Portal confluence vessels
The major branches of the portal confluence are the portal vein, superior mesenteric vein, and splenic vein (discussed above). The portal vein appears as a hypo-echoic structure posterior to the duodenal bulb. With the sector scan instrument, the portal vein is visualized along its longitudinal axis, adjacent to the bile duct which runs between it and the duodenal wall. Without too much difficulty, this vein can be traced to its bifurcation into right and left branches in the liver. Obtaining these images of the portal vein in relation to the common bile duct is considerably more difficult with the linear array instrument. With this instrument, constant adjustments to torque or change the transducer's position must be made because these structures follow a route almost perpendicular to that of the scanning plane in the duodenal bulb. The linear array transducer must be torqued a full 180 degrees in the first part of the duodenum to visualize the portal vein in the hilum of the liver.
As the transducer is moved from the horizontal into the descending duodenum, the mesenteric vessels come into view. From this position the superior mesenteric vein lies to the right and slightly posterior to the superior mesenteric artery. Since both vessels are anterior and superior to the duodenum, the superior mesenteric vein, which generally appears near the five o'clock position on a sector scan, will be counter-clockwise to the superior mesenteric artery, which generally appears near the six o'clock position. The superior mesenteric vein can be traced into the portal confluence in some patients with slow withdrawal of the scope. However, in many patients, the duodenal anatomy does not allow maintenance of the required acoustic window needed to see this junction. Although the images obtained on a 120 degree scan of the linear array instrument are similar to those of the sector scan instrument, the superior mesenteric artery tends to be further from the transducer and slightly posterior to the superior mesenteric vein when both vessels are in the image together. In the optimal view of the superior mesenteric vein in relation to the pancreas, the superior mesenteric artery tends not to be seen on the imaging plane.
The mesenteric vessels can also be seen from the stomach or from the withdrawn-wedged position. As in a conventional transcutaneous ultrasound view, the superior mesenteric artery appears as a circular structure below the splenic vein and to the left of the portal-splenic-superior mesenteric vein confluence.
The primary manner in which the two existing scanning instruments differ in imaging vessels is noted at the portal confluence. The splenic vein and portal vein can be seen on one image with the sector scanning scope, but only a short segment of the superior mesenteric vein is generally seen. With the linear array instrument, the portal vein is difficult to follow from the portal confluence, both with the transducer in the stomach or in the duodenal bulb, because the scope must constantly be torqued counterclockwise while advancing to follow the course of the vein. During this maneuver, the other vessels move off the screen. On the plus side, the linear array instrument allows about an extra 1 cm segment of the superior mesenteric vein and perhaps even the artery to be visualized simultaneously on screen with the portal confluence. When following this extra cm of mesenteric vessel, however, the pancreas is likely to disappear off-screen, limiting the clinical value of this information.
Color doppler scanning, available only with the linear array instrument, will identify arterial from venous blood flow in the portal confluence/superior mesenteric vein and superior mesenteric artery. Otherwise, the images of the portal confluence and superior mesenteric artery obtained with the two instruments are comparable.