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2. Volume-rendered 3D image in sagittal projection(A) shows stenosis of the proximal celiac artery (arrow). This stenosis has a characteristic hooklike appearance with poststenotic dilatation. (B) Axial contrast-enhanced image shows prominent vessels around the pancreatic head(arrows). (C) Coronal volume-rendered image shows that the prominent vessels are actually a dilated gastroduodenalartery (arrow), which is now supplying the celiac axis through the SMA. A patient with median arcuate ligament syndrome.

3. Acute Mesenteric Ischemia • It is caused by an abrupt reduction of either arterial or venous blood flow to the gut. • The condition requires urgent diagnosis and treatment. Almost all patients present with severe abdominal pain. In patients with emboli as the cause the onset of pain is usually sudden, whereas patients with thombotic etiology may have a more insidious onset of symptom.Nausea, vomiting, and diarrhea are also common complaints. • There are 4 major causes of acute mesenteric ischemia: SMA embolus, SMA thrombus, mesenteric venous thrombus, and nonocclusive mesenteric ischemia.

4. Acute emboli to the SMA are the most common origin of acute mesenteric ischemia.(40% - 50%) . • Most emboli originate in the heart and will lodge in the SMA a few centimeters distal to the origin, typically near the origin of the middle colic artery. Smaller emboli lodge more distally and may affect only small segments of bowel. • The arterial thrombus is visible as a low-density filling defect on CT . • Proximal thrombi are best visualized on the sagittal reconstructions,while distal thrombi may only be visible using volume rendering with comprehensive interrogation of all the distal mesenteric branches.

5. An 80-year-old man presenting with acute abdominal pain. (A) Sagittal MPR and (B) Coronal MIP show a large thrombus in the mid SMA (arrow); this was embolic, presumably from a cardiogenic source. Surgical embolectomy was performed.

6. Regardless of the cause of the ischemia, the affected small bowel loops may be dilated and fluid filled, as a result of an interruption in normal peristalsis and increased secretion. • The wall may be thickened, but in some cases will actually be normal or thinned. • Ischemia usually causes circumferential thickening of the bowel wall. The ischemic bowel wall is typically to 8 to 9 mm thick. • Bowel wall thickening is more pronounced in cases of venous thrombosis than in cases of arterial thrombosis.Therefore, a bowel wall measuring 1.5 cm, in the setting of suspected ischemia, most likely signals obstruction of venous blood flow.

7. An 80-year-old woman with an acute closed loop small bowel obstruction. • Coronal MPR and (B)sagittal MPR • show dilated fluid-filled small bowel loops. The wall appears thinned and has • decreased enhancement (arrows) compared with the more proximal and unaffected small • bowel loops (arrowheads).

8. Acute mesenteric ischemia with small bowel infarction in a 70-year-old man who complained of abdominal pain, nausea, and vomiting. (A, B) Intravenous contrast-enhanced CT scan shows hypo attenuating thrombus occluding of the superior mesenteric vein (arrow in panel A), an edematous small bowel mesentery, and extensive mural thickening throughout the small intestine with a target pattern of mural enhancement (arrow in panel B).

9. The bowel wall may appear of low density, reflecting decreased perfusion and edema, or may appear increased in density relative to normal bowel loops, related to hemorrhage or hyperemia. The halo sign may be present. Intramural hemorrhage may be present, and is often only appreciated if noncontrast scans are obtained. • Pneumatosis is a late finding, indicating transmural infarction, and may be accompanied by air in the mesenteric veins and/or portal vein . In patients with acute arterial ischemia, there may be stranding in the mesentery and ascites, also indicating severe ischemia and usually transmural infarction.

10. A 37-year-old postpartum woman with severe abdominal pain. Axial contrast-enhanced CT with soft tissues windows (A) and lung windows (B) shows a small bowel obstruction and pneumatosis (arrows). F, large necrotic uterine fibroid.

11. (A) Axial image through the pelvis in a patient who has mesenteric ischemia shows small bowel pneumatosis. (B) Axial image through the liver in the same patient shows portal venous gas in the liver

12. Small intestinal ischemia and infarction from multiple arterial emboli in a 52-year-old man who had atrial fibrillations. (A, B, C) Intravenous contrast-enhanced CT scan shows multiple splenic infarctions, mesenteric edema, ascites, and mural thickening in the small intestine. Lack of mural enhancement (arrow in panel B) and pneumatosis (arrow in panel C) are present.

13. Thrombosis of the SMA usually occurs in the setting of atherosclerotic disease, likely as the result of rupture of an unstable atherosclerotic plaque.(30% of all cases of acute ischemia) • Unlike emboli, thrombi typically develop at the origin of the SMA and within the first 2 cm, best visualized using sagittal reconstructions. • There is usually a combination of calcified plaque with superimposed thrombus.Because SMA thrombosis often occurs in the setting of patients with chronic ischemia, there may be associated arterial collaterals, which can be visualized well using CTA.

14. An 83-year-old woman with acute abdominal pain. (A) Sagittal MIP shows extensive calcified atherosclerotic disease involving aorta and proximal SMA (arrow). (B) There is also a filling defect in the proximal SMA(arrows), which is acute thrombus that has form in a region of calcified plaque.

15. Nonocclusive mesenteric ischemia occurs in patients with hypotension or cardiogenicshock. • Other conditions that may precipitate a low-flow state are heart failure, hypovolemia,dehydration, and chronic renal failure, particularly after dialysis.Certain drugs, such as digitalis, norepinephrine, cocaine, and ergot derivatives,also are known to cause low-flow states. • Patients present with abdominal distention and in some cases gastrointestinal bleeding, but they seldom complain of severe abdominal pain.

16. Severe hypoperfusion of the gut will cause severe vasoconstriction of the mesenteric arteries the SMA and its branches will appear small in caliber and pruned down, a result of the body’s attempt to maintain blood flow to the gut and there may be delayed opacification of the mesenteric veins . • The bowel is often dilated and fluid filled. The bowel wall may also be thickened.In severe cases pneumatosis or portomesenteric venous gas is present, indicating transmural infarction, which carries a dismalprognosis.

17. 65-year-old man in cardiogenic shock after an acute myocardial infarction. (C)Sagittal MIP image shows the small-caliber celiac axis and SMA, a typical finding in patients with hypotension (A) Axial contrast-enhanced image through the mid abdomen shows dilated small bowel and colon as well as poor perfusion of the kidneys. (B) Axial contrast-enhanced image shows a small-caliber SMA (arrow). A right pleural effusion is also present.

18. (A) Sagittal volume-rendered 3D CTA in a patient who has hypotension and sepsis shows marked narrowing of the celiac axis and SMA. (B) Coronal oblique volume-rendered 3D CTA in the same patient shows pruning of the SMA branches (arrows). The mesenteric veins (arrowheads) are prominent. The small bowel is dilated and fluid filled.

19. In patients with suspected infarcted bowel from emboli, therapy consists of exploratory laparotomy with resection of the nonviable bowel and reestablishmentof blood flow to the intestines. • interventional radiology techniques offer an alternative for patients with ischemia but no clear evidence of infarcted bowel. Intra-arterial thrombolysis,angioplasty, and stent placement are all available and effective. • Nonocclusive mesenteric ischemia can be treated with selective arterial administrationof vasodilating agents (ie, papaverine) • Patients with thrombus forming in the setting of chronic mesenteric ischemia may require a combination of percutaneous and systemic therapies.

20. (A) Coronal MPR in a patient with severe abdominal pain shows pneumatosis (arrow) in the right lower quadrant. Portal venous gas (arrowhead) is present also. (C) Sagittal volume-rendered 3D CTA shows extensive atherosclerosis of the celiac and SMA. At surgery the patient was found to have acute on cnronic ischemia. The infarcted bowel was resected, and a mesenteric bypass graft was placed. (B) Axial image of the liver shows extensive portal venous air.

21. Vasculitisis another cause of acute and chronic mesenteric ischemia and can be divided into three categories: large-, medium-, and small-vessel vasculitis • Involvement of the mesenteric arteries can result in pain, acute or chronic mesenteric ischemia,hemorrhage, and/or stricture. • The most common large-vessel vasculitis affecting the mesenteric vessels is Takayasuvasculitis , which targets the aorta and its major branches. The most common medium-vessel vasculitis is polyarteritisnodosum, a necrotizing form of the disease that weakens the vessel wall and can cause the formation of aneurysms.Approximately 50% of cases involve the small intestine and mesenteric vessels.The most common vasculidities to involve the small intestine are Henoch-Scho¨ nleinpurpura, systemic lupus erythematosus, and Behc¸et’s disease

22. (A) Axial CT in a 45-year-old patient with recurrent abdominal pain shows marked mural thickening of the SMA (arrow). (B) Sagittal MPR shows the extensive thickening along the proximal SMA (arrows). (C) Coronal volume-rendered 3D CTA shows the irregularity in the SMA and a small pseudoaneurysm (arrow). Based on the CT diagnosis of vasculitis, the patient was treated successfully with steroids

23. Chronic Mesenteric Ischemia • It is almost always a result of severe atherosclerotic disease involving the mesenteric arteries, and therefore occurs in older patients. • Even in the absence of symptoms, patients may have clinically significant atherosclerotic disease affecting the mesenteric arteries. • Patients with atherosclerotic stenosis of the mesenteric arteries will usually become symptomatic when 2 of 3 major mesenteric vessels, typically the SMA and celiac artery,become severely stenotic or occludeds. • Standard treatment involves revascularization, which can be surgical or catheter based.

24. Symptoms of chronic mesenteric ischemia develop slowly over time. Patients mesenteric typically experience epigastric pain 15 to 60 minutes after a meal, as a result of increased demand for mesenteric blood flow .(abdominal angina) • Weight loss is common, a result of both pain and a change in dietary habits. Patients may even develop sitophobia, a fear of food or eating. Weight loss also may be caused by damage to the intestinal mucosa, with malabsorption of nutrients. • Symptoms occur when collateral pathways no longer deliver an adequate supply of blood to the intestine.

25. (C) Sagittal volume-rendered 3D CTA nicely shows the occlusion of the proximal celiac axis (arrow) caused by atherosclerosis (A) Coronal volume-rendered 3D CTA shows a dilated gastroduodenal artery (arrow). This is a common collateral pathway between the SMA and celiac. The patient has cirrhosis, splenomegaly, and ascites. (B)Axial image shows occlusion of the proximal celiac axis (arrow)

26. CT will show significant stenosis of at least 2 of the major mesenteric arteries, usually the celiac trunk and SMA. • The stenosis is usually at the origin and may be a combination of calcified and noncalcified plaque. Because the process develops over a long period of time, collaterals are present. • CTA and volume rendering in particular are especially valuable in detecting and quantifying the degree of stenosis and displaying the collaterals.This technique can be used as a road map for the surgeon or interventional radiologist.

27. Coronal volume-rendered 3D CTA in a patient who has chromic mesenteric ischemia shows a dilated collateral vessel (arrow) connecting the IMA and SMA.

28. The detection of calcified atherosclerotic plaque on CT is not in itself diagnostic for chronic mesenteric ischemia,and it is common incidental finding in asymptomatic elderly patients. It is important, however, to document the presence of atherosclerotic plaque in the report to clinicians and to quantify the degree of stenosis. • Long-term studies have shown that as many as 86% of asymptomatic patients with greater than 50% stenosis of the mesenteric arteries eventually develop symptoms. • Other causes of chronic mesenteric ischemia unrelated to atherosclerotic disease include vasculitis,fibromuscular dysplasia, median arcuate ligament syndrome, and tumor encasement. Radiation therapy can cause scarring and narrowing of the mesenteric vessels.

29. Sagittal volume-rendered image demonstrates extensive calcified atherosclerotic plaque in the aorta and mesenteric arteries. Although significant plaque is present, there is no luminal narrowing. This patient has no signs or symptoms of ischemia

30. Sagittal volume-rendered 3D CTA shows extensive atherosclerotic plaque (arrows) in a diabetic patient.

31. VENOUS PATHOLOGY Mesenteric Vein Thrombosis (MVT) • It accounts for 5% -15% of all mesenteric ischemias. Thrombosis usually involves the SMV, only rarely involving the IMV.MVT can primary or secondary. • Primary or idiopathicMVT results when no underlying etiology can be identified. • Secondary is more common. Common causes include underlying coagulopathy, either hereditary or acquired. • Hereditary factors include Factor III deficiency, deficiencies in protein C,protein S, or antithrombin, or polycythemiavera. • Acquired coagulopathy is often related to cancer, intra-abdominal inflammatory conditions, postoperativepatients,OCP, cirrhosis and portal hypertension, pancreatitis, sepsis, or after splenectomy.

32. Clinical presentation varies depending on the location, extent, and cause of the thrombosis. Patients can present with acute,subacute, or chronic symptoms. • In acute Presentation,patient can often mimic the presentation of acute arterial ischemia. In acute presentations, patients present with severe pain and there is a high risk of both ischemia and infarction. Outcomes vary, based on the extent of thrombosis. • Acute thrombosis can result in venous hypertension depending on the residual drainage from the intestines. Severe venous hypertension will compromise the perfusion of the bowel.

33. On CT, thrombus will be visible in the mesenteric veins, typically associated with engorgement of the veins.The walls of the veins may be thickened with increased enhancement. Stranding in the mesentery and ascites are also often present.The bowel wall is usually thickening, often related to the venous obstruction. There may be decreased enhancement of the wall, or in some patients there may be increased due to hyperemia .A halo pattern has also been described. • Complete lack of bowel enhancement is uncommon, but does signify transmural infarction, especially when there is accompanying pneumatosis or portomesenteric gas.

34. An 80-year-old man with abdominal pain and history of Osler-Weber-Rendu syndrome. (A) Contrastenhanced axial CT shows small bowel thickening (arrows). (B) Axial image through the superior mesenteric vein shows a large clot (arrow). (C) Axial contrast-enhanced image through the mid abdomen shows extensive thrombus (arrows) in the branches of the SMV.

35. Partial thrombotic occlusion of the superior mesenteric vein in a 40-year-old woman with protein S deficiency. (A, B) Intravenous contrast-enhanced CT scan shows nonoccluding thrombus in the superior mesenteric vein (arrow in panel A), dilated small intestine, and an engorged mesentery. Hydronephrosis of the right kidney is present.

36. In subacute situations, patients may have abdominal pain, but typically do not show associated signs of ischemia, likely related to the development of Collaterals. Treatment in acute and subacute cases usually includes anticoagulation, alone or in combination with surgery. • Chronic MVT, often in cirrhotic patients, typically causes little symptoms because of the development of an extensive collateral network. However,these patients are at increased risk for GI bleeding. Treatment may include propranolol to decrease the risk of variceal Bleeding.

37. Coronal volume-rendered image in a patient presenting with acute abdominal pain demonstrates extensive thrombosis (arrows) of the mesenteric veins. The proximal jejunum is thickened, and there also is mesenteric stranding.