Image enhancement in intensity projected multichannel MRI using spatially adaptive directional anisotropic diffusion
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Anisotropic Diffusion is widely used for noise reduction with simultaneous preservation of vascular structures in maximum intensity projected (MIP) angiograms. However, extension to minimum intensity projected (mIP) venograms in Susceptibility Weighted Imaging (SWI) poses difficulties due to spatially varying baseline. Here, we introduce a modified version of the directional anisotropic diffusion which allows us to simultaneously reduce the noise and enhance vascular structures reconstructed using both M/mIP angiograms. This method is based on spatial adaptation of the diffusion function, separately in the directions of the gradient, and along those of the minimum and maximum curvatures. The existing approach of directional anisotropic diffusion uses binary switched diffusion function to ensure diffusion along the direction of maximum curvature stopped near the vessel borders. Here, the choice of a threshold for detecting the upper limit of diffusion becomes difficult in the presence of spatially varying baseline. Also, the approach of using vesselness measure to steer the diffusion process results in structural discontinuities due to junction suppression in mIP. The merits of the proposed method include elimination of the need for an apriori choice of a threshold to detect the vessel, and problems due to junction suppression. The proposed method is also extended to multi-channel phase contrast angiogram.
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