Moyamoya disease is a progressive disorder that affects the blood supply to the brain. Instead of one large carotid artery on each side, people with Moyamoya have a network of small, vulnerable arteries.  They look like a puff of smoke on a dye arteriogram, and moyamoya means 'a puff of smoke' in Japanese.

You can read more about Moyamoya disease here:

http://www.brain-aneurysm.com/mmd.html

http://www.stanfordhospital.com/clinicsmedServices/COE/neuro/moyamoyaDisease/default

http://www.ninds.nih.gov/disorders/moyamoya/moyamoya.htm

http://www.moyamoya.com/

http://en.wikipedia.org/wiki/Moyamoya_syndrome

http://www.emedicine.com/neuro/topic616.htm