Spectral waveform from the portal vein in a 10-year-old boy shows a typical flow pattern, with fairly uniform velocity and slight phasic variations due to respiration and cardiac motion.

The direction of normal portal venous flow is hepatopetal. The flow velocity is fairly uniform, although slight variations may occur in the spectral line because of respiration (Fig ). Some degree of periodicity or pulsatility resulting from cardiac activity also has been observed in normal portal veins . The mean portal venous flow velocity in a fasting adult is approximately 18 cm/sec (range, 13–23 cm/sec) . The volume and velocity of portal venous flow normally increase after meals , reflecting the increased flow in the superior mesenteric artery . The splenic vein and the superior mesenteric vein show Doppler flow patterns similar to that of the portal vein.


http://radiographics.rsna.org/content/28/3/691.full