Disclosed is a computer implemented method of interrogating volumetric data, the method including the steps of: defining a reference surface relative to the volumetric data; providing an interrogation window lying in a movable interrogation plane intersecting the reference surface and the interrogation window intersecting the volumetric data, said interrogation plane having a pole lying in it about which pole said interrogation plane is rotatable, wherein movement of the interrogation plane provides a corresponding movement of the interrogation window; for each movement of the interrogation plane, determining the point of intersection of said pole and said reference surface, determining the relative angle of rotation of said interrogation plane about said pole and determining the angle of inclination of said pole to said reference surface; and on the basis of said determinations, providing a sectional image through the volumetric data corresponding to the position of the interrogation window within the volumetric data. Also disclosed is a computer system for implementing the method according to any one of the preceding claims, the system includes a processor for performing the steps of the method and a visual display for displaying said sectional image.