“…gray level based techniques, learning techniques, model fitting techniques, probabilistic atlases, and level set) this problem is still open. Indeed, although the gray level based techniques proposed so far [11,12,5,13,14,15,16,17] obtain the most promising results, they are not robust to database variations; this is because their basic step of organ gray level estimation does not take into account the high variability observed both in the same and in different CT volumes. For this reason, when tested on larger and complex data sets, these methods' performance could decrease significantly.…”