For the purpose of enhancing a difference between pixels relating to a substantial
structure in an image and other pixels, with each pixel constituting an image defined
as a pixel of interest, the variance of pixel values is determined in a local region
to which the pixel of interest belongs (508); and the pixel value of the
pixel of interest is maintained when the variance of pixel values is significantly
larger than the variance of noise, otherwise the pixel value of the pixel of interest
is suppressed (510, 512).