as patchy opacities. There is progressive confluence of the opacities resulting in lobar
opacification. The process is often bilateral. Abscess formation occurs late in the infection and
is demonstrated by increasing demarcation of an initially ill-defined opacity with evolution into a
round cavity with an irregular thick wall and possibly an air-fluid level (37).
Gram-negative agents includeKlebsiellaandPseudomonas, each of which has relatively
specific radiographic features that can facilitate diagnosis, in addition to clinical history and
sputum culture. In general, Gram-negative pneumonia can present as ill-defined pulmonary
Figure 21 (A). CT scan of the chest demon-
strates an embolus in the left lower lobe pulmo-
nary artery (arrow) as well as a small left pleural
effusion. (B) An area of consolidation in the left
lower lobe posteriorly represents pulmonary
infarction. Although the appearance may be
similar to pneumonia in some patients, the
presence of embolus and absence of other
clinical signs of infection in this patient estab-
lishes the diagnosis pulmonary infarction with
certainty.Figure 22 CT scan of the chest demonstrates
bilateral alveolar and ground-glass opacities as
well as interlobular septal thickening in a 38-
year-old female with a history of lupus. These
findings were not present on a CT performed
four days earlier and are compatible with lupus
pneumonitis and/or hemorrhage. Bilateral pleu-
ral effusions as well as a pericardial effusion are
also present.Radiology of Infectious Diseases and Their Mimics in Critical Care 95