Colon: Vascular: Volume-Rendering 3D CT of the Mesenteric Vasculature: Normal Anatomy and Pathology

Karen M. Horton MD
Eliot K. Fishman MD

Spiral CT and multidetector CT (MDCT) offer distinct advantages over traditional dynamic CT for imaging the mesenteric vasculature (superior and inferior mesenteric artery and vein). The faster scanning speeds and narrow collimation allow data acquisition during optimal opacification of the mesenteric vessels and their branches, on both the axial and 3D scans. In addition, the 1mm collimation possible with MDCT coupled with faster scanning and rapid IV contrast injection, improves the quality of the data set available for 3D image reconstruction.

3D angiographic parameters can be optimized to routinely display in detail the SMA/SMV and IMA/IMV and their major branches. This will likely aid in the assessment of arterial and venous encasement in patients with pancreatic cancer and may come to play a role in the evaluation of patients with mesenteric ischemia or inflammatory bowel disease.

This paper reviews the axial and 3D anatomy of the SMA/SMV and IMA/IMV and discusses the use of volume rendering of the 3D data sets to improve evaluation and visualization of the mesenteric vasculature. As there is significant variability in the anatomy of gastrointestinal vasculature, we will focus on the most common branching patterns and variations.

ANATOMY AND NORMAL VARIANTS Superior Mesenteric Artery
The superior mesenteric artery arises from the abdominal aorta, usually at the level of the L1 vertebral body. The SMA typically arises approximately 1.5 cm below the celiac origin and approximately 7 cm above the origin for the inferior mesenteric artery. The origin of the SMA is typically close (usually above) the origin of the renal arteries. The left renal vein is located posterior to the origin of the SMA and anterior to the aorta. The SMA lies to the left of the superior mesenteric vein and crosses over the third portion of the duodenum. When the superior mesenteric artery enters the mesentery, it usually lies posterior to the mesenteric vein, although this relationship is variable.

The jejunal arteries (usually 4-6) arise from the left side of the SMA. The ileocolic artery which arises from the right side of the SMA marks the transition from jejunal to ileal arteries. There are usually between 8-12 ileal arteries. The branching pattern of the last jejunal artery, ileocolic, and ileal arteries does vary, sometimes forming a loop or tripod. The ileocolic artery has branches to the terminal ileum, cecum and lower ascending colon.

The right colic artery can arise from the SMA to aid the ileocolic and middle colic arteries in supplying blood to the ascending colon. However, it is absent in up to 80% of people. The middle colic artery usually arises from the right side of the SMA just before it enters the mesentery. It descends into the right lower quadrant where it anastomoses with the ileocolic artery. Other branches include artery for the right angle (arteria flexurae coli dextra), artery for the transverse colon all arise from the SMA. There are many variations. For instance, the artery for the right angle and artery for the transverse colon can arise from the middle colic artery or there may be accessory arteries for the artery for the transverse colon or accessory arteries for the left colic artery. There are also anastomotic connections between the artery to the transverse colon and the left colic artery which arises from the IMA.

The inferior pancreaticoduodenal artery is the pivotal point of embryologic gut rotation. It can arises from either the right or left side of the SMA. It may arise as one vessel or two(anterior and posterior). This vessel courses behind the SMV and superiorly to anastomose with the superior pancreaticoduodenal artery which arises from the gastroduodenal artery.

The marginal arteries of Dwight and Drummond supply the vasa recta to the small intestine and colon and provide a continuous channel of potential collateral blood supply to the entire gut. It is defined as the artery closest and parallel with the wall of the intestine, supplying the vasa recta. The vasa recta are fine branches that arise from the marginal artery and supply the bowel wall. The middle colic is often the marginal artery for the major portion of its distribution. The arc of Riolan is an inconstant artery which is parallel to a portion of the middle colic artery.

Aberrant branches from the superior mesenteric artery include: common hepatic artery, right hepatic artery, splenic artery, celiac trunk, cystic artery, gastroduodenal artery, right gastroepiploic, and left gastric artery. The right colic artery is only present in 13% of cases. In addition, the artery for the right angle and the artery for the transverse can arise from the middle colic or there may be accessory arteries for the artery for the transverse colon or accessory arteries for the left colic artery.

Superior Mesenteric Vein
The SMV is usually a single trunk of variable length (5 -50mm) which is formed by two large intestinal branches (right and left) which receive blood from several veins including : the ileocolic, gastrocolic, right colic and middle colic veins. In some patients a single trunk may not be present. Instead there may be a large right and left mesenteric branch, which both join the splenic vein to form the portal vein.

The SMV lies to the right of the superior mesenteric artery as it crosses over the third portion of the duodenum. When the superior mesenteric vessels enter the mesentery, the SMV usually lies anterior to the mesenteric artery, although this relationship is variable, especially in patients with malrotation or nonrotation of the gut. In patients with complete non-rotation of the gut, the relationship of the of the SMA and SMV will be reversed and the location of the small bowel and colon will be abnormal (typically the small bowel will be within the right abdomen and the colon in the left). In these patients, the duodenum will not cross the spine. However, patients can have a degree of malrotation and not complete nonrotation.

As with the SMA, there are jejunal and ileal branches of the SMV which receive blood from the intestine.

Inferior Mesenteric Artery
The inferior mesenteric artery arises from the aorta approximately 7 cm below the origin of the SMA, usually at the level of L3. It is a relatively straight vessel which has the following branches, all arising from the left. The left colic artery forms an anastomosis with the artery to the transverse colon which arises off the SMA. It is absent in 12 % of people, where its function is then performed by the colosigmoid artery. The colosigmoid artery also arises from the left side of the IMA and supplies blood to the descending and sigmoid colon. There are usually 2-4 sigmoid branches which can arise from IMA, colosigmoid artery or left colic. The next branch off the IMA is the rectosigmoid artery. Finally, after the origin of the rectosigmoid artery the IMA bifurcates into the superior rectal arteries.

The colosigmoid artery can arise from the left colic artery or from the angle between the left colic artery and the inferior mesenteric arteries. The left colic artery can be absent or can arise from the SMA.

Inferior Mesenteric Vein
The major tributaries of the inferior mesenteric vein include the superior hemorrhoidal vein, the sigmoid vein and the left colic vein. The hemorrhoidal vein and the sigmoid vein usually join to form a common trunk before uniting with the left colic vein. The IMV can terminate into the splenic vein, at the splenoportal angle or it may drain into the SMV.

PATHOLOGY
Staging Pancreatic Cancer.
In addition to detecting liver metastases, a crucial goal of CT imaging of pancreatic cancer includes the evaluation of adjacent vascular structures. Involvement of any of the major arterial (i.e. celiac axis or SMA) or venous structures (i.e. portal vein, splenic vein or SMV) will make the patient unresectable. Recent articles have described a vessel grading scheme based on the circumferential vessel involvement by tumor. Grading has been categorized as 1-4 based on 0-25%, 25-50%, 50-75% and 75-100% encasement.

In the past, all patients with pancreatic tumors underwent catheter angiography before surgery to assess the presence of mesenteric vessel encasement and to provide vascular maps. Several articles have shown similarity in results between spiral CT and angiography for this indication. Also, CT can be used to create angiographic style vascular maps, and is less expensive than traditional angiography. With the recent introduction of multidetector CT, the limitations of earlier CT angiography are eliminated. MDCT allows even faster scanning with very thin collimation (i.e. 1mm)which further improves the quality of the CT angiogram in terms of vessel detail and definition. In addition, unlike classic angiography CT is not limited by plane or perspective and often the "optimal" view is only determined in retrospect.

Invasion of the SMA is one of the contraindications to surgery in pancreatic cancer. Novick et al. previously showed that the traditional axial CT plane of scanning was not ideal for imaging the SMA due to its oblique course.

Involvement of either the portal vein, splenic vein or the SMV is typically a contraindication to surgery although limited involvement of the portal vein or confluence may not be an absolute contraindication to all surgeons who may elect to attempt resection using a vascular graft. Accurate assessment of the mesenteric veins requires proper timing of the IV bolus and scanning. For maximum opacification of the venous structure, scanning should be performed during the portal venous phase (approximately 50 sec after the start of the injection (after obtaining the imaged during the arterial phase). With MDCT there is better opacification and visualization of smaller vessels in the venous arcade around the pancreas including the gastrocolic vein, the anterior superior pancreaticoduodenal vein and the posterior superior pancreaticoduodenal vein.

MDCT, in particular, results in better visualization of arterial and venous branching and thus, detection of involvement of the more distal portions of the mesenteric vessels is also possible. Vessel visualization is greatly improved by the use of 3D volume rendering, which can display the vessel in the optimal plane, as evaluation of the arteries and veins may be limited if only axial images are obtained.
Vessel involvement can be defined as either occlusion or narrowing of the vessel usually with an associated soft tissue mass surrounding the area of involvement. Collateral vessels may be present and are a useful secondary sign of vessel invasion or occlusion.

Mesenteric Ischemia
Small bowel ischemia/infarction is an important diagnostic challenge, as clinical signs and symptoms are usually nonspecific. Common underlying causes of small bowel ischemia include: superior mesenteric artery narrowing or occlusion due to atherosclerotic plaque, thrombus or tumor encasement, mesenteric vein thrombosis or encasement and hypoperfusion due to low cardiac out-put or atherosclerotic disease. Although angiography was traditionally the procedure of choice for the diagnosis of mesenteric ischemia, CT, especially MDCT with 3D imaging can now play a significant role.

With proper technique and timing of the IV bolus and scanning, excellent opacification of the mesenteric vessels can be routinely obtained. Thrombosis of the main vessels can be easily seen on the axial images often with associated collaterals, depending on the chronicity. For evaluation of more distal branches of the mesenteric vessels and for evaluation of narrowing or plaque at the origin the arteries, 3D imaging is a distinct advantage. Also, 3D images allows a better representation of the complex vascular collaterals and provides an excellent roadmap for the surgeons.

CT with 3D imaging in also useful for evaluation of mesenteric shunts in patients after surgical intervention. The course and patency of the shunt can be evaluated.


Inflammatory Disease
Patients with inflammatory disease of the small bowel may demonstrate detectable changes in mesenteric blood flow. For instance, ultrasound with color Doppler imaging and Power Doppler has been used to attempt to differentiate active bowel inflammation (increased blood flow) from chronic wall thickening/fibrosis (no increased flow) in patients with Crohns disease. Similarly, Doppler ultrasound has been able to demonstrate hemodynamic changes in the mesenteric vessels in patients with active inflammatory bowel disease which are not present in patients with quiescent disease.

With the introduction of MDCT and improvements in3D CT imaging it may now be possible to detect some of these changes in patients with active inflammation and hyperemia of a bowel segment. Mesenteric vessels supplying the diseased loops of bowel may be enlarged, indicating increased blood flow to that region, compared with adjacent normal loops. In addition, MDCT with the ability to obtain volume data sets at defined enhancement points (i.e. arterial phase or venous phase) makes it possible to quantify small bowel enhancement over time and to calculate small bowel perfusion rates. This may help detect hyperemia, even before detectable changes in vessel size or configuration.

CONCLUSIONS
MDCT is a significant advancement in CT technology. By allowing thin collimation (i.e. 1-1.25mm) and rapid scanning, excellent opacification of the mesenteric vessels can be routinely obtained. Better 3D volume sets, in turn, lead to improved 3D vascular maps which are useful in staging vascular invasion by cancer, for surgical planning and in evaluation of patients with suspected mesenteric ischemia. This exhibit reviews the normal axial and 3D anatomy of the superior and inferior mesenteric arteries and veins. In addition, several conditions which affect the mesenteric vessels (pancreatic cancer, ischemia, Crohns were discussed and illustrated.