Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 109

Air in the lungs appears black on X‐ray. Consolidation appears as areas of opacification sometimes conforming to the outline of a lobe or segment of lung in which the air has been replaced by an inflammatory exudate (e.g. pneumonia), fluid (e.g. pulmonary oedema), blood (e.g. pulmonary haemorrhage) or tumour (e.g. carcinoma with lepidic growth). Bronchi containing air passing through the consolidated lung are sometimes clearly visible as black tubes of air against the white background of the consolidated lung: air bronchograms (see Fig . 17.2). Structures such as the heart, mediastinum and diaphragm are usually clearly outlined as a silhouette on an X‐ray because of the contrast between the blackness of aerated lung and the whiteness of these structures. When there is abnormal shadowing in the lung adjacent to these structures, there is loss of the sharp outline, and this is often referred to as the silhouette sign although it is the absence of the expected silhouette that indicates an abnormality (Fig. 4.6).

Figure 4.2 Diagram of chest X‐ray (lateral view). (a) Trachea. (b) Oblique fissure. (c) Horizontal fissure. It is useful to note that in a normal lateral view, the radiodensity of the lung field above and in front of the cardiac shadow is about the same as that below and behind (x). Ao, aorta.