Gadolinium-based intravenous contrast agents used in magnetic resonance imaging (MRI) appear hyperintense on T1 sequences. These agents do not cross the blood–brain barrier in normal patients, and thus T1 hyperintensity from contrast should primarily be present in the brain vasculature and regions of brain without a blood–brain barrier, such as the choroid plexus. When an enhancing lesion is present, this indicates either breakdown of the existing blood–brain barrier or angiogenesis of blood vessels without a blood–brain barrier (as may be seen with a tumor or after cerebral infarction). If a lesion has enhancement of its peripheral rim but not its central core, it is frequently referred to as a ring-enhancing (or more accurately “rim-enhancing”) lesion.
Active demyelinating lesions in multiple sclerosis can have different enhancement patterns, including complete enhancement, heterogenous enhancement, or an incomplete rim of enhancement ( Fig. 117.1 ). The latter refers to a pattern where the enhancing rim around the lesion is not completely circumferential; the segment of the rim that is not enhancing typically faces the ventricle. An incomplete rim of enhancement is unusual in a neoplasm or abscess, and thus its presence suggests demyelinating disease. Active demyelinating lesions in multiple sclerosis typically enhance for about 3 weeks. Thus, if clinical uncertainty persists as to whether a lesion with an incomplete rim of enhancement is secondary to demyelination, follow-up imaging in about one month can be helpful. If enhancement has resolved at that point, this is most consistent with demyelination.
Restricted diffusion on diffusion weighted imaging indicates decreased mobility of water molecules, which can be caused by increased intracellular water as seen in cytotoxic edema or by increased cell density as can be seen in high-grade malignancies. In glioblastoma, where the central core is typically necrotic, the enhancing rim may show restricted diffusion indicating high density of tumor cells at the growing periphery. This may also be seen in lymphoma, particularly in immunocompromised patients.
The classic appearance of an abscess is a complete rim of enhancement due to the angiogenesis required by the fibroblasts to create the wall of the abscess, and restricted diffusion in its core due to high cellularity of pus ( Fig. 117.2 ). An abscess also typically has associated vasogenic edema out of proportion to the size of the abscess.