Apparatus, method and computer program for use in generating a three dimensional image. A three dimensional image volume comprising a three dimensional image is displayed using a visualization device in a way that the position and orientation of an input surface is visible within the three dimensional image volume. User input indicative of a two dimensional image provided in the plane of the input surface is received via the input surface. First positional data indicative of a relative position and orientation of the input surface in relation to the three dimensional image volume is then determined. Subsequently, the three dimensional image is modified to include a three dimensional representation of the two dimensional image, the three dimensional representation having a position and orientation within the three dimensional image volume determined from the first positional data.