  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.
