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(18)F-FDG PET/CT allows to obtain a complete morpho-functional cartography of inflammatory active localizations and to follow treatment efficacy in patients with sarcoidosis, particularly in atypical, complex, and multisystemic forms.

Magnetic resonance (MR) diffusion imaging provides a valuable tool used for inferring structural anisotropy of brain white matter connectivity from diffusion tensor imaging. Recently, several high angular resolution diffusion models were introduced in order to overcome the inadequacy of the tensor model for describing fibre crossing within a single voxel. Among them, q-ball imaging (QBI), inherited from the q-space method, relies on a spherical Radon transform providing a direct relationship between the diffusion-weighted MR signal and the orientation distribution function (ODF). Experimental validation of these methods in a model system is necessary to determine the accuracy of the methods and to optimize them. A diffusion phantom made up of two textile rayon fibre (comparable in diameter to axons) bundles, crossing at 90 degrees , was designed and dedicated to ex vivo q-ball validation on a clinical scanner. Normalized ODFs were calculated inside regions of interest corresponding to monomodal and bimodal configurations of underlying structures. Three-dimensional renderings of ODFs revealed monomodal shapes for voxels containing single-fibre population and bimodal patterns for voxels located within the crossing area. Principal orientations were estimated from ODFs and were compared with a priori structural fibre directions, validating efficiency of QBI for depicting fibre crossing. In the homogeneous regions, QBI detected the fibre angle with an accuracy of 19 degrees and in the fibre-crossing region with an accuracy of 30 degrees .

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Abstract. Most of the approaches dedicated to fiber tracking from diffusionweighted MR data rely on a tensor model. However, the tensor model can only resolve a single fiber orientation within each imaging voxel. New emerging approaches have been proposed to obtain a better representation of the diffusion process occurring in fiber crossing. In this paper, we adapt a tracking algorithm to the q-ball representation, which results from a spherical Radon transform of high angular resolution data. This algorithm is based on a Monte-Carlo strategy, using regularized particle trajectories to sample the white matter geometry. The method is validated using a phantom of bundle crossing made up of haemodialysis fibers. The method is also applied to the detection of the auditory tract in three human subjects.

Magnetic resonance elastography (MRE) is an increasingly used method for non-invasive determination of tissue stiffness. MRE has shown its ability to measure in vivo elasticity or viscoelasticity depending on the chosen rheological model. However, few data exist on quantitative comparison of MRE with reference mechanical measurement techniques. MRE has only been validated on soft homogeneous gels under both Hookean elasticity and linear viscoelasticity assumptions, but comparison studies are lacking concerning viscoelastic properties of complex heterogeneous tissues. In this context, the present study aims at comparing an MRE-based method combined with a wave equation inversion algorithm to rotational rheometry. For this purpose, experiments are performed on in vitro porcine brain tissue. The dynamic behavior of shear storage (G') and loss (G ('')) moduli obtained by both rheometry and MRE at different frequency ranges is similar to that of linear viscoelastic properties of brain tissue found in other studies. This continuity between rheometry and MRE results consolidates the quantitative nature of values found by MRE in terms of viscoelastic parameters of soft heterogeneous tissues. Based on these results, the limits of MRE in terms of frequency range are also discussed.

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