Brain abscess

MRI findings and imaging semiotics of abscess

The central part of the abscess is filled with viscous detritus, which determines its signal characteristics on magnetic resonance imaging. On T1-weighted images, the signal is intermediate (between cerebrospinal fluid and white matter), whereas on T2-weighted images it becomes isointense or slightly hyperintense relative to the CSF.

The most reliable marker of an abscess is diffusion-weighted imaging (DWI). Due to the high viscosity of pus, the diffusion of water molecules is severely restricted, which is why the abscess core yields a bright (high) signal on DWI and a low signal on apparent diffusion coefficient (ADC) maps.

The abscess wall (capsule) has several specific features:

• Shape: smooth inner contour and typical thinning on the medial side.
• Enhancement: accumulates contrast agent in the form of a uniform ring. Unlike the early cerebritis stage, the abscess center does not accumulate contrast on delayed scans.
• T2-weighted images and SWI: the rim appears hypointense on T2, while on SWI it demonstrates pronounced signal loss (presumably due to the accumulation of free radicals during phagocytosis). A "dual ring sign" (dual rim sign) sometimes occurs — outer hypointensity and inner hyperintensity relative to the core.

A zone of prominent vasogenic edema forms around the capsule (bright on T2, dark on T1). The presence of gas within the cavity is rare and indicates either communication with the paranasal sinuses or previous surgery.

Differential diagnosis

The main challenge in neuroimaging is the non-specificity of ring-like contrast enhancement. A similar pattern can be produced by metastases, resolving hematomas, and malignant tumors (e.g., glioblastoma).

How to differentiate an abscess from a tumor:

1. Rim shape: tumors typically have a thick and irregular (lobulated) wall, whereas an abscess has a smooth outer edge.
2. Satellite lesions: the presence of small additional purulent foci near the main lesion is typical for infection.
3. Perfusion MRI: allows estimation of the relative cerebral blood flow (rCBV). In the contrast-enhancing rim of an abscess, rCBV values are significantly lower than in the richly vascularized wall of a glioma.

Although reduced ADC values are typical for pus, they are sometimes found in the necrotic center of metastases; therefore, imaging data are always evaluated comprehensively.

Epidural abscess and subdural empyema

Infection can localize not only in the brain parenchyma but also beneath the meninges.

Epidural abscess is a collection of pus between the inner table of the skull and the outer layer of the dura mater. It grows slowly, has a biconvex lens shape strictly limited by the skull sutures. When complicated by frontal sinusitis, it can cross the midline. On MRI, it yields a hyperintense signal on T2 and FLAIR, while the dura mater along its deep edge thickens and shows irregular contrast enhancement. The causative agent is frequently Streptococcus milleri.

Subdural empyema is more common and is located between the inner layer of the dura mater and the arachnoid mater. Pus spreads in a crescentic pattern over the convexity of the brain or along the falx cerebri. It tends to become encapsulated (the edges become scalloped). Unlike an epidural abscess, it is accompanied by prominent clinical symptoms: focal deficits, fever, and meningism.

When infection ruptures into the ventricles, ventriculitis develops. Detritus is visualized within their lumen (increased signal on FLAIR and higher density than CSF on CT), the ventricular walls enhance with contrast, and the ventricles themselves dilate.

Monitoring and treatment efficacy evaluation

Patients require comprehensive medical and neurosurgical treatment. Follow-up MRI scans are prescribed every 2 weeks or when new symptoms appear.

The main tool for evaluating treatment response is DWI, which is more accurate than standard sequences. If clinical symptoms improve, the signal on DWI decreases (correlating with a reduction of viable cells in the core). An increasing signal indicates re-accumulation of pus. The definitive imaging signs of successful therapy are considered to be the complete resolution of ring enhancement and the disappearance of the hypointense rim on T2-weighted images.

In the postoperative period in febrile patients, it can be difficult to distinguish a sterile fluid collection from an infection. In such cases, serial CT is relied upon: a gradual increase in the X-ray attenuation of the fluid indicates suppuration.

Specific infections: tuberculosis and listeriosis

In listerial meningitis (often occurring against the background of immunosuppression, e.g., in organ transplant recipients), a typical MRI finding is the lack of CSF signal suppression in the sulci on FLAIR sequences. The subarachnoid space is filled with pus, causing the sulci to appear abnormally bright.

In CNS tuberculosis (more common in individuals under 20 years of age), the basal parts of the brain become the favored location of the process — primarily the interpeduncular cistern of the midbrain. Contrast-enhanced CT reveals linear enhancement of the cisterns, with a Y-shaped enhancement pattern at the junction of the suprasellar cistern and the middle cerebral artery cisterns. The exudate impairs CSF resorption, causing communicating hydrocephalus in 50% of adults and 85% of children.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Pathophysiology of capsule formation: transition from early and late cerebritis to the encapsulation stage.
• The role of susceptibility-weighted imaging (SWI) in assessing macrophage free radicals.
• Differential diagnosis of non-specific ring enhancement: protocols and complex cases.
• Imaging management tactics for patients with sterile postoperative fluid collections.
• Mechanisms of communicating and obstructive hydrocephalus in basal tuberculous meningitis.

Other topics in the Neuroimaging section

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