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A neuroscience research center with very high field MRI equipments has been opened in November 2006 by the CEA life science division. One of the imaging systems requires a highly homogeneous 11.75T magnet with a 900 mm warm bore. We describe the principle of calculation of the main coil deformations in order to perform field homogeneity loss assessment and mechanical dimensioning. This coil is wounded as double pancakes laterally wetted by HeII. As preliminary calculation runs led to high hoop stress levels, and due to the composite nature of this winding pack, a detailed finite element model of the coil structure is needed to obtain accurate computation of the displacement and stresses. Also, this finite element model serves as reference solution to validate a homogenized model. Then, a mixed full resolution/homogenized model is constructed in order to reduce the mesh size. A good correlation between this mixed model and full resolution one is observed. These three kinds of models will be exploited during the development phase of the magnet, to conduct mechanical design and estimation of field homogeneity losses.