Infection

Infection and inflammation of the brain may be caused by all the known pathologic agents: bacteria, viruses, fungi, parasites, etc.
The reaction of the brain is, however, peculiar, due to the presence of the blood-brain barrier and specific immunological processes.
From a clinical point of view infections of the brain may manifest with specific neurological signs related to the location of the lesion and indirect signs of infection such as fever, malaise and meningeal signs.

Bacterial infections

The brain manifestations of bacterial infection are:
a. abscesses
b. meningitis
c. empyemas (subdural or epidural)

When a pyogenic organism, usually staphylococcus, succeeds in localising within the brain parenchyma, a cerebritis occurs. The lesion is poorly defined, usually hypodense at CT and on MRI it is hypointense in T1 and hyperintense in T2.

If the process is not eradicated by antibiotic therapy, the central area of the cerebritic lesion becomes necrotic and is surrounded by a capsule of collagen tissue, forming the abscess.

On CT (Fig. 35) the abscess usually appears as a round or ovoid lesion, hypodense, with an isodense capsule that enhances following contrast. The capsule is usually thin and regular. There is usually abundant surrounding oedema.

MRI (Fig. 35): the capsule of the abscess is not infrequently hyperintense in T1, probably due to paramagnetic effects of free radicals; the centre of the abscess is either hypo- or isointense. On T2 the abscess is usually hyperintense. The capsule enhances markedly following gadolinium injection.

Meningitis
Both CT and MRI may show leptomeningeal enhancement and associated cortical or brain involvement.

Empyema
Empyemas are characterised by presence of purulent material in the subdural or epidural space. In the majority of cases they represent an extension of an infectious process of the paranasal cavities. Both CT and MRI may demonstrate collections that have density and signal characteristics that may not be too different than those of chronic subdural haematomas.

Figure 37. Tuberculosis MR1, T1W1, following Gadolinium. Multiple small granulomas scattered within the brain parenchyma are seen (miliary tuberculosis).

Viral infections

These may produce minimal changes at CT and be better seen at MRI with non-specific T2 hyperintensity both involving the cortex and the white matter.

Herpes simplex encephalitis may have haemorrhagic components demonstrated by CT (Fig. 36) and occurs usually bilaterally in specific locations such as the temporal lobe, the hippocampus and the insula.

MRI (Fig. 36) clearly shows not only the T1 hypointensity and the T2 hyperintensity of the lesion but also the paramagnetic effect of the haemorrhagic component. Contrast enhancement is usually not necessary.

Microbacteria and fungi produce abscesses and granulomas with or without meningeal involvement; both CT and MRI are sensitive in demonstrating the lesions, particularly following contrast injection. Tuberculosis in the miliary form only may be appreciated following contrast injection (Fig. 37).

Parasitic infections

The most common parasitic infections are cysticercosis and echinococcosis.

In cysticercosis, both intraparenchymal and meningeal cysts are found which at different stages may include calcified nodules; CT clearly demonstrates the calcification; frequent meningeal enhancement is encountered.

a, b	Figure 38. Definite MS a, b) CT without contrast. A few small foci of hypointensity are barely seen. c, d) CT following intravenous injection of iodinated contrast. At least three nodular enhancing foci within the white matter are seen.
c, d

Among the parasitic brain infections, toxoplasmosis is today the most frequent, due to its high incidence in AIDS patients. Multiple nodules are found both in the basal ganglia and in the cerebral hemisphere. They tend to enhance and the differential diagnosis with multinodular lymphomas is frequently difficult.

AIDS has given rise an increase of all parasitic infections. Brain abnormalities directly related to AIDS are atrophy and subtle changes of signal intensity of the white matter.

Otherwise, the picture is that of parasitic, fungal or viral opportunistic infections.

Kjell Bergström and Giuseppe Scotti