Figure 3b. Preparation of the volume for analysis. (a) Posterosuperior image shows large veins (arrowheads), which are typically also visible in CT angiography of intracranial vessels and preclude an unobstructed view of the circle of Willis and the basilar artery (arrow). (b) Left lateroposterior image shows easy elimination of the most obscuring venous structures by using a clip plane (dotted white line) parallel to the clivus. (c) Posterosuperior image obtained after application of the clip plane (dotted white line) shows that the basilar artery is demonstrated completely (arrowheads).