Quoit filter-a new filter based on mathematical morphology to extract the isolated shadow, and its application to automatic detection of lung cancer in X-ray CT

We propose a new algorithm named Quoit filter (Q-filter) to extract the isolated but low amplitude shadow located in the background which has extremely high amplitude fluctuation. Q-filter is a kind of mathematical morphology and its formulation is quite simple. This simplicity brings about a unique merit that output from this filter is analytically expressive for the case of analytical input shapes like ball, cone, or rotation of cosine function, which have characteristics of rotation symmetry and monotonic decreasing from the origin. This Q-filter is composed of two sequential operations named Q Trans. and Q Inv. Trans., Q Trans. corresponds to extracting feature parameters like a matched filter from the input image having a nonideal isolated shadow, and Q Inv. Trans. corresponds to restoring isolated images using extracted feature parameters. This filter is applied to detecting the cancer candidate shadow automatically in the CT cross sections of lung areas, aiming to reduce drastically the number of cross sections to be diagnosed by the doctor.