Direct signs of vessel abnormality include increased MFV (>120 cm/s) or a >50% increase in velocity compared with an unaffected segment of the vessel. Severely reduced or absent MFV, increased PI, and increased contralateral MFV may be an indirect sign of VA dissection; however, variability in VA diameter is common in the population confounding this examination observation. Occlusion of the vessel detected by a focal absence of ow particularly in the intertransverse seg­ments should raise concern for vertebral dissection. An intraluminal abnormality such as an echo-lucent hematoma or double lumen sign may also suggest dissection in the right clinical context [23]. In this respect, color-coded Doppler can be useful in visualizing segmental dilation or an eccentric channel in the proximal or distal parts of the artery with an increased velocity in the residual channel [18]. Segmentally, the proximal VA (V1–V2) can have distinct ndings of increased arte­rial diameter and decreased PI.The atlas loop is a vulnerable, mobile segment of the VA to which additional attention should be paid, and signs of absent ow signals, low bidirectional ow signals or low post-stenotic ow signals should prompt fur­ther investigation with angiography [19]. Ultrasound has also been shown to be useful in determining the length of dissection in addition to being a practical means of follow-up in cases of angiographic conrmed dissection [18]. The sensitivity of sonographic evaluation of the posterior circulation for dissection is preserved only when all potential abnormalities are considered, and sensitivity decreases when denite abnormal ndings of absent or severely reduced ow velocities, absent dia­stolic ow, bidirectional ow or stenotic signals are used as strict criteria [19].