Patterns of abnormal magnetic susceptibility in the brain: an image-based review

This article is designed to facilitate a systematic approach to formulating a radiologic differential diagnosis based on the pattern of abnormal magnetic susceptibility on MRI. Susceptibility-weighted imaging (SWI) is a three-dimensional sequence with excellent spatial resolution and superior contrast resolution. It originated from and has largely replaced two-dimensional T2^* weighted sequences. Currently, SWI refers to any high-spatial resolution susceptibility-enhanced sequence from different MR vendors.

There are many entities that have specific patterns unique on SWI. We chose both entities that are commonly encountered in the clinical practice and unusual entities that may present as challenges in making the diagnosis. Each entity is discussed in detail, focusing on salient imaging features seen with SWI and key differences highlighted from other entities.

In the first category, lesions with randomly distributed susceptibility abnormalities are described. Further classification is made based on the presence of foci of susceptibility located diffusely through the brain (e.g. diffuse axonal injury) noting the subtleties of the shape, size, and preferential distribution of these foci. Special attention is also directed toward entities that, while random in location, are associated with a specific lesion (e.g. abscess). Finally, clues to correctly diagnose the various pathologies are provided. In the second category, the focus is on lesions that can be classified based upon anatomic locations whether peripheral (e.g. sulcal hemosiderin) versus central distribution.

Knowledge of the patterns of susceptibility on SWI and the physics behind this technique are essential for facilitating MR interpretation.