Mimic of Encephalitis
Restricted diffusion may be present, which, depending on clinical presentation, may rarely
lead to confusion of the entity with acute infarction. In such cases, MR spectroscopy and
nuclear medicine imaging may be helpful. Tc-99m HMPAO single-photon emission CT has
shown utility for the detection of both herpes encephalitis and Japanese encephalitis (36).
Clinical and Radiologic Diagnosis of HIV Encephalopathy/Encephalitis
HIV encephalopathy/encephalitis (HIVE) is a syndrome of cognitive, behavioral, and motor
abnormalities attributed to the effect of HIV infection on the brain in the absence of other
opportunistic infection. HIVE is the most common neurologic manifestation of HIV. Diffuse
cortical atrophy is the most common finding on both CT and MR. White matter disease is also
present, and the areas most affected are the periventricular regions and centrum semiovale, the
basal ganglia, cerebellum, and the brainstem. On T2-weighted MR images, white matter signal
changes may be focal or diffuse, and the distribution and extent of the lesions do not
necessarily correlate with clinical presentation. FLAIR sequences may demonstrate lesions not
detected on T2-weighted images, such as those smaller than 2 cm. HIVE lesions do not enhance
on MR examination after gadolinium administration, a characteristic feature (28).
Mimic of HIVE
The differential for white matter lesions is broad, encompassing infectious, inflammatory, and
autoimmune causes. Multiple sclerosis lesions are usually focal, although with severe illness
they can become confluent (Fig. 19A, B, and C). Unlike lesions in HIV, active multiple sclerosis
(MS) lesions do enhance. The lesions are isointense to hypointense on T1-weighted imaging,
whereas such lesions are not visualized on T1-weighted images in HIVE (28).
Acute disseminated encephalomyelitis (ADEM) is a condition whereby multifocal white
matter and basal ganglia lesions occur, typically 10–14 days after infection or vaccination. The
lesions involve both the brain and spinal cord. CT is initially negative, but with time
demonstrate low-density, flocculent, and asymmetric lesions. These abnormalities are better
visualized on FLAIR MR sequences. Contrast enhancement may be punctate or ringlike
(complete or incomplete). Again, contrast enhancement of the lesions is one helpful
differentiating feature from HIVE (28).
THORACIC INFECTIONS AND THEIR MIMICS
Clinical and Radiologic Diagnosis of Focal/Segmental Pneumonia
Bacterial pneumonia can be divided into three main categories: lobar, lobular or
bronchopneumonia, and interstitial. The causative organism generally determines what type
of pneumonia results. Bronchopneumonia is the most common type, with the prototype
causative agent being staphylococcus. The classic appearance on chest radiography and CT is a
“patchwork-quilt” pattern of air-space opacification, reflecting diseased and adjacent non-
diseased pulmonary lobules and the presence of air bronchograms, reflecting air-filled bronchi
within diseased parenchyma (Fig. 20A and B) (37,38).
Mimics of Focal/Segmental Pneumonia
Pulmonary Embolus
Although many chest radiographs in patients with pulmonary embolus (PE) are not entirely
normal, the findings are usually not specific for PE, and confirmation with additional
modalities, such as pulmonary CT angiography (the current imaging reference standard),
ventilation/perfusion (V/Q) scan, and lower extremity venous Doppler, are required for
diagnosis. Radiographic findings include right heart enlargement, central pulmonary artery
enlargement (usually when chronic, but occasionally when acute with a large clot burden),
localized peripheral oligemia with or without distention of more proximal vessels (“Wester-
mark sign”), and peripheral air-space opacification due to localized pulmonary hemorrhage.
When lung infarction occurs, in a minority of cases, a pleural-based, wedge-shaped opacity can
be identified, the “Hampton’s Hump.” Lung infarction can have a similar appearance to
segmental pneumonia, and correlation with CT angiography is usually needed to differentiate
the two entities (Fig. 21A and B). The utility of chest radiography is more for identifying
92 Luongo et al.