Cause of signal loss in MR images of old hemorrhagic lesions.

Old hemorrhagic lesions in the brain are characteristically surrounded by a band of hemosiderin-containing tissue. This region is typically of low signal intensity on long-echo-time (TE) radio-frequency (RF) spin-echo magnetic resonance (MR) images and on gradient-echo MR images. To determine the cause of signal loss in this band, the authors measured the signal that arises from imaging such a region with use of an RF spin-echo technique with a 180 degrees pulse incrementally displaced from TE/2. The incremental loss of signal was small. Using an agar phantom containing iron particles, the authors also showed that signal loss results primarily from diffusion in magnetic gradients. They conclude that most signal loss in the dark band surrounding areas of late-stage hemorrhage arises from diffusion in areas of magnetic inhomogeneity.