1. A region extraction system comprising: a temporary region extractor for temporarily extracting a to-be-extracted region from measured data with a region growing technique; an initial position determiner for determining an initial position of a standard model for said to-be-extracted region using said to-be-extracted region temporarily extracted; and a to-be-extracted region extractor for extracting said to-be-extracted region from said measured data with a model fitting technique using said standard model.