Brain tumour. Magnetic resonance imaging (MRI) scan of a coronal (front) section through the head of a 43 year old woman with low grade glioma. Glioma, a central nervous system tumour, arises from glial cells. It is seen here as a grey mass (top right) within a cyst (pale). About half of all primary brain tumours are gliomas. Low grade gliomas are slow growing tumours, which are much less likely to come back if they can be completely removed surgically. Radiotherapy may be kept in reserve as a treatment for if the glioma returns. MRI uses radio waves and a magnet to obtain inssliceins body images.