Erase the Cartesian coordinates from a visual field symbolic pattern image.

Hi, I'm working on the symbolic pattern of the visual field test chart which has the Cartesian coordinates as shown in this image. Getting only this pattern from the whole VF chart, these coordinates could have non black shadows. I tried to use the code of ImageAnalyst (median filter) but it just work for the black very straight lines, but not with lines having shadows and not perpendicular according to the fineness of the image scanning. Any help??