An apparatus for aiding in navigating or modifying an at least 3D image data set , the set including a plurality of cross sectional 2D image slices (110a-c), including first and second slices , the slices representing a cross sectional view on the 3D image date set, each of the two slices having one of a plurality of states, the apparatus comprising: - a database interface to access the 3D image data set and the slices therein; - a graphical user interface controller configured to display on a screen a graphical user interface (GUI), the graphical user interface (GUI), when so controlled and displayed on the screen, comprising: - a selection widget to select for display as an image a first slice of the 3D image data; - a window widget for displaying the first slice as an image; - a slice position indicator widget for i) displaying a visual indication of the spatial position of the first slice in the 3D image data set, and ii) for displaying a visual indication of the spatial position of the second slice in the 3D image data set and for indicating the relative positions between the first and second slices in the 3D image data set; - a graphical element for displaying a visual indication of a current status of the first slice and of a current status of the second slice, different states being represented by different visual indications; the graphical user interface controller further configured to i) receive a user request to change the current status of the first or the second slice into a new status from among the plurality of states and, when the user request to change the current status has been received, to ii) update the displaying of the visual indication of the status of the first or second slice to now represent the requested new status of the first or second slice, wherein each of the slices has one of two states, either a protected or an unprotected state, wherein, wherein each of the two slices includes a sub-volume delineation (117a-c) each representing a different cross section of a 3D or higher dimensional delineation of a region of interest in the at last 3D image data set , the 3D delineation extending across at least the two slices (110a,c), the 3D delineations being a result of an application to the first slice of a 3D delineation tool , the delineations (117a-c) of the slices (110a-c) being logically linked so that a later modification by the same or another 3D delineation tool of the delineation in the first slice automatically effects a corresponding co-modification of the delineation in the second slice when the second slice is in the unprotected state, , whereas, when the second slice is in the protected state, the co-modification is blocked. - a database interface to access the 3D image data set and the slices therein; - a graphical user interface controller configured to display on a screen a graphical user interface (GUI), the graphical user interface (GUI), when so controlled and displayed on the screen, comprising: - a selection widget to select for display as an image a first slice of the 3D image data; - a window widget for displaying the first slice as an image; - a slice position indicator widget for i) displaying a visual indication of the spatial position of the first slice in the 3D image data set, and ii) for displaying a visual indication of the spatial position of the second slice in the 3D image data set and for indicating the relative positions between the first and second slices in the 3D image data set; - a graphical element for displaying a visual indication of a current status of the first slice and of a current status of the second slice, different states being represented by different visual indications; <EOS>