High fractional anisotropy in brain abscesses versus other cystic intracranial lesions.

BACKGROUND AND PURPOSE It is known that intracranial mass lesions are relatively isotropic on diffusion-weighted imaging. The purpose of this study is to report an unusually high fractional anisotropy (FA) and mean diffusivity (D(av)) in the cavity of the brain abscess compared with other cystic lesions. METHODS We performed diffusion tensor imaging (DTI) in 12 patients with cystic intracranial lesions (pyogenic abscess, n = 5; cysticercus cysts, n = 2; and low-grade astrocytoma, n = 5). Mean FA, D(av) from the lesion core, perifocal edema, and corresponding contralateral normal-appearing regions were measured and compared for relative changes in these parameters. In the abscess cases, we placed regions of interest on areas with FA >0.2 and FA <0.2 to get FA and D(av) values. RESULTS There were two patterns of FA values in the abscess cavity in all five patients. Part of the abscess showed mean FA = 0.440 +/- 0.135, with D(av) = (0.993 +/- 0.185) x 10(-3) mm(2)/s, whereas other parts had FA = 0.131 +/- 0.039 with D(av) = (0.824 +/- 0.183) x 10(-3) mm(2)/s. The cystic tumors and neurocysticercosis showed very high D(av) = (2.806 +/- 0.25, 2.654 +/- 0.35)x 10(-3) mm(2)/s, with low FA = (0.108 +/- 0.037, 0.08 +/- 0.01), respectively. CONCLUSION Brain abscess cavity shows regions of increased FA values with restricted mean diffusivity compared with other cystic intracranial lesions. This information may prevent misinterpretation of the DTI information as white matter fiber bundle abnormalities associated with mass lesions.

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