Internal carotid artery stenosis (ICAS) is a well-known risk factor for stroke. While collaterals play a pivotal role in ischemic stroke, their effects in ICAS are not entirely understood. We assessed primary collaterals, i.e., configuration of the Circle of Willis by MR-Angiography, and high Coefficients of Variance in Dynamic Susceptibility Contrast MRI as surrogate of secondary collaterals. Moreover, individual watershed areas (iWSA) were determined in a group of ICAS patients and healthy volunteers. Our results demonstrate post-interventional shifts of iWSA, mainly influenced by primary collateral flow. No secondary collateral flow was found in patients similar to age-matched healthy participants.