Factors Affecting Appearance

CT images are produced with ionizing radiation, similar to radiographs. Therefore, some of the factors affecting the appearance of the CT images are similar to radiography (for example density).

On CT, the density of a structure, the image display, and the use of iv contrast enhancement are the main factors affecting the appearance of structures. Shape and thickness do not affect the final CT image as much as they do in radiographs, since CT images are summations of only thin slices of superimposed structures. 

The data obtained can be displayed (or "windowed") to highlight certain structures. The two chest CT images shown are from the same set of data at the same level. The top image is displayed in "lung" windows and the bottom in "soft tissue" windows. Note that lung windows show the lung parenchyma (small blood vessels, bronchi, and airspaces), but one is unable to distinguish the soft tissue structures well. Soft tissue windows show the muscles, fat and blood vessels well, but not the lung parenchyma.