Small Bowel: CT Evaluation of Intestinal Ischemia

Introduction

CT plays an important role in imaging patients with acute abdominal complaints. CT is a rapid, accurate and cost-effective imaging tool, which is especially valuable in the assessment of patients with vague clinical symptoms, nonspecific physical findings or confusing laboratory results. This chapter will review and illustrate the value of CT for the evaluation of patients with intestinal ischemia. Thromboembolic and non-occlusive mesenteric ischemia will be discussed as well as colonic ischemia and ischemia resulting from trauma and obstruction.

Mesenteric Ischemia

Mesenteric ischemia is a complex disorder, which is increasing in incidence as the population ages. It occurs in a variety of conditions, which result in insufficient blood flow to the intestines. Etiologies can be organized into three main categories. (1) Arterial compromise or occlusion. (2) Venous compromise or occlusion. (3) Low flow states. Mesenteric ischemia may be acute or chronic depending on the onset and clinical presentation1.

Thromboembolic arterial occlusion is the most common etiology of acute mesenteric ischemia, with emboli usually originating from the left atrium, left ventricle or cardiac valves. Embolism to the SMA accounts for 50% of cases2. Thrombosis of a pre-existing atherosclerotic plaque accounts for 25% of cases, and usually occurs in patients with a history of chronic mesenteric ischemia2. The degree of arterial narrowing must exceed 50%-80% in cross-section area before symptoms are expected. Non-occlusive ischemia accounts for up to 30% of cases of acute mesenteric ischemia and occurs in patients with low flow states (i.e. hypotension, sepsis, heart failure, digitalis therapy, etc.)1. In these cases, ischemia probably results from vasoconstriction and spasm. The least common cause of acute mesenteric ischemia is venous thrombosis, which can occur in patients with portal hypertension, hypercoagulable states or trauma. Patients with abdominal conditions such as pancreatitis or diverticulitis are also at risk3. Despite heightened awareness and sensitivity to the diagnosis of acute mesenteric ischemia, the morbidity and mortality of mesenteric ischemia have remained high over the last 30 years, due to the lack of an optimal diagnostic imaging tool. Mortality rates in patients with acute mesenteric ischemia exceed 60%1, 4.

Chronic mesenteric ischemia typically occurs in patients over 50 years of age5, 6. The majority of these patients are smokers and more than one third have hypertension, coronary artery disease and/or cerebrovascular disease5, 6. Patients typically present with recurrent postprandial abdominal pain usually located in the epigastric or midabdominal region, which subsides after 1-2 hours5. Most patients also report weight loss as a result5. The vast majority of cases of chronic mesenteric ischemia result from atherosclerosis of the mesenteric arteries, which results in luminal narrowing and ultimately compromised blood flow. However, it is important to realize that asymptomatic atherosclerosis of the mesenteric arteries is a frequent finding at autopsy, especially in older patients5-8. Whether or not a patient with mesenteric arthrosclerotic disease will develop symptoms probably depends on the degree of narrowing and the presence of adequate collateral vessels.

Traditional therapy for acute mesenteric ischemia consisted of exploratory laparotomy with resection of the nonviable bowel and re-establishment of blood flow to the intestines. However, recent advancement in interventional radiology techniques offer an effective, less invasive therapeutic alternative9. Intra-arterial thrombolysis, angioplasty and stent placement are all available and effective. Venous thrombosis can be treated with systemic, anticoagulation, or percutaneous transhepatic delivery of thrombolytics. Non-occlusive mesenteric ischemia can often by treated with selective arterial administration of vasodilating agents (i.e. papaverine). Chronic mesenteric ischemia is typically treated with surgical bypass although percutaneous transluminal angioplasty and endovascular stent have been attempted10.

CT Evaluation of mesenteric Ischemia

Since its introduction, Computed Tomography (CT) has been used with variable success for the evaluation of intestinal ischemia. Early reports of CT accuracy for the detection of mesenteric ischemia were not encouraging. But these studies were performed with first or second generation scanners, relatively thick collimation (i.e. 10mm) and with slow or no intravenous contrast11-13. More recent studies using better infusion techniques report a CT sensitivity of 64-82%13, 14. Continued improvements in CT technology with the introduction of spiral CT and now multidetector CT allows faster scanning coupled with thin collimation (i.e. 1mm). This in combination with timed rapid IV contrast administration should improve the CT evaluation of these patients. Today, the CT examination in patients with suspected ischemia is focused on two major areas: changes in the bowel wall and evaluation of the mesenteric vessels.

Mesenteric ischemia can result in changes in the affected bowel loops, which may be detected on CT. The most common reported CT finding is bowel wall thickening, which is a result of submucosal edema/inflammation and usually does not exceed 1.5cm in thickness15, 16 . The loops are typically circumferentially thickened. The bowel wall may appear low in density reflecting edema and inflammation or in patients with submucosal hemorrhage, the wall may appear high in density due to the blood or persistent enhancement16. Intramural hemorrhage, however, is not specific for ischemia, as it can occur as a result of condition including trauma, anticoagulation therapy or radiation16. Although the bowel wall thickening is usually homogeneous, a halo appearance to the bowel wall has also been described in patients with ischemia . Associated stranding and fluid in the mesentery may also be present . Although bowel wall thickening is a common findings in patients with ischemic bowel, it is very nonspecific, as it occurs in many inflammatory, infectious, or neoplastic conditions.

Thickened small bowel loops may demonstrate absence of enhancement or in some cases, delay in enhancement when compared to unaffected loops13. Visualization of the bowel wall and its enhancement may be improved if a low density oral contrast agent is administered. Low attenuation oral agents offer two major advantages over traditional CT oral contrast agents (i.e. Hypaque, or Bar-o-cat) when evaluating for mesenteric ischemia. First, low attenuation contrast agents do not interfere with manipulation of 3D volume sets and therefore the mesenteric vessels and their branches and be readily visualized without the need for extensive editing . Second, a low attenuation intraluminal agent allows better visualization of the enhancing bowel wall and therefore allows functional information to be acquired . Low attenuation contrast agents coupled with spiral CT and rapid intravenous contrast administration makes it possible to quantify small bowel enhancement17. Thus, low density oral contrast agents (water) along with the faster scanning obtainable with spiral and now MDCT allows better visualization of the enhancing bowel wall and can be utilized to obtain functional as well as anatomic information. Currently we use water as oral contrast in all patients with known or suspected ischemia. We administer 500-750 cc of water 30-60 minutes prior to the study in order to fill the intestines. An additional 250 cc of water is administered immediately prior to the scan to ensure maximal distention of the stomach.

Luminal dilatation of affected bowel loops is another common finding on CT scans, probably resulting from disruption of the normal peristaltic activity11, 18, 19 . The dilated bowel loops are often filled with fluid, which is most likely due to fluid and blood, which has seeped from the ischemic bowel wall. However, as with bowel wall thickening, small bowel dilatation is certainly not specific for ischemia.

Pneumatosis is a less common finding in patients with ischemia bowel, but is a much more specific finding16 . It occurs when intraluminal gas dissects into the friable ischemic bowel wall. Although pneumatosis has been reported in benign condition such as collagen vascular disease, steroid use, pulmonary disease, etc, the clinical presentation and history usually will allow differentiation20. On CT pneumatosis appears as air within both the bowel wall. In some patients the intramural air may then dissect from the bowel wall into the mesenteric vein or portal vein branches in the liver which can be easily detected with CT . Also, free intraperitoneal air has been reported, and as with pneumatosis is an ominous sign, usually signifying transmural infarction of the bowel16.

In addition to detecting ischemic changes in the bowel wall, CT can also determine the cause by evaluating the mesenteric vasculature for atherosclerosis, thrombus, occlusion, compression or invasion by tumor, trauma, etc. Multidetector CT offers distinct advantages over tradition spiral CT for imaging mesenteric vasculature. First, the faster scanning speeds (0.5sec) and narrow collimation (1mm) improve contrast opacification of the mesenteric vessels. Also MDCT scanners allow a minimal of four times the volume coverage when compared to single detector scanners, thereby allowing the study to be completed faster. The scanning can be timed to acquire data during both arterial or venous phases. This improves identification and evaluation of the mesenteric arteries/veins and their branches21. In addition to resulting in better quality axial scans, the 1mm collimation coupled with fast scanning and fast intravenous contrast injection improves the quality of the 3D images. The 3D settings can be optimized to routinely display in detail the celiac artery, superior mesenteric artery, inferior mesenteric artery and their major branches as well as the mesenteric veins21. The 3D display of vessels is much more useful than axial images for defining the course and caliber of small branching vessels. Both 3D volume rendering and MIP imaging can display vessels similar to traditional angiography, but allow greater flexibility. The CT data can be viewed at any angle and superimposed structures or vessels can be removed with cut planes, without the need for additional injections of contrast, as with angiography.

Patients with acute ischemia may demonstrate thrombus within the mesenteric arteries with or without associated atherosclerosis or narrowing of the arteries. For example, in most cases of acute mesenteric ischemia, emboli lodge at the origin of the SMA or within 3-10 cm of the origin, usually just distal to the middle colic artery branch. This portion of the SMA is well visualized with MDCT as well as more distal branches. Also, almost half of patients with SMA thrombus will demonstrate extra-mesenteric emboli22. CT may also be able to detect these.

Acute ischemia can also result from thrombosis of the mesenteric veins, which also can be visualized on CT with the proper technique . Tumors encasement of the mesenteric vessels as in patients with pancreatic cancer can also be detected with CT. Non-occlusive ischemia due to low flow state may demonstrate small atretic vessels on the CTA, presumably due to vasoconstriction and/or spasm. The CT appearance is similar to the findings on conventional angiograms.

In patients with chronic mesenteric ischemia, atherosclerotic plaque and calcification can be visualized in the arteries as well as collaterals vessels, which have developed between the celiac, SMA, and IMA, in an effort to maintain adequate perfusion to the entire gastrointestinal tract . MDCT with 3D imaging also allows visualization of these anastomotic pathways.

Colonic Ischemia

Ischemia of the colon is considered to be the most common vascular disorder of the intestines in elderly patients23. In contrast to patients with mesenteric ischemia, patients with colonic ischemia are usually not critically ill at the time of presentation. Patients usually complain of mild abdominal pain, followed by bloody diarrhea. In the majority of cases, the exact etiology cannot be established, and no evidence of vascular occlusion can be demonstrated23. Since most patients are over the age of 70 and have evidence of widespread atherosclerosis, it is believed that the ischemia results from decreased blood flow to the colon. This may occur when there is an increased demand for blood by the colonic tissue in patients with only marginal blood flow or as a result of an acute decrease in blood flow to the colon.

Non-occlusive colonic ischemia usually involves the watershed areas of the colon, i.e. splenic flexure or rectosigmoid junction23, but any part of the colon can be involved . The length of involved colon depends on the cause. For instance, non-occlusive ischemia usually will affect a larger segment of colon than ischemia resulting from atheromatous emboli. Most patients with non-occlusive colonic ischemia are treated conservatively with bowel rest, IV fluids and occasionally antibiotics. The injury resolves spontaneously in over 50% of cases in 1-2 weeks9. Strictures are common complications of more serous episodes. If severe, ischemic bowel may become gangrenous and infarcted, which is a life threatening condition requiring immediate surgical resection.

CT of Colonic Ischemia

The literature regarding the usefulness of CT for the diagnosis of colonic ischemia is limited. Most reports were written in the 1980’s and describe late findings in ischemia such as pneumatosis or bowel infarction, likely related the use of early generation scanners11, 12, 18, 19, 24, 25. Even in a more recent report by Balthazar in 1997, 35/54 patients underwent a non-helical scan. With the widespread use of spiral scanners and now multidetector scanners, it is likely that CT will be more successful in detecting early changes.

The most common CT finding in ischemic colitis is wall thickening. In a recent series by Balthazar et al of 54 patients with proven ischemia colitis, the average wall thickness was 8mm, with a range of 2-20mm26. The majority of cases demonstrated segmental involvement with a mean length of 19 cm26 . Any segment of the colon can be involved, depending on the etiology. As most cases are a result non-occlusive ischemia, the watershed regions are most commonly affected, usually at the splenic flexure, left colon or rectosigmoid junction. Right sided colonic ischemia and necrosis has been reported as a complication of hemorrhagic shock after blunt or penetrating trauma27. In addition, ischemic colitis has been described proximal to an obstructing colon cancer, which can be misinterpreted as tumor extension if it is adjacent to the primary tumor28. However, the ischemia can occur at site remote from the obstructing tumor28.

The colonic thickening is usually circumferential but may appear homogenous or heterogeneous depending on the extent of submucosal edema, inflammation of hemorrhage. A halo may also be present26 . As with mesenteric ischemia, if transmural ischemia or infarction has occurred, pneumatosis may be present. Pneumatosis with or without air in the mesenteric vessels or portal vein is an ominous finding in patients with colonic ischemia and suggesting necrosis20, 24 However, the CT findings of pneumatosis coli is not specific for colonic ischemic and has been reported in benign conditions, including as a late manifestation in patients with AIDS29, 30.

In addition to wall thickening, free fluid or mesenteric stranding may also be present, but are certainly not specific signs of ischemia. In the majority of cases no evidence of vascular occlusion can be demonstrated23 in patients with classic colonic ischemia.

Other Causes of Intestinal Ischemia

Obstruction

Interference of blood supply to a segment of bowel can result from various causes of mechanical obstruction, including neoplasms ,intussusception, and hernias. Strangulated intestinal obstructions can occur as a result of bowel loops twisted on the mesentery or adhesions or in internal or external hernias. The sensitivity of CT in detected strangulated bowel obstruction ranges between 83-100%9, 31-33. Vascular compromise to the affected loops primarily involves an obstruction to venous flow that then may result in arterial vasospasm34. CT findings include bowel dilatation, wall thickening, mesenteric stranding and fluid. Delayed enhancement of obstructed bowel loops has also been described, and signifies, strangulation. When present, urgent surgical intervention is required35.

Trauma

Shock bowel is causes by prolonged hypoperfusion usually occurring trauma patients in hypovolemic shock. The bowel is typically dilated and fluid filled and shows intense persistent enhancement following IV contrast injection36 . . Other findings will include a small caliber inferior vena cava and/or aorta and intense enhancement of the kidneys, pancreas and mesentery36-38. Shock bowel is reversible after restoration of fluid volume.

Conclusions

Intestinal ischemia continues to pose a diagnostic challenge to both the clinicians and radiologists. Traditionally, radiological studies, including CT, have relied on static imaging of the small intestine and findings such as bowel wall thickening and mesenteric stranding. These parameters are inadequate and insensitive for early and accurate detection of ischemia. Continued advancements in CT technology including multidetector CT and 3D CT imaging MDCT coupled with the use of water as an oral contrast agent, improves visualization of the bowel wall, enhancement and mesenteric vasculature. This will likely result in improvements in CT evaluation of patients with intestinal ischemia.