Direct spread  The direct spread of organisms from a contiguous site usually causes a single brain abscess. Primary infections that can directly spread to the cerebral cortex include :

• Subacute and chronic otitis media and mastoiditis (spread to the inferior temporal lobe and cerebellum)

• Frontal or ethmoid sinuses (spread to the frontal lobes)

• Dental infection (usually spreads to the frontal lobes)

Hematogenous spread  Brain abscesses associated with bacteremia usually result in multiple abscesses that are most commonly located in the distribution of the middle cerebral artery  . Abscesses usually form at the grey-white matter junction where micro infarction damages the blood-brain barrier.

Primary infections that lead to hematogenous seeding of the brain include :

• Chronic pulmonary infections, such as lung abscess and empyema, often in hosts with bronchiectasis or cystic fibrosis

• Skin infections

• Pelvic infection

• Intraabdominal infection

• Esophageal dilation and endoscopic sclerosis of esophageal varices

• Bacterial endocarditis (two to four percent of cases)

• Cyanotic congenital heart diseases (most common in children)

No primary site or underlying condition can be identified in 20 to 40 percent of patients with brain abscess depending upon the series .

The location of a brain abscess reflects the site of the primary infection that spreads to the cerebral cortex. These locations in the brain in order of decreasing frequency are :

• Frontal or temporal lobes
• Frontal-parietal
• Parietal
• Cerebellar
• Occipital

Computed tomographic scan  CT scan is not as sensitive as MRI for the diagnosis of brain abscess but can frequently be obtained more easily on an emergent basis. When looking for a brain abscess, this study must be performed with a contrast agent. The lesion has different appearances depending upon its age:

• Early cerebritis appears as an irregular area of low density that does not enhance following contrast injection.

• As cerebritis evolves; the lesion enlarges with thick and diffuse ring enhancement following contrast injection. The ring of contrast enhancement represents breakdown of the blood brain barrier and the development of an inflammatory capsule.

• The precontrast scan reveals a faint area of higher density than the surrounding edematous brain by the time the lesion has matured and become encapsulated. The contrast study for these late lesions demonstrates a thin ring which may not be uniform in thickness and is often less prominent on the medial surface, adjacent to the white matter, where vascularity is reduced.

Magnetic resonance imaging  MRI should be performed with gadolinium diethylenetriamine peta-acetic acid. This agent increases the T1 intensity and causes more prominent enhancement of lesions than CT scan. Compared to CT scan, MRI:

• Is more sensitive for early cerebritis

• Is more sensitive for detecting satellite lesions

• More accurately estimates the extent of central necrosis, ring

enhancement, and cerebral edema

• Better visualizes the brainstem

Diffusion-weighted MR imaging (DWI) is capable of differentiating ring-enhancing lesions due to brain abscess from neoplastic lesions . Abscesses are usually hyperintense on DWI (indicating restricted diffusion, characteristic of viscous materials, such as pus), while neoplastic lesions are hypointense or show variable hyperintensity that is lower than the intensity seen with an abscess.