The lungs are assessed and described by dividing them into upper, middle and lower zones
Refer to 'zones' not 'lobes'
Compare left with right
Compare an area of abnormality with the rest of the lung on the same side
It is difficult to fit the whole of both lungs into your field of view. When describing the lungs it is conventional to divide them into three 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 consists of middle and lower lobes.
Dividing the lungs into zones allows more careful attention to be paid to each smaller area. If this is not done it is easy to ignore important abnormalities.
Note that the lower zones reach below the diaphragm. This is because the lungs pass behind the dome of the diaphragm into the posterior sulcus of each hemithorax. Normal lung markings can be seen below the well defined edges of the diaphragm.
Assessing the lung zones
Each zone is compared with its opposite side paying attention to any asymmetry. If the lungs appear asymmetrical, it should be determined if this can be explained by 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, but some diseases cause reduced density (blacker). If there is an area that is different from the surrounding ipsilateral lung, then this is likely to be the abnormal area.
You should also be aware that 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 lung.