Collateral circulation refers to the occlusion of a large vessel or a severe stenosis of a large vessel, but small capillaries can emerge or be generated from the side to supply the area innervated by the large vessel, so that the area supplied by the large vessel can function normally, such as the microcirculation of the heart, collateral circulation, or collateral circulation of the cerebral vessels. However, a sudden occlusion of a large vessel or a short-lived stenosis will not generate collateral circulation. The establishment of collateral circulation is a chronic process that takes at least three months, and the gradual narrowing of the large vessels, which is more severe, can gradually generate peripheral collateral circulation. A functional MRI or high field strength MRI is required to visualize the vascular network of the collateral circulation, which is relatively slender and cloudy.