Specific disorders

Specific disorders : bacterial pneumonia : The major purpose of CXR in patients with suspected chest infection is to establish whether or not pneumonia is present.
Diagnosing the responsible infectious agent is rarely possible on radiological grounds.
The basic radiological feature of pneumonia is one or more areas of consolidation, varying from a small, ill defined opacity to a large opacity involving the whole of one or more lobes .
Consolidation may be accompanied by loss of volume of the affected lobe , a feature that is particularly common in children.
Cavitation may occur within the consolidated areas. Cavitation is a particular feature of infections with staphylococci, Gram negative and anaerobic bacteria and tuberculosis.
The differentiation between pneumonia and pulmonary oedema or pulmonary infarction may, at times, be difficult or impossible radiographically.
A common infecting organism in community-acquired lobar pneumonia is Streptococcus pneumoniae (pneumococcal pneumonia).
In pneumococcal pneumonia, there is usually 1- dense consolidation of a considerable portion of one lobe, 2- usually without loss of volume. 3- There may be an associated pleural effusion.
When the consolidation is patchy, involving one or more lobes, it is commonly referred to as bronchopneumonia, The most frequent causes of community acquired bronchopneumonia are Staphylococcus aureus, various Gram-negative and anaerobic bacteria and Mycoplasma pneumoniae.
Pneumonia may be secondary to obstruction of a major bronchus, carcinoma being a common cause of obstruction.
Bronchial obstruction should always be considered in any patient presenting with consolidation of one lobe, or of two lobes supplied by a common bronchus (e.g. the right middle and lower lobes), particularly if there is associated loss of volume.
Viral and Mycoplasma pneumonia: Viral pneumonia and pneumonia due to Mycoplasma pneumoniae may produce 1- widespread ill-defined consolidation and 2- loss of clarity of the vascular markings , which on occasion may resemble pulmonary oedema. 3- Alternatively, only a localized area of consolidation may be seen. 4- Pleural effusions are rare. 5- The radiological abnormality may persist for many weeks after clinical recovery.