1. A system for selecting a region of interest in an image, comprising: a user interface that receives input from a user that expressly defines a position and size of a region of interest in a two-dimensional image corresponding to an intended three-dimensional volume of interest; a region extender that extends the two-dimensional region of interest to a three-dimensional region of interest based on the size or shape of the two-dimensional region of interest; a slab selector that determines a position and a thickness of a slab of the image, based on the position and the size of the three-dimensional region of interest, the slab comprising at least part of the intended volume of interest; and a visualization subsystem that provides a visualization of the slab; wherein the user interface enables the user to alter at least one of the position and size of at least one of the two-dimensional and three-dimensional regions of interest when the determined slab does not encompass an entirety of the intended volume of interest.