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Bacterial Abscess

Last Updated: October 1, 2018

Open Table of Contents: Bacterial Abscess

Figure 1: MRI demonstrates multiple FLAIR hyperintense lesions with primarily central reduced diffusivity with spread into the posterior horns of the lateral ventricles (best demonstrated on the DWI and FLAIR sequences). The central location is fairly classic and suggests a systemic cause of the infection.

Description

  • Focal infection that begins as an area of cerebritis and develops into a collection of encapsulated pus
  • Mechanisms:
    • Direct inoculation and hematogenous spread

Pathology

  • Britt and Enzmann classification
  • Early cerebritis (1st-4th day)
    • Ill-defined focal infection
  • Late cerebritis (5th-10th day)

    • Undergoes central necrosis and vascular proliferation
  • Early capsule (10th-14th day)

    • Develops a well-defined capsule
  • Late capsule (2 weeks to months)

    • Thickened capsule with shrinkage of the central cavity

Clinical Features

  • Signs and symptoms
    • Only 40-50% of patients are febrile
    • Headache, seizures, altered mental status, focal deficits, nausea and vomiting
  • Demographics

    • 2x more common in men
    • Most common in 3rd and 4th decades
  • Prognosis

    • Mortality rates are highly variable but typically up to 30%
    • Based on virulence of the organism, size and location of the infection

Treatment

  • If < 2.5 cm antibiotics only
  • If > 2.5 cm surgical evacuation and antibiotics
  • Steroids for edema and mass effect

Imaging

  • General
    • Highly variable depending on stage of development
    • Most commonly supratentorial
    • Location may clue into site of origin (whether hematogenous or the primary direct spread infection site)
  • CT

    • Initially, appears as an ill-defined, hypodense lesion with mass effect, vasogenic edema and patchy enhancement
    • Develops a capsule and central necrosis
      • Capsule initially demonstrates irregular rim enhancement and over time becomes thickened with continuous peripheral enhancement
      • Inner rim of enhancement tends to be quite smooth, helping to differentiate from the irregular enhancement in the necrotic center of high grade tumors
  • MR

    • Variable depending on the stage
    • DWI is often the most helpful sequence
    • Resolving abscess: T2 hypointense rim resolves, central ADC increases, enhancement is the last to resolve
    • T1WI
      • Initially will be an ill-defined, heterogeneous hypo to isointense mass
      • As it walls off, will develop definable rim that is often iso to hyperintense and central hyperintense signal
      • The cavity will eventually shrink and the capsule will thicken
    • T2WI

      • Hyperintense mass that develops a hypointense rim and demonstrates significant surrounding edema until it begins to resolve
    • DWI

      • Restricted diffusion (bright on DWI, dark on ADC) throughout the necrotic core, often (though not always) strikingly homogeneous.
      • This is in contrast to restricted diffusion seen in hypercellular viable tumor, in which the necrotic center (unless hemorrhagic) only rarely demonstrates restricted diffusion
    • SWI

      • “Dual rim” sign
        • Hypointense along the periphery of the capsule and hyperintense along the inner portion of the capsule
    • Contrast

      • Early patchy peripheral enhancement that becomes intense, continuous and thickened with time
      • Inner rim of enhancement tends to be quite smooth, helping to differentiate from the irregular enhancement in the necrotic center of high grade tumors
    • 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), metabolites usually not present in tumor
    • PWI

      • Abscess capsule shows lower rCBV values than rims of enhancing tumors
  • Imaging Recommendations

    • Standard protocol MR with intravenous contrast to include DWI. MRS and PWI can be helpful
  • Mimic

    • Bacterial abscess can mimic most ring enhancing lesions, but classically can be differentiated by central diffusion restriction and potential secondary causative etiologies, such as mastoiditis or sinusitis.

For more information, please see the corresponding chapter in Radiopaedia.

Contributor: Sean Dodson, MD

DOI: https://doi.org/10.18791/nsatlas.v1.03.02.05

References

Bajpai A, et al. Multimodal approach for diagnosis of bacterial etiology in brain abscess. Magn Reson Imaging. 2014 Jun;32(5):491-6.

Chaudhary V, Bano S. MR Spectroscopy in the Diagnosis of Bacterial Etiology in Brain Abscesses. Magn Reson Imaging. 2014 Dec; 32(10):1434.

Rabelo NN, et al. Differential Diagnosis between Neoplastic and Non-Neoplastic Brain Lesions in Radiology. Arq Bras Neurocir. 2016. Doi: 10.1055/s-0035-1570362.

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