![]() Lobar pneumonias have specific appearances, which can be explained by the anatomical relationships of the lobes of the lung to their surrounding structures. Where the horizontal fissure is concerned, this can sometimes be seen as a sharp border in the mid-zone of the right hemithorax when there is consolidation in the alveoli either above it (in the right upper lobe) or below it (in the right middle lobe).īronchopneumonia appears on a chest X-ray as bilateral patchy shadows and typically predominates at the lung bases: This is termed the silhouette sign and is often described as causing a ‘loss of clarity’. In the lobes which abut the heart anatomically, it therefore, becomes impossible to distinguish where the lung ends, and the heart begins. Affected alveoli become as dense as soft tissue structures such as the heart. In the case of pneumonia, air within the alveoli is replaced by inflammatory exudate and pus. X-rays travelling through lung tissue cast less of a shadow than those passing through the soft tissues of the mediastinum or abdomen. If we apply this to a chest X-ray, it becomes evident that the only structures we can appreciate are those where there is a density change at their border with neighbouring structures. Shadow of two hands forming the shape of a bird, image sourced from Wikipedia The combination of their individual silhouettes adds up to become a bird, a rabbit, a dog and all manner of other possibilities. The game works because you cannot tell where one hand begins and the other ends. This sign can be difficult to conceptualise, but the simple children’s game of hand shadow puppets is a helpful way of assisting our understanding of it. Loss of these normal silhouettes on an AP chest X-ray is generally indicative of the site of the pathology. Much is made of this sign in radiology and, once it is properly understood, it is extremely useful in localising areas of airspace opacity, atelectasis, or a mass within the lung. It is something of a misnomer as it actually represents the loss of a normal silhouette. The silhouette sign refers to the loss of normal borders between thoracic structures on a chest X-ray. How can shadow puppets help us to understand chest X-rays? Chest X-rays are the initial modality of investigation in the majority of cases, and a sound understanding of the chest X-ray features of pneumonia is vital for all front-line clinicians that encounter and treat it. Streptococcus pneumoniae is by far the most common causative organism. Pneumonia is characterised by exudation and consolidation into the alveoli, and in the U.K. will develop community-acquired pneumonia. Every year between 0.5% and 1% of adults in the U.K. ![]() Pneumonia, which is defined as inflammation of the lung, is a common cause of morbidity and mortality around the world.
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