Conventional radiologic and endoscopic evaluations of the small bowel are often limited by the length, caliber, and motility of the small bowel loops. The development of new multidetector-row CT scanners, with faster scan times and isotropic spatial resolution, allows high-resolution multiphasic and multiplanar assessment of the bowel, bowel wall, and lumen. CT Enterography (CTE) is a variant of routine abdominal scanning, geared toward more sustained bowel filling with oral contrast material, and the use of multiplanar images, that can enhance gastrointestinal (GI) tract imaging. This article examines the techniques and clinical applications of CTE in comparison with CT enteroclysis, focusing on Crohn disease, obscure GI bleeding, GI tumors, acute abdominal pain, and bowel obstruction.
Imaging the small bowel has always been a challenge: conventional radiologic and endoscopic evaluations are often limited by the length, caliber, and motility of the small bowel loops. Computed tomography (CT) is used extensively in the abdomen for a variety of indications, but imaging evaluation of the bowel, especially the bowel wall, can be limited because of its length and orientation, the difficulty in sustaining distension and homogeneity of the oral contrast column, and variability in intravenous (IV) contrast enhancement. The development of new multidetector-row CT scanners, with faster scan times and isotropic spatial resolution, allows high-resolution multiphasic and multiplanar assessment of the bowel, bowel wall, and lumen. Conventional positive (high) attenuation oral contrast material shows mucosal detail while neutral attenuation oral contrast allows assessment of mucosal enhancement.
CT Enterography (CTE) is a variant of routine abdominal scanning, geared toward more sustained bowel filling with oral contrast material, and the use of multiplanar images, that can enhance gastrointestinal (GI) tract imaging.
Technique
The goal of CTE is to discriminate the bowel, distend the lumen, visualize the intestinal wall, identify the vessels supplying the bowel loops, and assess the mesentery. While oral gastrointestinal luminal contrast material is required, the use of IV contrast material is also very helpful and should be encouraged unless there are contraindications. The amount and timing of oral contrast administration affect the degree of distension of the bowel, while the timing and injection rate of IV contrast administration determine the degree of bowel wall enhancement.
Data are acquired with volumetric techniques using thin collimation. Prone positioning can help disperse the bowel loops but is rarely used.
Oral Contrast Agents: Positive and Neutral Attenuation
The small bowel must be adequately distended to perform CTE, because collapsed or poorly distended loops can obscure existing pathologic processes, or mimic pathology. To achieve optimal small bowel distension, a large volume of oral contrast must be administered within a short time. Two types of oral contrast agents can be used for CTE: positive and neutral ( Fig. 1 ). High-attenuation, or positive, contrast agents are routinely used in CT, and were used in the initial descriptions of CTE. Neutral, or near-water attenuation contrast, has been proposed as an alternative medium as it allows assessment of mucosal enhancement.
Positive oral contrast
For CTE, 1600 mL of 2% barium-based or 2% to 2.5% water-soluble iodine-based oral contrast are administered over 1 to 2 hours before scanning. This dose is 1.5 to 2 times that used for abdominal CT. High-attenuation oral contrast is helpful in patients with Crohn disease to evaluate for fistula and sinus tracks, or in those with suspected abscess. In suspected partial obstruction, high-attenuation oral contrast can be used in conjunction with low-dose sequential scanning. This technique may provide indirect information on bowel motility and degree of obstruction. Variations to this regimen are used, all with very good results. The authors routinely administer 400 to 600 mL over an interval of 40 to 60 minutes before scanning and another 200 to 400 mL in the last 20 minutes.
The use of positive contrast agents provides excellent background for detecting intraluminal filling defects, for example polyps, and depiction of mucosal detail. However, the high attenuation can impair detection of the mural features of the GI tract, identification of sources of obscure GI bleeding, and assessment of mucosal enhancement.
In a randomized controlled trial, Erturk and colleagues compared high-attenuation and low-attenuation oral contrast agents in 90 patients without small bowel disease, and concluded that neutral (low-attenuation) contrast agents provide equal or superior distension and bowel wall visualization compared with high-attenuation contrast media.
For CTE, most investigators nowadays favor the use of neutral oral contrast agents, but all agree that positive oral contrast is preferable when intravenous contrast material cannot be administered. In addition, a modified CTE technique can be used instead for routine abdominal or emergency scanning, in which 1 L of oral contrast (2% barium suspension + 5 mL of Gastrografin) is given as tolerated, 40 to 50 min before scanning. This modification is well tolerated by patients with abdominal pain, and provides satisfactory and consistent filling and homogeneity of intestinal lumen from duodenum to cecum in the majority of studies.
Neutral oral contrast
Several neutral enteric contrasts with near-water attenuation (0 HU) are available: water, whole milk (4% fat), polyethylene glycol (PEG), 12.5% corn-oil emulsion, and methylcellulose. Recently a low-attenuation barium solution with sorbitol (0.1% weight per volume barium sulfate suspension; VoLumen, E-ZEM) has gained popularity and is considered the neutral oral agent of choice. Sorbitol minimizes water resorption in the small bowel, improving lumen distension.
Young and colleagues and Kuehle and colleagues compared different neutral contrast agents, and concluded that water provides the poorest distension because it is absorbed more readily than the other agents, while VoLumen provided the best distension. Megibow and colleagues demonstrated that VoLumen significantly improved distension in all bowel segments compared with water and methylcellulose solution. Finally, Koo and colleagues compared milk and VoLumen without observing significant differences in bowel distension.
Several slight variations in oral contrast administration protocols have been proposed in the literature.
Fasting before the examination is generally advised to reduce the possibility of misinterpreting ingested hyperattenuating debris such as enhancing lesions or bleeding.
Kuehle and colleagues showed that the best bowel distension was achieved with 1350 mL of low-attenuation barium suspension with sorbitol, and that increasing the volume to 1800 mL did not improve distension, but led to a lower patient acceptance and a higher rate of side effects.
Tochetto and Yaghmai and Paulsen and colleagues propose the administration of 1350 mL of neutral enteric contrast over 60 minutes: 450 mL in the first 20 minutes, 450 mL in the second 20 minutes, 225 mL in the third 20 minutes, and 225 mL on the CT table. A similar protocol has been proposed by Huprich and Fletcher.
Optimal distension of the terminal ileum is achieved 45 to 60 minutes after the ingestion of oral contrast. The intake of oral contrast should be continuous and well timed. It is therefore advisable that patients are monitored by technologists or nurses while ingesting contrast, to avoid the risk of a poor study.
Positive versus neutral attenuation oral contrast media
Positive oral contrast agents are better bowel markers and can assess obstruction more accurately. In addition, they provide mucosal detail and can depict intraluminal defects, strictures, ulcers, perforation, and abscess. Neutral agents provide more consistent bowel dilation and are unique in assessing mucosal enhancement, which is associated with exacerbation of chronic inflammatory bowel disease. In addition, they depict mucosal folds and can be used in imaging of GI bleed.
CT Enteroclysis
Enteroclysis is performed after insertion of a nasoduodenal tube just beyond the ligament of Treitz and administration of more than 2000 mL (60–100 mL/min) of enteric contrast with an automatic pump. Previous preparation of the bowel is advised. As described by Maglinte and colleagues, this technique provides superior distension of the small bowel. However, placement of a nasoenteric tube is associated with patient discomfort and longer examination times.
IV Contrast
The combination of high iodine concentration and relatively fast injection rates produces excellent and consistent vascular, intestinal wall, and organ enhancement.
Most often, CTE includes single-phase scanning known as the “enteric phase.” Schindera and colleagues examined the attenuation of the aorta and the small bowel wall at 5-second intervals after injection of contrast medium at 5 mL/s, and observed peak small bowel wall enhancement 50 seconds after the start of the contrast medium injection. Wold and colleagues did not observe significant differences between arterial and venous phase images in patients with Crohn disease, which supports single-phase imaging.
The authors vary the amount of IV contrast between 120 mL and 150 mL of IV depending on patient weight (under or over 75 kg, respectively). The authors routinely use a 320 mgI/mL solution. For patients who weigh more than 90 kg, 350 mgI/mL solution is used. Nonionic contrast is now universally used, and is preferred in order to avoid nausea or vomiting in patients with gastrointestinal tracts already overdistended by oral contrast. The authors use a split-bolus injection and single, combined-phase scanning regimen: first, 40 mL of contrast is injected at 2 mL/s. After a delay of 2 to 3 minutes, 80 mL is injected at 2 to 3 mL/s. For patients heavier than 75 kg, 50 mL is injected first, followed by 100 mL. Scanning starts 60 seconds after the beginning of the second dose. This split-bolus injection regimen results in a combined phase with good enhancement of the bowel wall, the solid organs, the mesenteric and retroperitoneal vessels, as well as the kidneys and ureters. Other regimens are also effective, and one may not need to change the general IV contrast regimen used for routine abdominal scanning in the particular institution.
For the evaluation of occult GI bleeding, a multiphasic CTE protocol has been proposed by Huprich, based on a bolus triggering technique. The first “arterial” phase is triggered automatically when aortic attenuation reaches 150 HU after contrast injection by placing a cursor over the descending aorta 2 cm above the diaphragm. The second, “enteric” and third, “delayed,” phases are acquired 20 to 25 seconds and 70 to 75 seconds, respectively, after the beginning of the injection. However, this repeated scanning is associated with high radiation dose and should be used only when other methods have failed.
Scanning Techniques
The abdomen is scanned from the dome of the diaphragm to below the symphysis pubis. Volumetric isotropic scanning with thin collimation (0.5 or 0.625 mm) is important so that meaningful multiple projection reformatted (MPR) images can be obtained. To improve quality and decrease image glut, thicker slices are used for image interpretation: 3- to 5-mm thick axial and MPR images, reconstructed from thin overlapping slices.
Multiplanar reformations
Jaffe and colleagues showed that, with regard to the presence of intra-abdominal abnormalities, coronal reformations from isotropic voxels (same resolution in all planes) are equivalent to transverse scans in terms of interpretation time and reader agreement. Similarly, Sebastian and colleagues have demonstrated that the use of coronal reformats increases reader confidence, and these investigators suggest that coronal views could be used for primary interpretation. Coronal images are useful for quantifying the length of involved bowel and improve visualization of the terminal ileum, whereas sagittal reformations are useful in evaluating the rectum and the presacral area, as well as the mesenteric vessels.
Multiplanar CTE Versus CT Enteroclysis
As with conventional fluoroscopic enteroclysis, CT enteroclysis provides a consistent and high degree of bowel distension. The resultant studies are of superb quality and can provide the fine mucosal and luminal detail comparable to conventional double-contrast fluoroscopic enteroclysis. CT enteroclysis is contraindicated in patients with suspected bowel perforation or small bowel obstruction; however, it is very helpful in patients with partial small bowel obstruction and Crohn disease. Despite high image quality, CT enteroclysis may be uncomfortable for the patient and the cost is increased because of the tubing, the pump, and the longer use of the CT room. The risk of complications is also increased because, in contrast to fluoroscopic techniques, the bowel is filled without visual monitoring. Thus, the additional information provided may not be worth the effort in the majority of indications. In the past few years magnetic resonance (MR) enteroclysis techniques have been developed, providing an alternative to CT enteroclysis and CTE. In addition, these studies can provide real-time functional information allowing assessment of peristalsis.
Although multiplanar oral CTE provides less distension of the bowel, this may be more physiologic than the unnaturally overdistended small bowel achieved with CT enteroclysis. Furthermore, the examination is more comfortable to the patients and requires less preparation and room/equipment use. Although the fine anatomic mucosal details fall behind fluoroscopic studies, other signs that help characterize small bowel disease are available, including wall enhancement patterns, extraluminal abnormalities, and assessment of the mesentery. Therefore, the authors favor the use of CTE with high-attenuation oral contrast, which they use as a routine protocol for CT evaluation of gastrointestinal abnormalities, including acute abdominal pain. The authors use neutral attenuation CTE selectively for occult GI bleeding and for evaluation of active inflammatory bowel disease.
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