Blood flow
Moyamoya disease is a disease in which certain arteries in the brain are constricted. Blood flow is blocked by constriction and blood clots (thrombosis). A collateral circulation develops around the blocked vessels to compensate for the blockage, but the collateral vessels are small, weak, and prone to bleeding, aneurysm and thrombosis. On conventional MR angiography, these collateral vessels have the appearance of a "puff of smoke" (described as "もやもや ("moyamoya")" in Japanese).