256 scan–matrix, 18 time frames, resolution 0.4x0.4x1.4mm3, duration 27min). k–t BLAST encoding with k–t acceleration factors of up to 9 were evaluated to reduce the scan time down to 3.3min. 2D CSPAMM images (256x151 scan–matrix, in 6min 21 time frames of 70ms, 0.3x0.3x4mm3, tag–line distance 2mm) were acquired with the same setup. Data were post–processed to get displacement maps.
The dynamic images without using k–t BLAST were moderately blurred due to movement compared to the static images. k–t BLAST did not further degrade the sharpness of the slower moving muscles. Only fast moving tissue like the optic nerve and the lens of the eye were further blurred. These images allowed a reasonably good visualization and segmentation of the extraocular eye muscles with an 8–fold scantime reduction compared to equivalent static images. The CSPAMM motion encoded images provided information of the movement also within homogeneous regions like the muscle and the orbital fat. The data revealed the region of greatest contraction along the muscles. The movement of the orbital fat was also clearly depicted.
k–t BLAST offers a unique possibility for high–resolution fast dynamic 3D images of the orbit with acceptable quality for segmentation of all extraocular muscles. However, it does not show movement within homogenous tissues. CSPAMM allows visualization of the muscles contractions, which are essential for modeling extraocular muscle movement. The combination of both methods will improve the understanding of the dynamics of the eye movement.
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