Brain lesion. Coloured axial (horizontal) magnetic resonance imaging (MRI) scan through the head of a patient with a brain lesion (red). The front of the brain is at top in this view from below. The lesion is in the brain's left hemisphere. It is a wart-like growth (papilloma) on an internal brain membrane (the choroid plexus). Warts are caused by a viral infection, and are not normally malignant. However, when their growth threatens an organ such as the brain, surgical removal is required. MRI scanning uses powerful magnetic fields and pulses of radio waves to form slice images of the body.