1. A method of producing a ventilation image, comprising: receiving two image data sets, wherein at least a portion of a patient's lung is represented by the two image data sets; determining an initial volume, V, for each of a plurality of local volume elements based on a first of the two image data sets; calculating a local volume change, ΔV, between the two image data sets for each local volume element; calculating ΔV/V for each local volume element; producing a ventilation image having a plurality of voxels, wherein each ventilation image voxel corresponds to one or more of the local volume elements and is represented by an indicator corresponding to the one or more ΔV/V value for the corresponding one or more local volume element.