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A 34-year-old man presented with one day of fever and a cough productive of yellowish sputum. He complained of generalized myalgia and had vomited twice.

He had a history of intravenous drug use and had last injected drugs over one year earlier. He was tested and found to be PPD negative and HIV negative two months earlier.

On examination, he appeared healthy but had persistent cough. His vital sign were: blood pressure 118/78 mm Hg, pulse 88 beats/min, respiratory rate 24 breaths/min, temperature 100.8°F (oral), 103.4°F (rectal). Pulse oximetry oxygen saturation was 96% on room air.

Lung auscultation revealed bibasilar crackles. His neck was supple and oropharynx was clear. He was anicteric and had no rash, oral thrush, or lymphadenopathy. He had no heart murmur or hepatosplenomegaly.

A chest radiograph was obtained and interpreted as normal (Figure 1).

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In most patients, pneumonia is easy to detect. However, when the infiltrate is small or has only slightly greater opacity than adjacent normal lung, more subtle radiographic signs must be sought.

Silhouette signs occur when the airspaces of the lung are filled with fluid resulting in either the formation of a new abnormal air/fluid interface or obliteration of a normal air/fluid interface (loss of a silhouette). This silhouette effect, which occurs with pneumonia as well as other conditions that cause airspace filling, is one of the basic principles of chest radiography.

For an air/fluid interface to be visible on a radiograph (i.e., form a silhouette), it has to have a relatively sharp margin that is parallel to the direction of the x-ray beam. When the margin of a fluid collection is either gradual or not parallel to the x-ray beam, it appears indistinct (ill-defined) or may be completely invisible.

There are four types of silhouette signs (Table 1). The first and most well known is loss of a normal air/fluid interface that occurs when an abnormal fluid collection such as pneumonia lies adjacent to a soft tissue structure such as the heart or diaphragm. For example, a right middle lobe pneumonia obliterates the right heart border because the right middle lobe lies against the right atrium. This is often referred to as “the silhouette sign,” although it is actually a misnomer because there is loss of a normal radiographic silhouette (air/fluid interface). In addition, a silhouette sign does not necessarily mean that the fluid collection is within the lung. A large pleural effusion can also obliterate the margin of the diaphragm.

Table 1 Four Silhouette Signs...

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