Colon: Non Neoplastic Disease: Multidetector CT evaluation of mesenteric ischemia: can it be done?

Karen M. Horton MD and Elliot K. Fishman MD

ABSTRACT

Mesenteric ischemia continues to pose a diagnostic challenge to both 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 small bowel ischemia.

Multidetector CT (MDCT) is the latest advancement in CT technology and is now increasingly available. MDCT combines multiple rows of detectors and faster gantry rotation speeds with narrow collimation. MDCT, therefore, potentially offers distinct advantages over classic spiral CT for imaging the mesenteric vasculature and small bowel. Faster scanning speeds and narrow collimation coupled with a timed bolus injection of IV contrast, increase contrast opacification of the mesenteric vessels which improves identification of distal branches on both the axial and 3D scans. The faster scanning also allows evaluation of bowel wall enhancement over time, which may be important in detecting ischemia.

This paper explores the potential use of MDCT for the evaluation of 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 the onset and clinical presentation. 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 cases. Thrombosis of a pre-existing atherosclerotic plaque accounts for 25% of cases, and usually occurs in patients with a history of chronic mesenteric ischemia. 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.). In these cases, ischemia probably results form vasoconstriction and spasm. The least common cause of 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 risk.

Despite heightened awareness and sensitivity to the diagnosis, 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%. In some series, the mortality rate has been reported to be over 90%.

Computed Tomography (CT) has been used with some success for the evaluation of small bowel ischemia. The CT examination in patients with suspected ischemia has traditionally focused on two major areas. First, ischemia can result in changes in the affected small bowel loops and mesentery which can be detected on CT. These include: bowel wall thickening and edema, submucosal hemorrhage, increased or decreased enhancement of the bowel wall, mesenteric stranding /fluid or pneumatosis. Second, CT can sometimes determine the cause by evaluating the mesenteric vasculature for atherosclerosis, thrombus, occlusion, compression or invasion by tumor, trauma, etc.

The introduction of Spiral CT definitely improved the ability of CT to image the mesenteric vessels and bowel wall by allowing narrow collimation and faster scanning coupled with timed bolus contrast administration. However, despite these improvements spiral CT was still not sensitive for the early detection of small bowel ischemia and in most cases where there was a high clinical suspicion, angiography would be performed. In a recent study by Taourel, the CT sensitivity and specificity for diagnosing mesenteric ischemia was only 64% and 92%, respectively.

Although angiography is considered to be the gold standard for the diagnosis of mesenteric ischemia, it is invasive and expensive. Angiography is also not without morbidity in the population where intestinal ischemia is the most common. However, with the recent introduction and increasing availability of multidetector CT (MDCT) many of the prior limitations of CT are overcome. MDCT improves the CT examination of the small bowel and the mesenteric vasculature, both of which are crucial when evaluating for small bowel ischemia and may come to play a more important role in evaluating patients for mesenteric ischemia.

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 branches. 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 veins. 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.

Better visualization of the mesenteric vessels may aid in the evaluation of patients with suspected intestinal ischemia by allowing better imaging of the mesenteric vessels anatomy and pathology. Patient with ischemia may demonstrate thrombus within the mesenteric arteries or veins, atherosclerosis or narrowing of the arteries, tumors encasement or compression. 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. Patients with non-occlusive ischemia due to low flow state may demonstrate small atretic vessels and/or spasm.

In patients with chronic mesenteric ischemia, collateral vessels develop between the celiac, SMA, and IMA, in an effort to maintain adequate perfusion to the entire gastrointestinal tract. MDCT with 3D imaging allows visualization of these anastomotic pathways.

CT imaging of the small bowel traditionally has utilized high attenuation contrast agents for opacification of the gastrointestinal tract and has relied on structural changes in the appearance of the bowel for diagnosis (wall thickening, pneumatosis, etc.). However, a low attenuation oral agent offers two major advantages when evaluating for mesenteric ischemia.

First, 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 enhancement. For example, in a study by Harvey et al a group a patients with Crohn's disease underwent, CT enteroclysis or CT pneumocolon with intravenous contrast (10). Kinetic curves of small bowel enhancement were obtained. Elevated perfusion rates were demonstrated in small bowel segment which were actively inflamed. Thus, low density oral contrast agents (water) along with the faster scanning obtainable with MDCT allows better visualization of the enhancing bowel wall and can be utilized to obtain functional as well as anatomic information.

Second, low attenuation contrast agents do not interfere with manipulation of 3D volume sets. The use of high attenuation agents require extensive, time consuming editing of the data sets because the high attenuation bowel contents obscure other structures. Therefore, when water is used as an oral contrast agents in patients undergoing CT evaluation of suspected mesenteric ischemia, simultaneous 3D examination of the bowel and mesenteric vessels can be performed without the need for extensive editing.

Currently we use water as oral contrast in all patients with known or suspected mesenteric ischemia. First 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.

Traditional therapy for 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 alternative. 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).

Mulitdetector CT is an exciting advancement in CT technology. MDCT can not only perform routine studies much faster than single detector CT scanners, but also enables new applications, especially in the field of CT angiography. The increased speed and narrow collimation possible with MDCT coupled with the use of water as an oral contrast agent, improves visualization of the bowel wall and mesenteric vasculature. This may aid in CT evaluation of patients with suspected mesenteric ischemia.