The present invention provides a system of extracting a visual feature from a dataset, comprising a storage means for storing said dataset; retrieval means for retrieving said dataset from said storage means; display means for displaying an image of said retrieval dataset; means for defining a block of voxels corresponding to a selected portion of said displayed dataset, said block containing said visual feature therein; means for removing from said block voxels not containing said visual feature, to generate a feature block; means for generating a mask from said feature block; and means for rendering said dataset using said mask. A method of extracting visual features is also provided.