Brain abscess. Sequence of axial magnetic resonance imaging (MRI) scans showing the internal structure of the patient's head and neck. The abscess (swelling caused by accumulation of pus) is seen in the left frontal lobe; it is large, well defined, peripherally enhancing (bright around edges), and surrounded by parenchymal oedema (fluid build up). There is some accumulation of mucus in the sinuses (left frontal, left ethmoid, and both maxillary sinuses) and mild opacification of the mastoid air cells (seen as black areas), bilaterally (both sides). The abscess and surrounding oedema are causing various distortions to structures within the brain, most notably a midline shift to the right and local sulcal effacement (obscuring of the grooves between folds in the cerebrum).