Neuroimaging of Brain Abscess

The imaging characteristics of brain abscess using CT are time-dependent and roughly correlate with the histopathological findings

[9] (Table 37.1). The evolution of a brain abscess progresses through four stages: early cerebritis, late cerebritis, early capsule formation and late capsule formation. During the cerebritis phases, there is progressive destruction of tissue, with formation of pus and cerebral edema. Capsule formation is exceedingly important, since it limits the destruction of brain parenchyma. Capsule formation is influenced by several factors, including the type of organism, the pathogenesis of infection and the use of steroids. Aerobic bacteria tend to promote the formation of a thick capsule, while anerobic organisms produce enzymes that may retard encapsulation and exacerbate cerebral edema. Post-traumatic abscesses are frequently better encapsulated than metastatic abscesses. The latter are often associated with vegetative emboli that produce micro-infarcts and secondary tissue hypoxia that impedes neovascularization and migration of fibro-blasts. Corticosteroids also can impede capsule formation. However, the potential benefit of steroids on severe cerebral edema generally outweighs any disadvantage of their use in this setting.

Experience with MRI has shown it to be more sensitive than CT in detecting the very early changes of cerebritis and very early patterns of ring-enhancement. This advantage may be of limited benefit, since most patients present with an established lesion. Perhaps of more practical value is the suggestion that MRI is more specific than CT in differentiating cerebral edema from liquefactive necrosis, which would be valuable in planning timing of aspiration

[10]. Diffusion weighted imaging (DWI) and magnetic resonance spectroscopy (MRS) have shown promise in differentiating abscess from a necrotic or cystic brain tumor. Abscess contents appear bright on DWI, while cystic tumors appear dark. Using MRS, tumor spectra are characterized by an elevated choline peak, while abscess spectra are not. Abscesses also demonstrate elevated acetate and amino acid resonances, which are not seen in tumor spectra. For maximum accuracy, spectra should be obtained so that the wall of the lesion is dominant in the acquisition voxel. Given that DWI and MRS may be performed simultaneously with routine MRI, a combination of MR techniques may provide the diagnostic study of choice in the evaluation of suspected abscesses [11].

Occasionally, CT and/or MRI may be inconclusive. In these cases, Indium111-radiolabeled white blood cell (WBC) imaging may help clarify the diagnosis. Indium111 WBC imaging should theoretically be able to distinguish abscess from neoplasm. The technique is easy and non-invasive and can provide information that might influence management. The main disadvantage is that optimal images are obtained 24 hours following injection (although positive images are sometimes obtained at 6 hours) and, therefore, this test is best suited to patients who do not require emergent surgery. The diagnostic accuracy has been reported to be around 96%, with a sensitivity and specificity 100 and 94%, respectively [12]. In spite of the high sensitivity, false-negative scans may occur in tumors that have undergone extensive necrosis of such magnitude as to incite an inflammatory response sufficient to be detectable by labeled leukocytes. Other radionuclide scans such as thallium201 SPECT are useful for differentiating lymphoma from toxoplasmosis in AIDS patients. Thallium201 is taken up by lymphoma but not by toxoplasma abscesses. SPECT thallium may also help differentiate tumor from abscess in non-AIDS patients, although false-positive results have been reported [11].

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