Hairy cell leukaemia. Coloured magnetic resonance imaging (MRI) scan of a section through the pelvis of a 70-year-old patient with hairy cell leukaemia, showing the presence of bony structures and multiple lesions affecting the lacunae (spaces) within the bone tissue and bilateral haematomas (collections of blood) in the muscle tissue. Hairy cell leukaemia is a rare blood cancer characterised by an accumulation of abnormal B lymphocyte white blood cells.