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Optimal small bowel evaluation includes use of neutral contrast, 3D processing, and use of dose-reduction techniques
Optimal small bowel evaluation includes use of neutral contrast, 3D processing, and use of dose-reduction techniques
CT enterography is simply defined as the utilization of CT scanning specifically to evaluate the bowel. While bowel abnormalities have been characterized and diagnosed with CT scanning for many years, CT enterography uses a neutral (close to water attenuation) oral contrast agent to distend the bowel. Beyond bowel wall distention, neutral contrast allows the radiologist to carefully examine the folds of the bowel and the bowel wall itself.
CT enterography is not a new idea. In 1988, Raptopoulos and colleagues described the use of an oral corn-oil emulsion with resultant exquisite bowel wall visualization.1 Widespread use of this agent was limited, however, by poor patient acceptance and the cost associated with its use. In 1996, Winter and colleagues reported on the value of oral water as an agent to mark and distend the duodenum and proximal bowel for CT evaluation of the pancreas.2
Raptopoulos and colleagues were the first to describe the technique of CT enterography, exploiting the multiplanar capabilities of single-slice spiral CT. In their study, these capabilities did not add new diagnostic information but rather improved reader confidence in diagnosis.3 Investigators quickly realized the limited resolution afforded by single-slice CT technology but began to explore neutral contrast CT evaluation of the bowel using the enteroclysis technique as multislice scanners became more widely available.4 The advantage of this technique was its ability to reliably and uniformly distend the entire small bowel, minimizing errors due to collapsed bowel loops that could create a spuriously thickened wall or mask significant pathology.5 While the technique produced exquisite results, the necessity of nasojejunal intubation hindered patient acceptance.
Peroral CT enterography required development of oral contrast agents that could achieve patient tolerance, provide low-attenuation distention of the bowel, and undergo limited absorption from the intestinal lumen. Currently, a low-Hounsfield unit barium suspension (Volumen, E-Z-EM, Westbury, NY) satisfies these conditions. We have demonstrated that this agent provides significantly superior distention and visualization of mural features of the stomach, duodenum, and small bowel when compared with water. The agent was also shown to result in superior distention of small bowel loops compared with conventional CT barium.6
As with any multislice CT application, successful clinical results are directly related to proper patient preparation and selection of correct acquisition parameters. Patients must be able to consume at least 1000 mL of the contrast agent, with 1200 mL optimal. The patient is instructed to drink the contrast steadily. Centers vary in the time over which they recommend agent administration. We ask patients to drink two doses of 450 mL of contrast over 20 to 30 minutes. They are then given an additional 150 to 200 mL just before they change for the examination and a final 150 to 200 mL when they enter the CT scanning suite. Other institutions give the agent over the period of an hour. Regardless of the timing of administration, the patient must drink steadily and consume the entire volume of the oral agent.
We employ a single-phase study unless we are evaluating mesenteric ischemia. We use the narrowest detector configuration, creating three data sets: continuous 4-mm-thick axial images for PACS or film, continuous 3-mm coronal multiplanar reformatted images for PACS or film, and 0.75 to 1-mm continuous slices sent to the 3D workstation.
The study must be performed with intravenous contrast, which provides the enhancement of the bowel wall necessary for detecting abnormalities against the water attenuation intestinal lumen. The IV contrast should be administered at a rate of at least 3 cc/sec. The study cannot be performed on patients who cannot receive IV contrast, whether due to medical contraindication or lack of safe venous access. These patients are all studied with positive-contrast oral agents. We begin our data acquisition 65 to 70 seconds after the initiation of the bolus. When patients are referred for intestinal/mesenteric ischemia, we use a dual-phase acquisition, with an arterial phase at 55 seconds and a mural phase at 70 seconds. This approach allows creation of CT angiograms and helps accentuate regional variation in blood flow to the bowel wall.
These acquisition protocols facilitate multiplanar and 3D visualization of the data set. Interpretation of these studies requires that the imaging data be viewed in multiple planes. Predetermined coronal MPR images may be adequate; however, when attempting to trace complex fistulae in Crohn's disease patients or present the images for surgical decision-making, however, tailored 3D images are the most effective means to communicate diagnostic information.
CT enterography is indicated for the evaluation of patients with Crohn's disease, suspected small bowel tumors, bowel obstruction, mesenteric/intestinal ischemia, and suspected GI bleeding. It is also used in evaluation of bowel patency before capsule endoscopy.7 The principles of diagnosis by CT enterography are similar to those for conventional CT scanning. Mural thickening is the primary diagnostic feature signaling pathology. With enterography, the radiologist can also incorporate an assessment of the nature of the thickening (presence or absence of mural stratification) and levels of enhancement in refining the differential diagnosis.
- Crohn's disease. The most extensive experience in the utility of CT enterography is the evaluation of patients with known or suspected Crohn's disease. Three-D displays are ideally suited to depict the location and extent of bowel involvement. Changes in the perienteric fat, including fistulae and abscesses, are readily diagnosed and can be evaluated for surgical planning. Hara and colleagues have shown that CT enterography may depict nonobstructive Crohn's disease when techniques such as ileoscopy and small bowel follow-through have negative or inconclusive findings.8
The higher sensitivity of CT enterography compared with other radiologic imaging procedures establishes it as a first-choice procedure. Several centers have begun to use CT enterography prior to capsule endoscopy to screen patients for possible strictures that could result in obstruction of the capsule, necessitating surgical removal.
One of the most exciting areas of investigation has been in the radiologic determination of disease activity in Crohn's patients. Bodily and colleagues correlated the degree of mural enhancement with ileoscopic observations of disease activity. Because the intestinal lumen is of lower attenuation than the intestinal wall, quantitative evaluation of the degree of mural enhancement can be recorded. These researchers found that high attenuation (mural hyperenhancement) had a sensitivity in diagnosing active disease identical to ileoscopy.9 As these results are reproduced, CT enterography will emerge as a rapid noninvasive method to obtain this determination.
- Small bowel tumors. CT enterography can display small bowel tumors with the same degree of accuracy as positive-contrast MSCT.10 Advantages of the 3D technique include the ability to display the information in a plane useful to the operating surgeon and the ability to create angiographic images isolating the major blood supply to the tumor. CT enterography is particularly effective in localizing an abnormality detected by capsule endoscopy. The superior distention of the duodenum and small bowel achieved with properly performed CT enterography can minimize confusion between a truly thickened bowel wall and a partially collapsed loop.
Radiologists must be extremely cautious when interpreting CT enterography studies in patients with peritoneal malignancies and pelvic tumors. Small peritoneal implants and cystic pelvic tumors may harbor masses whose Hounsfield values can be close to that of the intestinal lumen. When the bowel is well distended, IV contrast administered properly, and 3D imaging incorporated into the evaluation, pathologic masses can usually be distinguished from adjacent bowel loops.
- Mesenteric ischemia and gastrointestinal bleeding. CT enterography facilitates the diagnosis of mesenteric ischemia by providing a clear window through distended bowel loops to the mesenteric vasculature. Occlusive disease in mesenteric arteries may or, more likely, may not be causative of ischemic symptoms due to the extensive collateral circulation that provides a redundant blood supply throughout the small bowel. By applying currently available 3D volume rendering capabilities with the CT enterography acquisition, the radiologist can not only create high-quality angiographic images but can simultaneously assess regional perfusion of the intestinal wall.11 Intensive investigation is under way in many institutions trying to determine protocols for diagnosing mesenteric ischemia and establishing pathophysiologic significance to anatomic occlusions. A biphasic (arterial and "mural" phase) acquisition protocol is thought to be the most sensitive and specific for establishing the diagnosis.12
In current clinical practice, patients with occult GI bleeding are frequently referred for conventional barium small bowel follow-through studies when upper GI endoscopy and/or fiberoptic colonoscopy are normal. CT enterography may become the follow-up procedure of choice. CT scanning can detect intestinal bleeding at rates of 0.5 mL/sec or less.13 An area of contrast pooling on arterial phase MSCT scanning within the intestinal lumen is diagnostic.14 The ability to analyze the affected bowel at the site of contrast extravasation, as facilitated by MSCT, can establish the etiology.
- Other indications. MSCT performed with neutral contrast and 3D imaging can be extended to evaluation of gastric and colonic diseases. Distention of the stomach improves detectability and, therefore, staging of gastric neoplasms. When the protocols for oral contrast administration are followed, the large bowel may be filled as well. Without bowel preparation, however, colonic findings must be carefully evaluated before rendering a final diagnosis.
Well-performed CT enterography is rapidly becoming a first-line procedure for evaluation of small bowel diseases, but the technique may be suboptimal in several situations. The most common is in the diagnosis of acute appendicitis. We find that distal ileal loops on CT enterography can have an appearance identical to the abnormal thick-walled, fluid-filled distended appendix. Potential pitfalls associated with cystic pelvic neoplasms and tiny peritoneal implants have already been encountered.
Radiologists must also consider the radiation dose associated with this procedure. Without dose correction, our protocols result in a CT dose index of approximately 10 mGy, comparable to the dose delivered by a conventional MSCT scan performed with 16- or 64-slice systems. By using dose reducing technology with a slightly lower reference mA and accepting slightly noisier images, particularly in patients under 45 years, we can perform these studies with a 10% to 20% reduction in dose. These doses are in the range of those delivered during traditional fluoroscopic evaluation of the small bowel.
Dr. Megibow is a professor of radiology and director of outpatient imaging at the New York University Medical Center in New York City. He is a consultant with E-Z-EM and on the CT medical advisory board of Siemens Medical Solutions.
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