Mechanisms for image processing are provided. A computing device comprises a memory, an input device, a display, and a processor. The processor is configured to: acquire a three-dimensional image of an anatomical structure and store it in the memory. The processor renders on the display (i) an initial volume of the three-dimensional image corresponding to an initial portion of the anatomical structure, and (ii) a moveable control element. The initial volume has an outer surface defined by a position of the control element. The processor receives input data updating the position of the control element relative to the initial volume; and renders on the display, in place of the initial volume, a further volume of the three-dimensional image, corresponding to a further portion of the anatomical structure and having a further outer surface defined by the updated position of the control element.