Year : 2006 | Volume
: 16 | Issue : 4 | Page : 431--432
MDCT enteroclysis new kid on the bloc
Head Dept of Radiology, Breach Candy Hospital and Research Centre, 60, Bhulabhai Desai Road, Mumbai - 400 036, India
Head Dept of Radiology, Breach Candy Hospital and Research Centre, 60, Bhulabhai Desai Road, Mumbai - 400 036
|How to cite this article:|
Kohli A. MDCT enteroclysis new kid on the bloc.Indian J Radiol Imaging 2006;16:431-432
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Kohli A. MDCT enteroclysis new kid on the bloc. Indian J Radiol Imaging [serial online] 2006 [cited 2020 Aug 14 ];16:431-432
Available from: http://www.ijri.org/text.asp?2006/16/4/431/32417
The small bowel is the most challenging part of the alimentary tract to image due its length, contoursity and overlapping loops. This is also the only organ not fully accessible by endoscopy. The conventional double contrast small bowel enema/enteroclysis is accepted as the gold standard for assessing small bowel pathologies. However as time goes by and technologies improve we find the so called gold standards have limitations and deficiencies. Similarly the conventional small bowel enema has its share of limitations. The small bowel enema evaluates only the lumen of the small bowel, it does not provide any information about the wall or extraluminal pathology. It is a 2 D modality, as a result there is significant overlap of small bowel loops. These loops need to be separated by manual compression, loops which are deep in the pelvis are difficult to compress due to the bony pelvis. Even with manual compression it is impossible to be sure that every loop has been visualized separately. Occasionally there may be diagnostic problems in differentiating a stricture from a peristalsis as the bowel wall is not seen on a conventional small bowel enema. One of the main disadvantages is the radiation dose, not only to the patient but the operator also, especially if a lot of manual compression of bowel loops is required.
Conventional small bowel enema has a new competitor, MDCT of the small bowel. This is a cross sectional imaging technique so every loop can be seen. With thin collimation available, the bowel can be imaged in multiple planes. Most importantly, MDCT visualizes not only the lumen but also the wall of the bowel as well as the structures external to the bowel. The only disadvantage of MDCT is that there is inadequate distension of the bowel lumen, as a result mucosal abnormalities are not well visualized. To counter this disadvantage, when MDCT is used to study the small bowel a CT enteroclysis technique is added to the examination. It is ideal to intubate the proximal jejunum with a nasojejunal tube. 12 Fr tubes are routinely available, with the possibility of 8 Fr tubes to be available in the near future. Similar to the initial steps of a conventional small bowel study, the nasojejunal tube is placed beyond the ligament of treitz under fluoroscopy. 150 to 200ml of contrast is injected via the tube per minute to a total contrast quantity of 1500-2000 ml. If the patient complains of discomfort the rate can be reduced or contrast stopped. 20 mg of buscopan just prior to doing the CT exam helps to reduce artifacts from small bowel peristalsis. Occasionally patients object to insertion of a naso jejunal tube, in these indivuals a noninvasive CT Enteroclysis may be performed. In this technique no NJ tube is used, 450 ml of contrast is administered orally every 15 min. At the end of an hour CT is performed. The disadvantage of this over the invasive technique is that the distension is not as adequate as the invasive technique, as a result there may be false positive results for bowel wall thickening. There are three types of enteral contrast agents that can be used positive - hyperdense contrast agents like iodinated contrast medium, neutral which are isodense such as water and negative such as air. The negative contrast agents are very useful in evaluating the stomach, duodenum and colon. In the small bowel there are numerous technical difficulties in insufflation of air, patients experience significant discomfort especially as there is late elimination of air after enteroclysis. Neutral contrast agents are very useful in good delineation of contrast enhanced mucosa, therefore early mucosal abnormalities such as tumors, polyps, vascular malformations can be imaged well. Also opacified mesenteric vessels can be seen well through the isodense neutral contrast. Water is a very good neutral contrast agent especially for the stomach and duodenum as it is inexpensive and readily available. Unfortunately it provides suboptimal distension of the small bowel as it is absorbed by the intestinal mucosa. This absorption from the intestinal mucosa creates problems in indivuals with cardiac and renal failure as it increases the preload, therefore contraindicated in these indivuals. Paraffin methylcellulose suspension may also be used as it provides an excellent bowel wall and lumen attenuation separation, however if the patient vomits there is a significant risk of an aspiration chemical pneumonitis. The ideal neutral contrast agent is 5% methylcellulose or mannitol. Positive contrast agents have been conventionally used for opacifing the small bowel, these are losing their lusture, as enhancement of bowel wall/mucosa cannot be visualized with positive contrast agents. Positive contrast agents are useful especially if there is an enteral leak/fistula as even a small quantity of extra luminal contrast can be easily detected. In addition to enteral contrast IV contrast is also useful. 75 ml is injected at a rate of 3 ml/sec, images are obtained in three phases, an early arterial phase at 20 sec to obtain a arteriogram, a late arterial phase at 35 sec to detect any hypervascular hepatic lesions and assess vascularity of bowel wall and finally a portal venous phase approx 70 seconds after onset of administration of contrast. This also helps to assess bowel wall vascularity as well as provides a CT Venogram of the mesenteric circulation.
Wire less capsule endoscopy is a new technique for imaging the small bowel. In this technique a small capsule with a recorder is swallowed and propelled by peristalsis, endoluminal images are transmitted to an external recorder. The advantage of this technique over all other techniques is its higher sensitivity in detecting small mucosal lesions especially angiodysplasia and aphthous ulcers. It is contraindicated in indivuals with a stricture or partial small bowel obstruction as the capsule can be retained. Lesion location and evaluation are difficult as the camera can not be rotated, size of lesion can not be measured, other disadvantages are its inability to visualize lesions inside and outside the bowel wall, it has an 8 hr recording time, 2 hr download time and 2 hr viewing time for the clinician as well as costs 40000 rupees.
CT Enteroclysis is now considered the modality of choice for evaluating inflammatory lesions, tumors, obstruction and ischemia of the small bowel. The conventional small bowel enema has still a role in evaluating functional disorders, wireless capsule endoscopy is useful in chronic/obscure causes of GI bleeding.