Color Doppler. Color Doppler mapping showing the origin of the right branch of the portal vein in one dog. Note that the axis of the vessel is very close to that of the ultrasound wave, which provides an optimal insonation angle, with the flow running towards the transducer. Longitudinal section at approximately the right 10th intercostal space (RBPV: right branch of portal vein; CVC: caudal vena cava; AA: abdominal aorta; PV: portal vein).