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The silhouette signDescribed by I'elson & l elson (1950), the 'silhouette sign' is the

l oss of an interface by adjacent disease and permits localisation of al esion on a film by .studying the diaphragm, cardiac and aortic outlines.

These structures are normally seen because the adjacent lung

i s aerated and the dilIerence in radiodensity is demonstrated. Whenair in the alveolar spaces is replaced by fluid or soft tissue. there isno longer a difference inradioden.sity between that part of the 11.11 1 2

t he adjacent struetures.'I'herefore the silhoutte is lost and the

silhoette sign' is present. Conversely if the border is retained andthe abnormality is superimposed, the lesion must he lying either

anterior or posterior. In 8-IUr4 of people a short segment of the

right heart border is obliterated by the bit pad or pulmonary vessels.

Obliteration of these borders may occur with pleural or mediastinallesions as well as pulmonary pathology. The right middle

l obe and lingula lie adjacent to the right and left cardiac borders,

the apicoposterior ,segment of the Icfl upper lobe lies adjacent tothe aortic knuckle, the anterior segment of the right upper lobe and

the middle lobe lie against the right aortic border, and the basal segments

of the lower lobes lie adjacent to the hemidiaphragills.

Pulntonarv disease in these lobes and segments cam obliterate theborders (Figs 1.23- 1.25).

Using the same principle. a well-delned mass seen ahem the

clavicles is always posterior whereas an anterior mass, being incontact ss ith soft tissues rather than aerated Iung. is ill defined. This

i s the cerricolltnrat is sigtr.

The hi/um ot e rluv sign helps distinguish a large heart from a

media.stinal mass. With the latter the hilum is seen through the masswhereas ssith the former the hilunr is displaced so that orals its

l ateral border is visible.

The air bronchogramOriginally described by I'Ieischner ( 1941 ), and named by Fclson

11973). the air hronchoggram is an important sign shoss ing that an

opacity is intrapulmonary. The bronchus, if air filled but not Iluidl illed, becomes visible when air rs displaced from the surrounding

parenchyma. Frequently the air bronchogran is seen as scattered

linear transluceneies rather than continuous branching structures. It

i s most commonly seen within pneumonic consolidation and pulmonaryoedema. An air hronchogram is not seen within pleural fluid and rarely within a tumour, with the

exception of alveolar cell distal to a malignancy if the bronchus remains patent (Fig. 1.26).

carcinoma and rarely lymphoma. It may he seen in consolidation An air bronchogram is usually

a feature of air-space filling but is described accompanying severe interstitial fibrosis such asmay

develop with sarcoidosis (Box 1.4).

Air-space (acinar/alveolar) pattern (Box 1.5)Few disease processes truly only involve the jnterstitium or

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acinus on histological examination, but air-space shadowing on

t he chest radiograph has distinctive features . When the distal

airways and alveoli are filled with fluid, whether it is trmsudate,exudate or blood, the acinus forms a nodular 4-f; mm shadow.

These shadows coalesce into ITulTy ill-dclincd round or irregular

cotton-wool shadows, non-segmental, homogeneous or patchy.but frequently well defined adjacent to the fissures (Fig. I.28).

The