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PubMed2001

Restricted diffusion within ring enhancement is not pathognomonic for brain abscess.

BACKGROUND AND PURPOSE: Recent experience suggests that diffusion-weighted MR imaging may be decisive in the differential diagnosis of ring-enhancing cerebral lesions. Whether restricted diffusion within a ring-enhancing cerebral mass lesion is pathognomonic for abscess was studied. METHODS: Seventeen patients with ring-enhancing cerebral lesions (three abscesses, six glioblastomas, eight metastases) on conventional contrast-enhanced T1-weighted images were examined with echo-planar diffusion-weighted MR imaging. Apparent diffusion coefficient (ADC) maps and the ADCs were calculated for all lesions. Lesions with signs of intralesional hemorrhage on unenhanced T1-weighted images were excluded. RESULTS: The central portion of all six glioblastomas and seven of eight metastases showed unrestricted diffusion, whereas two of three abscesses showed restricted diffusion (low ADC values) in their cavity. However, restricted diffusion also was found in one metastasis, and one abscess within a postoperative cavity showed unrestricted diffusion within a larger nondependent portion. CONCLUSION: In patients with ring-enhancing cerebral mass lesions, restricted diffusion might be characteristic but is not pathognomonic for abscess, as low ADC values also may be found in brain metastases.

Brain Abscess↗
PubMed1958

Brain abscess.

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Abscess↗
PubMed1989

MR imaging of brain abscesses.

The MR images and CT scans of 14 patients with surgically verified pyogenic cerebral abscesses were reviewed. The MR findings correlated well with those seen on CT and were believed to be sufficiently characteristic to allow early and accurate diagnosis with MR alone. These features include (1) peripheral edema producing mild hypointensity on short TR/short TE and marked hyperintensity on long TR/intermediate to long TE scans; (2) central necrosis with abscess fluid hypointense relative to white matter and hyperintense relative to CSF on short TR/short TE scans and hyperintense relative to gray matter on long TR/intermediate to long TE scans (the fluid had concentric zones of varying intensity in seven cases, a finding not previously identified in other lesions); (3) extraparenchymal spread (intraventricular or subarachnoid), which was detected more easily on MR than on CT and was manifested by increased intensity relative to normal CSF on both short TR/short TE and long TR/intermediate TE scans; and (4) visualization of the abscess capsule, which was iso- to mildly hyperintense relative to brain on short TR/short TE scans and iso- to hypointense relative to white matter on long TR/intermediate to long TE scans. On the long TR scans, the relative hypointensity of the rim allowed for visualization of the typical morphologic features of the capsule, which in turn aided in differentiation of abscesses from other lesions (as it does on CT). To investigate the cause of the capsular intensity, pathologic studies of the capsules were reviewed when available (10 cases). Fibrosis was identified in all mature abscess capsules, but the combination of the intensities seen on short TR/short TE and long TR/intermediate to long TE scans as well as the temporal changes in intensity were believed to be incompatible with fibrosis as a cause of the capsular changes. Intensity patterns were suggestive of hemorrhage, but neither acute nor chronic hemorrhage was identified on routine H and E stains, while iron stain revealed scant hemorrhage in only two of the eight patients in whom these stains were used. We believe the capsular intensity (in particular the hypointense rims on long TR scans) may reflect paramagnetic T1, and to a greater extent T2, shortening, possibly due to the presence of heterogeneously distributed free radicals that are products of the respiratory burst produced by actively phagocytosing macrophages in the capsule wall. Distinctive MR features of pyogenic abscesses should afford early and accurate diagnosis.

Brain↗