For the purpose of description the lungs are divided into zones: upper, middle and lower. Each of these zones occupies approximately one third of the height of the lungs.
The lung zones do not equate to the lung lobes. For example, the lower zone on the right comprises the middle and lower lobes.
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Each zone is compared with its opposite side. If the lungs appear asymmetrical it should be determined if this is due to asymmetry of normal structures, technical factors, such as rotation, or lung pathology.
If there is genuine asymmetry, decide which side is abnormal. Often a dense (whiter) area is abnormal, such as in pneumonia or lung cancer, but some diseases cause reduced density (blacker), such as a pneumothorax. If there is an area that is different from the surrounding areas of the same lung, then this is likely to be the abnormal area.
Some diseases result in bilateral lung abnormalities making comparison of left with right difficult. In these cases it is still important to assess each zone in turn, to avoid missing subtle abnormalities on the background of abnormal lungs.