In registration of images in digitized subtracted angiography, X-rays
irradiate a patient's body. A first digitized mask image is acquired
without an injection of the contrast agent into the body, then the
contrast agent is injected into the body, and a second digitized contrast
image is acquired. The mask image is divided with regular geometrical
patterns and a registration is made of the mask image relative to the
contrast image through the use of landmarks defined at the corners of the
geometrical patterns. These two images are subtracted after registration,
and the subtracted image is displayed. The robustness of the landmarks is
furthermore tested, and if the robustness is insufficient, a subdivision
is made of the regular geometrical patterns.