The prestenotic ow is often normal. The higher the degree of the stenosis is, the more likely it is that proximally, the ow is lower and more pulsatile. In the intra­stenotic portion, the ow is increased and the velocities are associated with the degree of the stenosis. The latter can be determined based on the ratio of the peak systolic and end diastolic velocities and the velocities of the internal/common carotid arteries. The poststenotic ow velocity is signicantly decreased with prom­inent turbulence and loss of the systolic spectral window (the spectrum becomes broader and the area below the envelope becomes full), the spectrum becomes sharper and notched, and retrograde ow can be detected in the lateral parts of the poststenotic, slightly dilated lumen. Systolic acceleration decreases (the systolic upslope becomes less steep and the curve becomes attened – delta sign). The higher the stenotic degree, the more distally from the stenosis the spectrum begins to regenerate. The ow velocity in the internal carotid artery is often higher if the contralateral common or internal carotid is occluded [4]. This phenomenon can be observed if there is collateral ow from the intact internal carotid toward the contra­lateral middle cerebral artery (that is above the occlusion) through the anterior com­municating artery. On the normal side, the ow is hyperkinetic and the diastolic velocity is more increased than the systolic; therefore, pulsatility is increased (in these cases, the vascular resistance of the supplied areas of the two internal carotids is coupled in parallel; therefore, the resistance decreases). If there is a stenosis con­tralaterally to the occlusion, then the above-mentioned phenomenon causes a ow velocity that indicates a falsely high-degree stenosis. In this case, the degree of the morphological stenosis, that is, visualized during color mode, should be given prior­ity to [4, 7, 12, 14, 16].