FIG 6. A, 3D CT angiogram (axial, stereoscopic views) and B, posteroanterior stereoscopic views. The bilateral BVRs drain via the lateral mesencephalic vein () into the superior petrosal sinus. In the presence of a hypoplastic third segment of the BVR, the right inferior ventricular vein () enters the lateral mesencephalic vein. The left third segment enters the great vein of Galen.