2. Bilateral progressive steno-occlusive changes of supraclinoid ICA segments involving carotid fork region; Unilateral involvements (overall 20%) are also seen “Puff of Smoke”; (1) Classical basal collaterals recruiting leptomeningeal vessels and deep parenchymal vessels of striatum (2) Collateral vessels are formed during prenatal period Pathophysiology; Fibrocellular intimal thickenings with wavy internal elastic lamina and thinning of media Moyamoya disease (MMD)