Chest Imaging

Lung abscess

Certain virulent bacteria may create focal areas of necrosis or lung abscess. The abscess may not be recognized on chest films because of surrounding lung consolidation or because there is no communication with a conducting airway and thus no cavitation is appreciated. However, lung abscess as a complication of pneumonia must be suspected when patients are not responding to otherwise appropriate antibiotic therapy. If abscess is considered, CT scanning may help to define its presence and extent.

Abscess formation is most likely with anaerobic and Gram-negative bacteria, Staphylococcus aureus, tuberculosis and atypical mycobacterial species, fungal organisms such as Coccidioides, Aspergillus and Mucor, and unusual bacteria such as Rhodococcus equi.

Lung abscess may occur because of inflammatory necrosis of otherwise normal lung or occasionally secondary to vascular involvement leading to ischaemic necrosis and death of lung.

Radiographically, as stated above, a lung abscess may be difficult to appreciate secondary to surrounding lung consolidation but will generally be recognized when air enters the abscess cavity creating either a large single abscess or multiple smaller microabscesses. With the larger abscess cavities the inner walls are thick and irregular (Fig.1). Resolution of an abscess is recognized when the abscess cavity decreases in size or the amount of fluid within an abscess fluctuates. This is not related to healing necessarily but instead to the ease of communication between the abscess cavity and a conducting airway. Fluid levels will rise in the cavity if the exit is blocked and will fall if the opening remains patent.

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Fig.1

A PA chest film demonstrates a thick-walled cavity in the right midlung. A small fluid level is noted in the cavity. The outer margins are irregular as is typical of lung abscess which in this case was related to anaerobic infection following an aspiration.
Lung abscess, Fig.1