Small Bowel: CT Angiography of the Gastrointestinal Tract

Karen M. Horton, MD and Elliot K. Fishman, MD
Russell H. Morgan Department of Radiology and Radiological Science

Introduction

Since its introduction in the late 1970’s, computed tomography (CT) has undergone significant technological advancements. For example, early scanners could only acquire one 10mm slice every 15 seconds and required 60 seconds to reconstruct each image. This slow speed and suboptimal resolution limited CT vascular applications in the abdomen. However, with the introduction of spiral/helical CT in the late 1980’s and multidetector row CT (MDCT) in the late 1990’s scanners have gained considerable speed and resolution. The most modern scanners can acquire eight 1.25 mm slices every second. Due to these advancements, CT data sets can now routinely be viewed as a volume of information as opposed to a series of individual slices. This paradigm shift along with the availability of powerful computers and 3D software packages, led to the introduction CT angiography which has revolutionized the way we image the abdominal vasculature.

The clinical role of CT has continued to expand along with the changes in technology. Examinations that had always been done with CT are now done with increased accuracy. Other examinations that were clearly in the realm of other modalities such as conventional angiography are now being routinely performed with CT. This chapter will focus on the current role of CT angiography (CTA) for GI tract pathology including its role in the evaluation of mesenteric ischemia, Crohn disease and gastrointestinal bleeding.

Applications

CT Evaluation of Mesenteric Ischemia

Early reports of CT accuracy for the detection of mesenteric ischemia were not encouraging. However, these studies were performed with first or second generation scanners, relatively thick collimation (i.e. 10mm) and with slow or no intravenous contrast . More recent studies using better infusion techniques report a CT sensitivity of 64-82%. Continued improvements in CT technology with the introduction of spiral CT and now multidetector CT 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.

Ischemia of the bowel usually results in changes in the affected bowel segments which may be detected on CT. The most common reported CT finding is bowel wall thickening, which is very nonspecific . The bowel wall may appear low in attenuation reflecting edema and inflammation or in patients with submucosal hemorrhage, the wall may appear of high CT attenuation due to the blood products. Visualization of the dynamic enhancement pattern of the affected bowel loops may improve diagnosis. Affected bowel loops may demonstrate absence of enhancement, delay in enhancement, or persistent enhancement when compared to unaffected loops. Pneumatosis is a less common finding in patients with ischemic bowel, but is a specific sign. It occurs when intraluminal gas dissects into the friable bowel wall. Although pneumatosis has been reported in benign conditions such as collagen vascular disease, steroid use, pulmonary disease, etc, the clinical presentation and history usually will allow differentiation.

In addition to detecting ischemic changes in the bowel wall, CT can also attempt to identify the cause by evaluating the mesenteric vasculature. Multidetector CT offers several advantages over tradition spiral CT for imaging mesenteric vasculature. First, the faster scanning speeds and narrow collimation allow optimization of contrast opacification of the mesenteric vessels. Second, MDCT scanners allow increased volume coverage when compared to single detector scanners. Third, the scans can be more accurately timed to acquire data during both arterial or venous phases. This improves identification and evaluation of the mesenteric arteries/veins and their branches. Fourth, in addition to resulting in better quality axial scans, the thin collimation coupled with faster scanning and bolus 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 branches as well as the superior and inferior mesenteric veins. The 3D display of vessels is much more useful than axial images for defining the course and caliber of small branching vessels. The mesenteric vasculature can be displayed similar to traditional angiography, but the CT 3D software allows greater flexibility. For example, the CT data can be viewed at any angle without the need for additional contrast injections. In addition, superimposed structures or vessels can be removed with cut planes so that the vessel of interest can be displayed optimally.

Intestinal ischemia results from compromised blood flow to the gut. Patients with acute ischemia may demonstrate clot within the mesenteric 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. This segment of the SMA is well visualized with MDCT. Also, small branches are easily identified on CT angiography and therefore more distal emboli may also be detectable. Acute ischemia can also result from thrombosis of the mesenteric veins, which also can be visualized on CT with the proper technique. Tumor encasement of the mesenteric vessels can also be easily identified on CT with 3D imaging. 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 symptoms suggesting chronic mesenteric ischemia, atherosclerotic plaque may be identified in the mesenteric arteries. This is especially easy to detect if the plaque is calcified. However, atherosclerotic plaque affecting mesenteric arteries is a common finding in older patients and does not necessarily signify significant disease. In addition to the presence of atherosclerotic plaque, patients with chronic mesenteric ischemia may also demonstrate collaterals, which have developed between the celiac, SMA, and IMA, in an effort to maintain adequate perfusion to bowel. MDCT with 3D imaging also allows visualization of these anastomotic pathways and can also be used to evaluate patients after bypass surgery.

CT Evaluation of Crohn Disease

The role of CT in the evaluation of patients with Crohn disease is well accepted. The ability of CT to identify bowel involvement as well as extraluminal disease (e.g. abscess, obstruction, fistula, etc) makes it an essential imaging tool for patient management.

The earliest CT finding in patients with Crohn disease is bowel wall thickening, usually involving the distal small bowel and colon, although any segment of the gastrointestinal tract can be affected. Typically, affected bowel loops will demonstrate wall thickening ranging between 5 to 15 mm . Ulcerations in the mucosal surface can sometimes be detected on thin section CT although small bowel series or enteroclysis are much more sensitive the early mucosal changes of Crohn. In addition to bowel wall thickening, mesenteric stranding, increased mesenteric fat, local adenopathy and fistulae are common findings. CT plays an important role in the detection of extraluminal complications such as obstruction, fistulae and abscess.

One limitation of CT has been its inability to reliably distinguish active from inactive disease. The presence of mesenteric stranding does not reliably signify active disease, as residual mesenteric thickening can remain even in remission. However, the introduction of faster MDCT scanners and thinner collimation, along with faster IV contrast injection has allowed a more detailed evaluation of the bowel wall. Investigators are now able to measure the enhancement the bowel wall, which may prove valuable in imaging patients with Crohn disease. For example, in an article by DelCampo et al, bowel wall attenuation was measured in 50 patients with Crohn disease after IV contrast. Patients with active disease demonstrated and average bowel wall attenuation of 95HU compared with the 65 HU in patients in disease remission. The ability to measure bowel wall enhancement may prove valuable for the detection and management of patients with Crohn disease . More investigation of these findings is needed.

CT Evaluation of Gastrointestinal Bleeding

CT scanning is usually not indicated in patients with active gastrointestinal bleeding, who typically require emergent intervention. However, CT may be helpful in patients with chronic or subacute bleeding. For example, CT with 3D imaging can detect the presence of gastroesophageal varices, small bowel neoplasms, enteritis/colitis and pseuodaneurysms, all of which may result in bleeding. In addition, there has been recent interest in the ability of CT to detect vascular lesions of the bowel such as hemangiomas, arteriovenous malformations, or angiodysplasia . The detection of such lesion requires fast scanning, thin collimation and fast IV contrast administration. However, the sensitivity and specificity of CT for the detection of such lesions is not yet known. It is likely that CT will play a complimentary role to endoscopy and nuclear medicine examination in this patient population.

Conclusions

Continued advancements in CT technology including multidetector CT and 3D CT imaging have improved CT evaluation of the intestines, bowel wall enhancement and mesenteric vasculature. This, in turn has lead to an expanded role of CT in the diagnosis of mesenteric ischemia, Crohn disease and gastrointestinal bleeding, often obviating the need for additional, more invasive diagnostic tests, e.g. angiography. The ability of CT to image the gastrointestinal tract and its vascular supply will continue to improve with the introduction of even faster scanners. The newest scanners which will be available in 2002 utilize 32 detector rows which allow up to 16 0.5mm slices to be acquired in less than 0.5 seconds. This will result in even better 3D data sets for improved CT vascular imaging. These faster more detailed acquisitions will also allow more dynamic/functional information to be obtained and therefore may improve the CT diagnosis of many inflammatory conditions as well as vascular pathology.