| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Editorial |
1 From the Department of Radiology, Methodist Hospital of Indiana, 1701 N Senate Blvd, Indianapolis, IN 46202 (D.D.T.M., F.M.K.), and the Department of Radiology, University of Pennsylvania Medical Center, Philadelphia (E.S.S.). Received February 29, 2000; accepted March 3. Address correspondence to D.D.T.M. (e-mail: dmraddoc@aol.com).
Index terms: Crohn disease, 74.262 • Editorials • Enteritis, 74.23, 74.261, 74.262 • Enteroclysis, 74.1269 • Intestines, diseases, 74.23, 74.261, 74.262 • Intestines, MR, 74.121411, 74.121412 • Intestines, stenosis or obstruction, 74.297
The radiologic assessment of the mesenteric small bowel is somewhat analogous to separating out and identifying a large number of writhing snakes in a crowded reptile tank at the zoo (1). This segment of the gastrointestinal tract has remained terra incognita as regards direct visualization, still defying the depredations of enteroscopists. The limitations of small-bowel radiologic investigation with oral contrast material have long been recognized (2). The dilemma of choosing between the oral approach, which is more acceptable to the patient, and the intubation-infusion method (enteroclysis), which is less well tolerated in the absence of conscious sedation, has not been resolved by the radiologic community. On the basis of long-term follow-up results (3,4), enteroclysis has been shown to be a reliable method for investigation of an organ where a high negative predictive value is needed.
In current clinical practice, magnetic resonance (MR) imaging has played a limited role in the evaluation of the small bowel, although its applicability in the diagnosis of diseases of the small bowel has been shown (5). The article on MR enteroclysis by Umschaden et al (6) in the current issue of Radiology provides a promising evaluation of a technology that has revolutionized diagnostic evaluation of other organs of the human body. MR imaging has the potential to change how we assess the small bowel, because of the functional information and soft-tissue contrast it can provide, the direct multiplanar imaging capabilities, and the lack of ionizing radiation. Although the number of patients described by Umschaden et al was small, their results show the advantage of MR imaging when combined with infusion-induced distention of the small bowel. Radiologists who are interested in performing MR enteroclysis should have a basic understanding of MR technology and the nuances of enteroclysis to utilize the full potential of this promising technique.
MR enteroclysis has been performed with both negative iron–based (7) and positive gadolinium-based (8,9) contrast agents. The use of methylcellulose in water as the enteric contrast agent, together with the intravenous administration of gadopentetate dimeglumine, is a practical alternative to the use of a positive enteric contrast agent. Distention of the small-bowel lumen with methylcellulose solution and intravenous administration of gadopentetate dimeglumine provide optimal contrast between the bowel wall and lumen (10).
Although cross-sectional imaging has been shown (10) to be relatively sensitive for high-grade small-bowel obstruction, its accuracy for the diagnosis of low-grade obstruction is poor. The ability of real-time MR imaging (MR fluoroscopy) to demonstrate the enteric agent makes it ideal for assessment of the severity of small-bowel obstruction, a question frequently posed by physicians who treat patients with mechanical small-bowel obstruction (11). The bowel wall enhancement provided by a gadolinium-based agent allows assessment of inflammatory disease activity (12), as well as the detection of small mural masses.
Barium sulfate has been investigated and allows the potential to perform barium sulfate–enhanced enteroclysis followed by MR imaging while the small bowel is still distended (13,14). To date, an ideal and universally available enteric contrast agent has yet to find widespread clinical use. The combination of a methylcellulose solution with intravenously administered gadopentetate dimeglumine, as used by Umschaden et al (6), appears to be the ideal enteric contrast agent. Methylcellulose in water is inexpensive and readily available, and it is a biphasic enteric contrast agent (low signal intensity on T1-weighted MR images and high signal intensity on T2-weighted images).
Despite precautions, vomiting and inadvertent rectal evacuation were limitations of MR enteroclysis in the report by Umschaden et al (6). Positioning of a physician near the MR unit to intervene quickly when vomiting is imminent and the use of a rectal tube in anticipation of inadvertent emptying will likely be impractical in many practices in the United States. These undesirable events can be diminished by understanding the response of the small bowel to different rates of infusion (15). In our experience, the injection rate used in this report (mean, 150 mL/min) was excessive and, together with the use of a catheter without an antireflux mechanism, likely accounted for the above-mentioned events. This rate of infusion usually abolishes peristalsis of proximal loops of small bowel, which results in duodenogastric reflux and vomiting. Because of inhibited peristalsis, the small bowel is not uniformly distended: The proximal portion of the small bowel is markedly distended, while the distal portion is collapsed.
To challenge the distensibility of the entire small bowel, which is the aim of enteroclysis, intermittent adjustments of flow rates during fluoroscopy are needed, increasing flow when excessive peristalsis is noted and decreasing flow when a segment becomes aperistaltic (15). The goal is to produce moderate distention from the jejunum to the distal portion of the ileum. In our experience, injection rates of 55–150 mL/min, adjusted constantly during fluoroscopy, will help decrease the aforementioned undesirable events, result in adequate distention, and allow optimum volume challenge of the entire small bowel. Higher rates are rarely necessary, in our experience. A total of less than 1,500 mL of methylcellulose solution is usually all that is needed if flow rates are appropriately adjusted. A larger volume will result in a higher frequency of vomiting. Uniform moderate distention of the entire small bowel challenges the distensibility of the wall of all segments and results in less contrast agent reaching the colon during image acquisition. The use of a balloon nasoenteric catheter (MEC; Cook, Bloomington, Ind, or Lafayette Pharmaceutical, Lafayette, Ind) to diminish reflux and control the degree of distention will result in a decrease in these untoward events (15).
The maintenance of adequate distention of the entire small bowel during MR imaging should not require the administration of a hypotonic agent (glucagon or hyoscine butylbromide) if the appropriate rates of flow are utilized and the injection is continued during image acquisition. Although these agents diminish or abolish peristalsis, they do not distend the bowel. It is the infusion that distends the lumen, and increasing the rate of flow as desired can diminish or abolish peristalsis. This is possible with the patient in the bore of the magnet and administration of the aqueous methylcellulose solution during real-time MR imaging. We have used hypotonic agents in the past during image acquisition with computed tomographic (CT) enteroclysis, but further experience showed that continued controlled infusion during image acquisition results in more uniform and better distention of the small bowel, as compared with that achieved by stopping the injection and administering a hypotonic agent (16). The small-bowel folds are shown to better advantage when infusion of the contrast agent is continued throughout the imaging phase. If administration during image acquisition is impractical, however, a hypotonic agent will be of value.
The diagnostic images that are acquired in the coronal plane at MR imaging represent a distinct advantage over the current CT enteroclysis technique. Although CT enteroclysis has facilitated diminishment of the subjective factor inherent in barium sulfate enteroclysis, one benefit of MR enteroclysis relative to CT enteroclysis is real-time functional information provided by MR imaging (13). The need for fluoroscopic experience is a disadvantage of barium sulfate enteroclysis. With real-time MR imaging, this influence can be reduced. Real-time imaging of the small bowel during injection could be obtained by using breath-hold T2-weighted sequences, the images from which could then be reviewed in a cine-loop format to obtain functional information concerning bowel obstruction. This is important at teaching institutions, where inexperienced residents perform real-time assessment and where on-site staff supervision may not always be possible.
One important variable that is lost with both MR and CT enteroclysis, as compared with barium sulfate enteroclysis, is the ability to compress the distended bowel. Are small intramural nodules going to be overlooked at MR enteroclysis because compression is not used, or will the direct multiplanar capabilities and the distention produced by contrast agent administration make compression dispensable? It is unlikely that compression will be critical, as long as the small bowel is distended and multiplanar images are acquired. Further experience will answer this question fully. The single false-negative diagnosis of small-bowel obstruction secondary to overlapping bowel segments reported by Umschaden et al (6) may be related to a limitation in the imaging technique they used. If multiple, thin, tomographic sections had been obtained during a breath hold, these images might have helped prevent the false-negative result.
Whether MR enteroclysis will readily replace currently used methods of small-bowel imaging will depend on how this method can be integrated in the clinical setting in a practical manner that will be acceptable to patients, referring clinicians, and surgeons. Economic considerations and the interest or expertise of radiologists will be important factors. Although Umschaden et al (6) emphasize the merits of intubation for administration of the contrast agent at small-bowel imaging along with the added distinct advantages of MR, it is unfortunate that their method has the disadvantage common to all enteroclysis methods: namely, the need for a nasoenteric tube.
This disadvantage can be minimized and turned to an advantage for radiologists. Among procedures commonly performed at an adult emergency department, nasogastric intubation is the most painful, as assessed by both patients and practitioners (17). Strategies for reducing the pain and discomfort of nasogastric intubation have been emphasized (18). The use of a smaller-sized catheter, such as that used by Umschaden et al (6), and conscious sedation for those who request it are appropriate measures to help reduce discomfort. An important strategy that has been proposed (18) to decrease patient discomfort is the prevention of repeat intubation. This is a situation where the performance of MR enteroclysis can be made practical for the radiologist and less uncomfortable for patients.
As shown in the study of Umschaden et al (6), some patients with small-bowel obstruction and Crohn disease will require further imaging, and some will need therapeutic nasogastric or nasointestinal decompression. Invariably, when such patients are admitted to the hospital, a nasogastric tube is introduced by an emergency physician, general surgeon, or nurse. Should these patients require MR enteroclysis, the commonly used conventional nasogastric tube must be replaced by an enteroclysis catheter, thereby subjecting patients to repeat intubation and the attendant additional discomfort and pain. It is no longer considered to be acceptable for patients to experience such discomfort and pain (18). To prevent this, radiologists should educate "first-contact" physicians and nurses about which patients may later require further diagnostic imaging and inform the practitioners that the conventional nasogastric tube cannot be used for this purpose. The initial use of a multipurpose tube (MDEC; Cook) capable of nasogastric or nasointestinal decompression and enteroclysis instead of the conventional nasogastric tube will help simplify the performance of MR enteroclysis, both for the patient and for the radiologist. After introduction of the multipurpose tube by a first-contact physician or nurse, nasogastric decompression is carried out initially, and the radiologist simply advances the tube to the small bowel for MR or CT enteroclysis or for long-tube decompression, if needed (19). This spares the patient the discomfort and pain of multiple intubations and lessens the cost because only one tube is used; the examination is rendered easier for the radiologist.
Is MR enteroclysis the future of optimal small-bowel imaging? Radiologic services are now evaluated with criteria to assess whether a particular diagnostic method influences clinical management, improves patient outcome, and lowers medical care costs (20,21). Gadolinium-enhanced MR imaging with dilute orally administered barium sulfate and rectally administered water accurately demonstrates intestinal and extraintestinal findings of Crohn disease. It is also better than single-phase helical CT for demonstration of mural thickening and enhancement and for overall gastrointestinal tract evaluation (22). Undoubtedly, the added benefits of volume challenge, such as better depiction of mucosal folds, increased sensitivity for the diagnoses of low-grade obstruction and small mural nodules, and demonstration of small sinus tracts and fistulas, will help improve its diagnostic accuracy.
The functional information, soft-tissue contrast, direct multiplanar capabilities, and lack of ionizing radiation suggest that MR enteroclysis has a greater potential than other techniques to become the ideal diagnostic method for imaging of the small bowel. Further research and experience will help clarify whether it should be the primary method for investigation of the small bowel or used only as a problem-solving examination. To be the primary method for investigation of small-bowel disease, MR enteroclysis will have to provide reliable evidence of normalcy, allow diagnosis of early or subtle structural abnormalities, influence treatment decisions in patient care, and be cost-effective (1).
Umschaden et al (6) should be congratulated for their feasibility study showing that MR enteroclysis can be performed with a commercially available MR system. Research venues are open; the future of small-bowel imaging looks bright.
Acknowledgments
We thank Fran Shaul for secretarial assistance.
Footnotes
See also the article by Umschaden et al (pp 717–725 ) in this issue.
References
Related Article
This article has been cited by other articles:
|
|
 |
|
  H. Arslan, O. Etlik, M. Kayan, M. Harman, Y. Tuncer, and O. Temizoz Peroral CT Enterography with Lactulose Solution: Preliminary Observations Am. J. Roentgenol., November 1, 2005; 185(5): 1173 - 1179. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. B. Hunter and M. S. Taljanovic Medical Devices of the Abdomen and Pelvis RadioGraphics, March 1, 2005; 25(2): 503 - 523. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. Wittenberg, M. G. Harisinghani, K. Jhaveri, J. Varghese, and P. R. Mueller Algorithmic Approach to CT Diagnosis of the Abnormal Bowel Wall RadioGraphics, September 1, 2002; 22(5): 1093 - 1107. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. Prassopoulos, N. Papanikolaou, J. Grammatikakis, M. Rousomoustakaki, T. Maris, and N. Gourtsoyiannis MR Enteroclysis Imaging of Crohn Disease RadioGraphics, October 1, 2001; 21(90001): S161 - 172. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| RADIOLOGY | RADIOGRAPHICS | RSNA JOURNALS ONLINE |
