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Abscess


KEY FACTS

Terminology

  • Focal pyogenic infection of brain parenchyma, typically bacterial; fungal or parasitic less common

  • 4 pathologic stages: Early cerebritis, late cerebritis, early capsule, late capsule

Imaging

  • Ring-enhancing lesion with T2 hypointense rim and central diffusion restriction characteristic

  • Imaging varies with stage of abscess development

    • Early cerebritis: Ill-defined T2 hyperintense mass

  • Findings with contrast enhancement

    • Early cerebritis: Patchy enhancement

    • Late cerebritis: Intense, irregular rim enhancement

    • Early capsule: Well-defined, thin-walled, enhancing rim

    • Late capsule: Cavity collapses, capsule thickens

  • MRS: Central necrotic area may show presence of amino acids (0.9 ppm), lactate (1.3 ppm), acetate (1.9 ppm), succinate (2.4 ppm)

  • Multiplanar MR ± contrast, DWI, ± MRS, PWI

  • Dual-rim sign on SWI (hypointense outside, hyperintense inside) helpful in from other ring-enhancing lesions

Top Differential Diagnoses

  • Glioblastoma

  • Parenchymal metastases

  • Demyelinating disease

  • Resolving intracerebral hematoma

  • Subacute cerebral infarction

Clinical Issues

  • Headache (up to 90%); may have seizures, altered mental status, focal deficits, nausea, vomiting

  • Potentially fatal but treatable lesion

Diagnostic Checklist

  • DWI, MRS helpful to distinguish abscess from mimics

Axial graphic shows the early capsule formation of an abscess with central liquified necrosis and inflammatory debris. Collagen and reticulin form the well-defined abscess wall
are shown. Note the surrounding edema
.

Axial T2WI in a 66-year-old woman with headaches, fever 4 weeks following a dental procedure, shows a mixed signal intensity mass in the left posterior temporal lobe
. The lesion has a “double rim” (hypointensity on the outside, hyperintensity on inside), commonly seen in pyogenic abscesses.

T1 C+ FS in the same case shows irregular rim enhancement
surrounding a nonenhancing center. Note: The lesion in the right occipital lobe
is a remote infarct.

DWI in the same case shows that the lesion strongly restricts. The imaging findings are very characteristic of a pyogenic abscess. Diffusion restriction in other rim-enhancing lesions such as metastasis, glioblastoma, and multiple sclerosis is uncommon.

TERMINOLOGY

Definitions

  • Localized infection of brain parenchyma, typically bacterial; fungal or parasitic less common

  • 4 pathologic stages: Early cerebritis, late cerebritis, early capsule, late capsule

IMAGING

General Features

  • Best diagnostic clue

    • Imaging varies with stage of abscess development

    • Early capsule: Well-defined, thin-walled enhancing rim

      • –

        Ring-enhancing lesion: DWI high signal, low ADC

      • –

        T2 hypointense rim with surrounding edema

  • Location

    • Typically supratentorial; up to 14% infratentorial

    • Frontal lobe (sinusitis, odontogenic infection), temporal lobe (otomastoiditis)

    • Usually at gray-white junction (hematogenous)

    • Multiple lesions may represent septic emboli

  • Size

    • 5 mm up to several cm

  • Morphology

    • Thin-walled, well-delineated, ring-enhancing, cystic-appearing mass

CT Findings

  • NECT

    • Early cerebritis: Ill-defined, hypodense subcortical lesion with mass effect; CT may be normal early

    • Late cerebritis: Central low-density area; peripheral edema, increased mass effect

    • Early capsule: Hypodense mass with moderate vasogenic edema and mass effect

    • Late capsule: Edema, mass effect diminish

    • Gas-containing abscess rare

  • CECT

    • Early cerebritis: ± mild patchy enhancement

    • Late cerebritis: Irregular rim enhancement

    • Early capsule: Low-density center with thin, distinct enhancing rim

      • –

        Medial or ventricular wall of capsule thinnest; thickest near cortex

    • Late capsule: Cavity shrinks, capsule thickens

      • –

        May have “daughter” abscesses

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