Small Bowel: MDCT in the Diagnosis of Mesenteric Ischemia

Abstract

Mesenteric ischemia is a complicated disorder which is increasing in incidence as the population ages. It is a often a difficult condition to diagnose, both clinically and radiographically. In the past, CT has had only limited success in the early detection of ischemia. However, with the introduction of multidetector CT (MDCT) and 3D imaging, it is now possible to obtain a detailed CT examination of the small bowel as well as mesenteric vessels. This may improve our ability to diagnosis ischemia early and to identify the cause. This comprehensive MDCT exam also has the potential to eliminate the need for additional studies such as angiography. This article reviews the role of MDCT in the diagnosis of mesenteric ischemia.

Introduction

Mesenteric ischemia is a complicated disorder which is increasing in incidence as the population ages. It occurs when blood flow (arterial or venous) to the intestines is compromised and can be classified as acute or chronic. It is estimated that nearly 1% of patients presenting with acute abdomen have ischemic intestinal disease. Despite heightened awareness and sensitivity to the diagnosis, the morbidity and mortality of mesenteric ischemia have remained high over the last 30 years with mortality rates in patients with acute mesenteric ischemia exceeding 60% . This is due in part to the lack of an accurate diagnostic imaging tool. Although angiography is considered to be the gold standard for the diagnosis of mesenteric ischemia, it is an invasive, lengthy and expensive. Angiography is also not without morbidity in the population where intestinal ischemia is the most common.

Since its introduction in the late1970’s, Computed Tomography (CT) has been used with variable 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 etiology of the ischemia by evaluating the mesenteric vasculature for atherosclerosis, thrombus, occlusion, compression or invasion by tumor, trauma, etc.

Early reports of CT accuracy for the detection of mesenteric ischemia using first and second generation scanners were not encouraging. . 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 intravenous contrast administration. However, despite these improvements spiral CT was still not sensitive for the early detection of reversible small bowel ischemia and in most cases where there was a high clinical suspicion, angiography would be necessary. In a study by Taourel, the CT sensitivity and specificity for diagnosing mesenteric ischemia was only 64% and 92%, respectively.

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, offers advantages over classic spiral CT for imaging the mesenteric vasculature and small bowel.

Scanning Technique

Our protocol for the evaluation of suspected mesenteric ischemia is listed in Table 1. It includes the administration of 500-750 cc of water as oral contrast. In the past, 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 (water, methylcellulose or air.) offers two major advantages when evaluating patients with suspected 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 make 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. Kinetic curves of small bowel enhancement were obtained. Elevated perfusion rates were demonstrated in small bowel segment which were actively inflamed. Using the same technique, it may be possible to detect early changes in perfusion to ischemic segments, before irreversible damage has occurred.

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 opacified vessels. 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.

In patients with suspected mesenteric ischemia, it is crucial to visualize the mesenteric vessels in addition to a thorough examination of the intestines. Multidetector CT offers distinct advantages over traditional spiral CT for imaging mesenteric vasculature. First, MDCT scanners are faster than single detector spiral CT scanner. Depending on the manufacturer (Siemens Medical Systems, General Electric Medical Systems etc.) and model of the MDCT scanner as well as on the parameters selected, the multidetector scanner can be up to 8 times faster than a 1 second, single slice spiral scannerThis basically eliminates motion and respiratory artifact. Also, faster scanning allows more accurate timing of the contrast bolus so that data can be obtained during both the arterial and venous phases. This faster scanning combined with rapid administration of intravenous contrast (3-5cc/sec) allows visualization of the more distal branches of the mesenteric vessels, which is especially important in patients with embolic disease . Second, modern MDCT scanners allow thinner collimation than single detector spiral scanners. A slice thickness of 0.5-1 mm is possible, thus reducing volume averaging of small vessel branches. The thinner collimation possible with MDCT coupled with overlapping reconstruction creates improved 3D volume sets and improves visualization of distal branches of the mesenteric arteries and veins.

3D Imaging of the Small Intestine and Mesenteric Vessels

A major advantage of performing MDCT for the evaluation of suspected mesenteric ischemia is the improved quality of the 3D reconstructions. However, the choice of 3D rendering algorithm is crucial in this clinical setting. There are currently 3 main rendering algorithms available: shaded surface (SS), volume rendering (VR), and maximum intensity projection (MIP).

With shaded surface, a specific attenuation thresholds are set. Voxels within the thresholds are displayed while voxels outside the thresholds are excluded. Therefore the entire data set is not being utilized. Shaded surface is simple to use and can be performed on inexpensive computer platforms, but is not adequate for visualization of the bowel wall because of the artifact, noise and loss of intramural detail since only the surface is displayed. In a study of mucosal detail at CT virtual reality by Hopper et al, volume rendering was found to be superior to SS.

Maximum intensity projection is a simple algorithm which displays the brightest voxel along computer generated ray in a specified orientation. This can be a valuable tool for visualizing distal branches of vessels. However, with MIP, spatial relationships are lost.

Volume rendering is the most advanced and computer intensive rendering algorithm available. Unlike SS, it incorporates all of the relevant data into the resulting image. For example, parameters can be applied to the volume set to affect the appearance of the small intestine in order to best demonstrate anatomy and pathology. These parameters include widow width, level, opacity and brightness and can be adjusted interactively by the user. The display parameters can be manipulated to optimize visualization of the enhancing bowel wall. In addition, setting can be adjusted in order to simulate a traditional small bowel series or enteroclysis . Volume rendering is also a powerful tool when performing CT angiography of the mesenteric vessels. In a recent study of CT angiography of the peripancreatic vessels, VR was found to be superior to both SS and MIP. The major advantage of VR is its ability to accurate display the vessels as well as demonstrate adjacent structures. In addition, spatial relationships are preserved with VR, unlike MIP. The quality of the CTA obtained with VR is comparable to classic angiograms at a fraction of the cost and time . When viewing the 3D data set of patients with suspected mesenteric ischemia, we utilize volume rendering and occasionally MIP. Our 3D soft ware is the Siemens Virtuoso.

MDCT Findings in Mesenteric Ischemia

The CT examination of patients with suspected mesenteric ischemia involves a thorough examination of the bowel as well as of the mesenteric vessels. The CT findings typically differ in patients with acute vs. chronic ischemia. Acute Ischemia

Acute mesenteric ischemia can result in changes in affected bowel loops. The most common CT finding is circumferential thickening of the bowel wall . The bowel wall may appear low in density reflecting submucosal edema and inflammation. Alternatively, the bowel wall may appear high in density due to submucosal hemorrhage which often accompanies ischemia . It can appear homogeneous or may demonstrate a halo. After the administration of intravenous contrast affected loops may show decreased enhancement compared with normal loops due to compromised blood flow or in some patients the affected loops may show increased enhancement due to hyperemia. Delayed and persistent enhancement of the affected loops has also been described. In cases where infarction has occurred, pneumatosis may be present, signifying irreversible disease . At this stage, urgent surgical resection is necessary, before perforation and sepsis occurs. In addition to bowel wall thickening ischemic small bowel may demonstrates luminal dilation and mesenteric standing . In patients’ with acute mesenteric ischemia, the portion of the intestine affected is dependent on the etiology of the ischemia and the availability of collaterals. If a major artery or vein is compromised (i.e. SMA/SMV), the entire mesenteric small bowel (jejunum and ileum) may be involved as well as the right and transverse colon. On the other hand, patients with atrial fibrillation may throw emboli which affect only a small segment of the small intestine.

In addition to detecting changes in the small bowel, MDCT with 3D imaging can evaluate the mesenteric vessels in patients with acute mesenteric ischemia. Over the last several years, there has been considerable investigation of the use of spiral CT with 3D imaging for the diagnosis and quantification of vessel stenosis, in both the renal and carotid arteries. CT has been shown to be accurate in grading renal artery stenosis in comparison with digital subtraction angiography and more accurate than Doppler ultrasound in this patient population. More investigation is necessary to assess the accuracy of CT angiography in grading the degree of stenosis of the mesenteric arteries, but the results should be similar.

Ischemia can occur when either the superior mesenteric vein or artery are acutely compromised. This could be due to thrombosis or severe narrowing of a mesenteric artery or vein . The mesenteric vessels can also be encased and occluded by adjacent tumors such as pancreatic cancer. Low flow states can also cause diffuse ischemia of the intestines. In these patients the mesenteric arteries may appear attenuated and narrowed due to hypovolemia and spasm.

Chronic Ischemia

Patients with chronic mesenteric ischemia usually have a more indolent presentation compared to patients with acute mesenteric ischemia. These patients typically present with a history of recurrent abdominal pain after meals. In contrast to patients with acute ischemia, most cases of chronic mesenteric ischemia result from atherosclerosis of the mesenteric arteries. Atherosclerotic plaque, in turn, results in luminal narrowing and ultimately compromised blood flow. Since this process occurs slowly over years, patients with chronic mesenteric ischemia will typically develop collateral pathways in an effort to keep the intestine adequately perfused. Symptoms occur if adequate collaterals are not present.

CT can detect calcified plaque in the aorta and mesenteric arteries in patients with chronic mesenteric ischemia in addition to the present of collaterals . However, calcified atherosclerotic plaque at the origin of the mesenteric arteries is relatively common in older individuals most of whom do not have symptoms of ischemia. Unlike patients with acute mesenteric ischemia, the small intestine usually appears normal in patients with chronic mesenteric ischemia, unless there is also an acute thrombus present. In patients with chronic mesenteric ischemia, mesenteric collaterals may form in order to perserve adequate blood flow to the intestines. These can also be idenitified on MDCT with 3D imaging .

Conclusions

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. We currently perform MDCT with 3D imaging in all patients with suspected acute of chronic mesenteric ischemia. In many cases the CT has eliminated the need for additional imaging studies such as Doppler ultrasound or angiography. Further investigation is necessary to determine the exact utility MDCT in this clinical setting.