A system for editing a 3-D medical diagnostic ultrasound image dataset displays both a 3-D reconstruction of the dataset and an editing object. A user moves the editing object relative to the 3-D reconstruction with a six degrees of freedom input device that allows the user to control both the position and the orientation of the editing object. The 3-D reconstruction and the associated 3-D dataset are edited to reduce opacity of a portion of a 3-D reconstruction on a selected side of the editing object. The disclosed editing system is fast, efficient and intuitive, and it allows the user to designate the portions of the 3-D dataset to be removed simply and reliably.