Anatomy of Alimentary Tract Vasculature

Ultrasound

The use of abdominal ultrasound in the evaluation of the GI tract vasculature is limited to visualization of the larger vessels such as the major branches of the aorta (celiac trunk, superior mesenteric artery) and the portal venous system. The segmental branches of the small bowel mesentery and inferior mesenteric artery are usually not visualized due to their small caliber. Other limitations of ultrasound include patient body habitus, the experience of the sonographer, and overlying bowel gas. The two most common clinical situations where ultrasound is helpful in making a diagnosis is in the evaluation of chronic mesenteric ischemia due to median arcuate ligament syndrome and when evaluating the portal venous system. Using color and Doppler sonography, both entities can easily be diagnosed by measuring flow velocities, the direction of flow, and vessel patency.

Computed Tomographic Angiography and MRA

Computed tomographic angiography (CTA) and MRA are the two most commonly used noninvasive imaging modalities for the diagnosis of mesenteric ischemia in the acute setting. Both examinations involve the administration of iodinated contrast medium for CTA and gadolinium for MRA, and can only be performed in patients with normal renal function. With the advent of volumetric acquisition of images and 3D reconstruction algorithms, both the mesenteric arterial and venous systems can be adequately visualized, especially when using dual-phase imaging (arterial and venous phases). Both CTA and MRA are highly accurate in the diagnosis of acute mesenteric artery thrombosis and mesenteric and portal venous thrombosis by demonstrating either a filling defect within the occluded vessels or an abrupt cutoff of the affected vessels. In addition, these modalities may show secondary signs of ischemia, such as bowel-wall thickening and mesenteric edema or hemorrhage. In patients with renal impairment, non–contrast-enhanced MRA using balanced steady-state free precession can be used as an alternative to imaging the aorta and its branches. Some advantages of using CTA over MRA include its greater availability in most institutions, faster acquisition time, and improved spatial resolution allowing better visualization of smaller peripheral vessels of the mesenteric circulation. Furthermore, CTA can evaluate the presence of atherosclerotic disease by visualizing calcified plaque, which is very difficult to perceive on MRA.

Catheter Angiography

Catheter angiography is still considered by many radiologists to be the gold standard for imaging the aorta and the mesenteric arteries and veins, even though it remains an invasive test and involves the use of ionizing radiation and the administration of iodinated contrast material. It remains the study of choice when nonocclusive mesenteric ischemia is suspected. In patients with renal impairment, carbon dioxide can be safely substituted for iodinated material as the contrast medium. One of the biggest advantages of using conventional angiography over other imaging modalities is that it can serve as a diagnostic and therapeutic tool when necessary. Vasodilators or thrombolytics can be administered at the time of the examination, in addition to visualizing possible collateral circulations.

Celiac Axis

The celiac axis arises from the ventral surface of the aorta at the level of the T12-L1 disk space. It immediately divides into three branches–—the left gastric artery, splenic artery, and common hepatic artery—and supplies the liver, spleen, stomach, and pancreas ( Fig. 25.1 ). Both the splenic artery and the left gastric artery can arise directly from the aorta as separate branches, but in more than 95% of patients, the common hepatic artery arises from the celiac trunk.

The left gastric artery is the smallest branch of the celiac axis. It supplies the gastroesophageal junction (abdominal part of the esophagus) and anastomoses with esophageal branches from the thoracic aorta. It also supplies the fundus and a portion of the body of the stomach and anastomoses with the right gastric artery, a branch of the hepatic artery.

The splenic artery is the largest branch of the celiac trunk. It has a tortuous course to the left along the superior border of the pancreas and has three branches:

• • Pancreatic branches. As the hepatic artery passes along, it gives off small branches to supply the neck, body, and tail of the pancreas.

  • Short gastric arteries, which supply blood to the fundus of the stomach.

  • Left gastroepiploic artery (terminal branch of the splenic artery), which runs along the greater curvature of the stomach and anastomoses with the right gastroepiploic artery.

The common hepatic artery is a medium-sized branch of the celiac trunk and runs to the right. It divides into two major branches:

• • Proper hepatic artery, which ascends toward the liver and is located on the left side of the bile duct and anterior to the portal vein. It gives off two branches: the right and left hepatic arteries.

  • Gastroduodenal artery, which descends posterior to the superior part of the duodenum. At the lower part of the superior duodenum, the gastroduodenal artery divides into the right gastroepiploic artery (which supplies both surfaces of the stomach and the greater curvature) and the superior pancreaticoduodenal artery. The second branch divides into anterior and posterior branches and supplies the head of the pancreas and the duodenum.

Other branches of the common hepatic artery include the supraduodenal and right gastric arteries.

Arterial anastomoses in the stomach include:

• • Left gastric, right gastric, and short gastric arteries from the spleen. This anastomosis makes an arcade along the lesser curvature of the stomach.

  • Right gastroepiploic artery (terminal branch of the gastroduodenal artery), which forms anastomoses with the left gastroepiploic artery (terminal branch of the splenic artery). These vessels form an arcade along the greater curvature of the stomach.

Superior Mesenteric Artery

The superior mesenteric artery (SMA) is one of the major blood supplies to the lower GI tract ( Figs. 25.2 and 25.3 ). It is located inferior to the celiac axis (usually within 2 cm and at the level of L1). Anatomically, the SMA is crossed anteriorly by the splenic vein and the neck of the pancreas, and as it descends, it passes anterior to the left renal vein, the uncinate process of the pancreas, and the inferior part of the duodenum. The SMA supplies the duodenum, pancreas, entire small bowel, appendix, ascending colon, and approximately two-thirds to three-fourths of the transverse colon. Its branches are the:

• • Inferior pancreaticoduodenal artery, the first branch of the SMA. It divides into anterior and posterior branches and, as mentioned earlier, forms an anastomosis with the superior pancreaticoduodenal artery. This arcade supplies the head and uncinate process of the pancreas and the duodenum.

  • Jejunal arteries, which arise from the left side of the SMA and supply the jejunum.

  • Ileal arteries, located on the left side of the SMA. These arteries supply most of the ileum.

  • Ileocolic arteries, which arise from the right side and supply the terminal ileum, cecum, and lower ascending colon. The superior branch anastomoses with the right colic artery, and the inferior branch continues toward the ileocolic junction and divides into colic, cecal, appendicular, and ileal branches.

  • Right colic artery, which arises from the right side of the SMA and supplies the ascending colon. It divides into ascending and descending branches that anastomose with the middle colic artery.

  • Middle colic artery, which arises from the right side of the SMA and divides into right and left branches. The right branch forms an anastomosis with the right colic artery, and the left branch forms an anastomosis with the left colic artery, a branch of the inferior mesenteric artery.

Inferior Mesenteric Artery

The inferior mesenteric artery (IMA) is another major blood supply to the lower GI tract ( Fig. 25.4 ). It is located at the level of L2-L4 (most often at the L3-L4 disk space level, 2–3 cm above the aortic bifurcation). The IMA supplies the distal transverse colon, descending colon, sigmoid colon, and rectum. The following are branches of the IMA:

• • Left colic artery, with ascending and descending branches. The ascending branch supplies the upper part of the descending colon and the distal part of the transverse colon. The descending branch supplies the lower part of the descending colon and anastomoses with the first sigmoid artery. The left colic artery is absent in approximately 12% of patients.

  • Sigmoid arteries, which descend in the sigmoid mesocolon and supply the lowest part of the descending colon and the sigmoid colon. These branches anastomose superiorly with the left colic and inferiorly with the superior rectal artery.

  • Superior rectal artery, the terminal branch of the IMA. At the S3 level, this vessel divides into two terminal branches on either side of the rectum and finally anastomoses with the middle and inferior rectal arteries (from the internal pudendal and internal iliac arteries).

Important collateral vessels of the lower alimentary tract consist of the:

• • Marginal artery of Drummond, which anastomoses with the SMA and IMA. It runs along the mesenteric border of the colon and supplies the vasa recta.

  • Arc of Riolan, an intraarterial bridge between the left colic and middle colic arteries.

  • Superior hemorrhoidal branches, which form anastomoses between the IMA and internal iliac arteries.