Susceptibility-weighted imaging in patients with pyogenic brain abscesses at 1.5T: characteristics of the abscess capsule

Abstract

Background and purpose: SWI is a high-resolution 3D, fully velocity-compensated gradient-echo sequence that uses both magnitude and phase data. The purpose of this study was to investigate the phase behavior of the capsule of pyogenic brain abscesses with noncontrast SWI.

Materials and methods: Fourteen patients with pyogenic brain abscesses were studied at 1.5T. In all of the patients, SWI images were obtained and reviewed in addition to conventional MR images. Phase values within the abscess capsule were measured and compared with those from the abscess cavities and contralateral normal white matter using 1-way repeated measures ANOVA with post hoc Bonferroni analysis.

Results: SWI phase images showed mild hypointesity in 6 patients, isointensity in 3 patients, and mixed iso- to mild hypointensity in 5 patients. The means of phase in the cavity, rim of abscesses, and contralateral normal white matter were -7.552 × 10(-3) ± 0.024, -0.105 ± 0.080, and +0.029 ± 0.011 radians, respectively. Post hoc comparisons showed significant differences between any pair of the 3 regions (abscess cavity, rim capsule, and normal white matter) in SWI (all Ps < .005).

Conclusions: SWI phase imaging shows evidence of paramagnetic substances in agreement with the presence of free radicals from phagocytosis. SWI may provide additional information valuable in the characterization of pyogenic brain abscesses.

Fig 1.

Box plots of the phase values (radians) of corrected phase images on high-resolution SWI in the cavity, rim of the abscesses, and contralateral normal white matter in patients with pyogenic abscesses. The middle horizontal line is the median, the upper and lower ends of the boxes are the 3rd and 1st quartiles, and the vertical lines show the full range of values in the data.

Fig 2.

A 57-year-old man with surgically proved pyogenic brain abscess in right midbrain. A, T1WI; B, T2WI; C, GRE T2*WI; D, DWI; E, ADC; F, postcontrast T1WI. The original nonmagnified image is shown in A (right bottom). The rim is isointense on T1WI, hypointense on T2WI and GRE-T2*WI, and homogeneously marginally enhanced on postcontrast T1WI. DWI shows hyperintensity, and ADC map reveals hypointensity in the cavity, suggesting restricted water diffusion.

Fig 3.

A 57-year-old man with right midbrain pyogenic abscess. A, magnitude image; B, corrected phase image; C, SWI image. SPGR SWI corrected phase image shows mild hypointense rim delineates the periphery of the lesion, suggesting an increased venous vasculature with rich deoxy-hemoglobin, blood products due to hemorrhage, or presence of paramagnetic free radicals from phagocytosis.