Figures 1-3 (above): 3d TOF and axial T2 images of probable (as unilateral) moyamoya. Note the carotid and MCA occlusion on the right with extensive new vessel formation.
Moyamoya means puff of smoke in japanese and this refers to the appearance of multiple, fine collateral vessels seen on cerebral angiography in patients suffering from this disease. There is a progressive occlusion of the distal internal carotid arteries and the arteries of the circle of Willis.
Children usually present with ischaemia (TIA or infarction) and adults with haemorrhage from the delicate abnormal vessels.
Further reading: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1151702/