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The use of computed tomography in the study of lung diseases is well established. Many reports have indeed emphasised its role not only in the detection and diagnosis but also in the quantification and follow-up of both focal and diffuse lung diseases. Moreover, CT has helped to better understand the clinical and pathological course of some diseases while some CT classifications are used now to categorise disease.

Good knowledge of lung anatomy is mandatory to understand the CT features of lung diseases, not only because it permits a better understanding of the CT features of the disease (appearance pattern), but also because it helps to understand the specific distribution in the lung of the disease (distribution pattern). Comprehensive knowledge of the lobes and segments of the lung has of course always been a very important part of radiologists’ armamentarium, but it was the introduction of CT and especially thin-slice CT that made the significance of the subsegmental lung anatomy apparent. Indeed, the high anatomic detail obtained with thin-slice CT allows the recognition of anatomical structures at a subsegmental level and the identification of lung units as small as the secondary pulmonary lobule. These secondary pulmonary lobules have turned out to be very important in the interpretation of lung changes seen on CT and abnormalities of these units are more or less the building blocks of which the CT patterns are constructed.

Generally the diagnosis of lung disease on a chest CT is based on three elements (Fig. 3.1):

Increased lung attenuation patterns develop when the density of the lung parenchyma increases. As mentioned in Chapter 2, the normal lung density on CT is slightly higher than air and is determined by the balance between air in the airspaces and in the small airways, on one hand, and the soft tissue structures that have a higher density than air but that are as such not individually visible on the other hand. These soft tissue structures include the interstitial lung tissues, the wall of the alveoli, small airways and capillaries and the blood in these capillaries.

The decreased lung attenuation pattern develops when the density of the lung parenchyma decreases. As mentioned in Chapter 2, the normal lung density on CT is slightly higher than air and is determined by the balance between air in the airspaces and in the small airways, on one hand, and the soft tissue structures that have a higher density than air but that are as such not individually visible on the other hand. These soft tissue structures include the (interstitial) lung tissues, the wall of the small airways and blood vessels and the blood in these vessels. A decrease in lung attenuation can be observed when this balance is disturbed by either an abnormal increase in the amount of air, an abnormal decrease in the intravascular blood volume with, as a result, a smaller calibre of the small vessels, and in case of loss of (interstitial) lung tissue. Each of these phenomena may result in a different decreased lung attenuation pattern (Fig. 5.1).

The nodular pattern is characterised by the presence of multiple nodular opacities with a maximum diameter of 3 cm. A nodule with a diameter less than 1 cm can be defined as a small nodule, whereas a nodule larger than 1 cm is often called a large nodule (). The term “micronodule” usually refers to nodules no larger than 7 mm in diameter (; ). The term “miliary pattern” indicates the presence of multiple small (1–3 mm) micronodules with sharp contours distributed in a major part of the lungs (; ). Generally, the nodules in the nodular pattern range from 1 mm to 1 cm. Larger nodules are often the result of the fusion of several small nodules.

The linear pattern is characterised by the presence of lines that occur when elongated structures or compartments of the lung that traverse the lung parenchyma parallel to the CT scan plane are involved. The introduction of multidetector CT and its ability to produce high-detail coronal and sagittal reconstructions in addition to the axial slices has improved the recognition of these linear opacities. When these lines cross each other, a netlike appearance can occur, which explains why the pattern is also often called the reticular pattern.

In this chapter, the most common and characteristic features of several diseases that affect the lungs are demonstrated. Some diseases show a typical presentation while others are less typical or show only a part of their spectrum of CT signs. These cases are in the first place intended for exercising pattern recognition and will also help to understand why diseases appear as they do on CT.