1. A method for 3D visualization of a pair of lungs, comprising: segmenting image data of the pair of lungs and lung parenchyma; generating a 3D map as a function of the segmented image data; and rendering the 3D map to produce a visualization of a color-coded semi-transparent 3D volume in its entirety, wherein areas of low, average and high perfusion are represented by different degrees of opacity in the visualization of the 3D volume at the same time so that they can be differentiated from each other, and wherein the opacity is varied according to perfusion or density values in the 3D map overlaying a 2D slice of the image data with the color-coded semi-transparent 3D volume.