Focal signal-intensity variations in the posterior internal capsule: normal MR findings and distinction from pathologic findings.

Cranial magnetic resonance (MR) imaging studies in 117 control patients were reviewed to evaluate for focal signal-intensity variations in the posterior internal capsule. Rounded foci of increased signal intensity were found near the junction of the posterior limb and retrolenticular portion of the internal capsule on axial T2-weighted images in 56% of patients imaged at 1.5 T and in 50% imaged at 0.5 T. Corresponding hypointense foci were found on T1-weighted images in 64% of control patients imaged at 1.5 T and in 69% imaged at 0.5 T. With all sequences, the foci were homogeneous and well defined, without mass effect, and bilaterally symmetric. Comparison was made with MR imaging studies in 32 patients with pathologic involvement of the posterior internal capsule region. Symmetric morphology and signal intensity, sharply defined margins, confinement to a characteristic location, signal intensity approximating that of cortical gray matter on T2-weighted images, and isointensity or hypointensity on spin-density-weighted images appear to characterize normal posterior capsular foci and distinguish them from foci of pathologic lesions. A region that stained less intensely than the surrounding internal capsule was noted in tissue blocks from two normal brain specimens, corresponding in morphology and position to the signal-intensity changes seen on MR images. The properties of this focus suggest that it may represent fibers of the parietopontine tract.