Patient presented with headaches, fever and neck rigidity. CSF findings were suggestive of pyogenic meningitis.
There is a well defined hyperintense lesion with a hypointense rim on the T2W images in the left para-atrial region. It is hypointense on the T1W images and reveals rim enhancement. Perilesional edema is noted with resultant mass effect. The CSF in the posterior body and atrium of the left lateral ventricle appears more hyperintense than normal and the ventricular wall in that region shows a hyperintense signal on the FLAIR images. This wall is also seen to enhance.
Pyogenic bacterial infection of the CNS may present as focal cerebritis, abscess, meningitis and/or subdural/epidural empyema. It may result from direct spread of infection, either ENT infection or meningitis or from hematogenous spread from an extracranial source of infection.
Pathogenesis: |
Cerebritis is a localized yet poorly demarcated area of parenchymal softening with scattered necrosis, edema, vascular congestion, petechial hemorrhage and perivascular inflammatory infiltrates. It progresses to an abscess when the central zone of necrosis within, becomes liquefied, better defined and encircled by a collagen capsule (is surrounded by a zone of gliosis). The collagen capsule is less well developed on its ventricular side than on its cortical side, probably related to slight differences in perfusion. The time required to form a mature abscess varies from 2 weeks to several months. In adults, abscesses arising from hematogenous spread are most often caused by anaerobic bacteria or a mixture of anaerobes and aerobes. In children, staphylococci, streptococci, and pneumococci are the most common pathogens. In patients with a history of trauma or prior neurosurgical procedure, abscesses are usually due to Staphylococcus aureus.
On MRI: |
Cerebritis appears as an area of hyperintensity, or slightly hypointense to surrounding edema on the T2W images. On T1W images, cerebritis appears isointense to slightly hypointense to normal brain parenchyma, with associated mass effect (sulcal effacement or ventricular compression). T1W images may also reveal foci of subacute hemorrhage (hyperintense). Contrast enhancement is minimal and inhomogeneous.
Pyogenic abscesses possess characteristic MR features, which are frequently sufficient to make an accurate diagnosis. In the mature abscesses with central liquefactive necrosis, the center of the cavity is slightly hyperintense to CSF whereas the surrounding edematous brain is slightly hypointense to normal brain parenchyma on T1W images. On T2W images the signal intensities are quite variable depending on the TE chosen and the protein composition and fluidity of the material in the central cavity.
On unenhanced MR images, the mature abscess often has a rim that is isointense to slightly hyperintense to white matter on the TW1 images and is hypointense on the T2W images. The signal properties of the rim seen on MR have been attributed to collagen, hemorrhage, or paramagnetic free radicals within phagocytosing macrophages, which are distributed at the periphery of the abscess. In the latter hypothesis, localized shortening of T1 and T2 relaxation times accounts for the signal properties. However, it is also well established that free radicals promote the formation of methemoglobin, so in fact, both hemorrhage and free radicals may play a role in the generation of the described signal characteristics. The hypointense rim resolves with successful surgical and/or medical treatment of the abscess with a reduction in phagocytic activity. Thus, the rim may be a better indicator of response to treatment than residual enhancement, which can persist on contrast studies for months after completion of therapy. Other lesions that may have a hypointense rim on T2-weighted images include evolving hematomas and infrequently metastases.
The ring enhancement of an abscess capsule on post gadolinium MR images is usually smooth and thin walled and is often thinner along the medial margin. Less often nodular or solid enhancement, incomplete thin rings, or thick and irregular rings may be observed. Daughter abscesses appear as adjacent smaller enhancing rings, often along the medial margin of the parent abscess.
Edema surrounding an abscess may be greater in volume than the abscess itself, and causes much of the associated mass effect. If an abscess ruptures into the ventricular system and ependymitis develops, there is enhancement of the ventricular margin in addition to the ring enhancement of the abscess, which heralds a poor prognosis (199). Purulent material within the CSF may show increased signal intensity on both T1W and T2W images. Patients receiving steroid therapy or antibiotic treatment may show a significant reduction in the degree of ring enhancement associated with an abscess.
The standard treatment of a mature brain abscess is surgical drainage and/or excision, whereas cerebritis and some cases of abscess may be managed with antibiotics. In cases with daughter abscess formation, intra-operative ultrasound guidance is recommended to ensure that all abscesses are identified and drained. After successful surgical and/or medical treatment, serial MR scans reveal a decrease in the edema, mass effect, and degree of enhancement associated with the abscess. A small focus of enhancement may persist even after a full course of antibiotics, yet the lesion usually resolves within the succeeding 3 to 4 months. With healing, an area of gliosis, and occasionally focal calcification is all that remains. Postoperatively, the MR appearance of a resolving abscess may be complicated by the presence of subacute or chronic hemorrhage.
Differential Diagnosis:
Primary brain tumor (high-grade astrocytoma)
Granulomas
Metastasis
Infarction (bland or septic)
Resolving hematoma
Thrombosed aneurysm or an Arteriovenous malformation
Radiation necrosis
AIDS-related lymphoma
Demyelinating disease
Helpful radiographic clues that may differentiate one lesion from another include the pattern of enhancement, the presence of "daughter" rings, time course, location, ependymal and/or meningeal enhancement, and extra-axial collections. Also, an abscess will often exhibit mesial thinning of the ring.
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