MDCT as a Diagnostic Tool in Evaluating Small Bowel

Disorders

By Rania Mohammed Refaat Abd El-Hamid

(MSc. )

Imaging the small bowel is challenging technically:-The organ is long and serpentine

-A large field of view and a large volume is needed to display in entirety.

-Another problem for imaging is motion, both intrinsic motion of peristalsis and the positional changes caused by breathing making their tracing very difficult.

In addition, because small bowel diseases have a low incidence, their appearance is less well known and there is an increased risk of missing them. Ever most of the common diseases in the small bowel, early changes are subtle making their diagnoses difficult

The availability of MDCT along with advancements in 3D CT imaging systems has greatly expanded the role of CT in evaluation of suspected small bowel pathology. Applications which were once routinely performed with barium studies (e.g. evaluate suspected small bowel obstruction) or modality angiography (e.g. evaluate for mesenteric ischemia) have now been replaced with CT scans. CT is now considered a first line for the evaluation of a wide variety of small bowel diseases

For an optimal display of the bowel, two things are essential in imaging: intraluminal contrast and distension. Intraluminal contrast is needed to delineate bowel loops in the abdominal cavity and to depict the bowel wall. Distension in small bowel imaging is essential to unfold the bowel tube and separate out the bowel wall. In imaging the small bowel, intraluminal contrast and distension are inseparable.

Positive intraluminal contrast Positive oral contrast are Barium sulfate or iodinated solutions. Barium should only be used in very low doses to prevent artifacts. The iodinated contrast agent meglumine diatrizoate (Gastrographin) is the

most widely used agent for CT. They have wide acceptance and a low adverse-event rate. Their use leads to a diminished display of the bowel wall because of the high

density of the lumen and hence to miss an enhancing tumour. One of the other problems with these agents is their relatively low distending capability. These positive oral contrast agents can be problematic when performing 3D imaging and 3D angiography in particular since the high-density bowel contents can obscure the opacified blood vessels and therefore need to be edited.

Copyright ©Radiological Society of North America, 2004

Furukawa, A. et al. Radiographics 2004;24:689-702

Use of positive intraluminal contrast medium

Copyright ©Radiological Society of North America, 2004

Furukawa, A. et al. Radiographics 2004;24:689-702

Use of positive intraluminal contrast medium

Neutral intraluminal contrast Contrast agents that have an intermediate density (10-30 HU). They

are used with increasing frequency. with IV contrast, they provide very good display of the bowel wall and

thus better visualization of the enhancing bowel wall is obtained. The most widely used neutral oral contrast agent is water. it is

inexpensive and universally available and well tolerated but water alone is not an ideal contrast agent, because it is absorbed early in the gastrointestinal system and is not available in the mid and distal section of the small bowel so it does not always result in optimal distention of the distal small bowel. The administration of agents such as Glucagon may improve distension but is not routinely done.

To overcome early absorption of water as a neutral contrast agent, additives that increase the osmolarity of the water are used without changing the contrast characteristics. Mannitol or other long- chain sugars can be used. The adverse effects of these additives are nausea and diarrhea.

A newly introduced neutral oral contrast agent (VoLumen) is used which is based in oral barium sulfate solutions containing all the additives but with only 0.1 % of barium sulfate. With such a low concentration of barium it has no possible contrast effect in MDCT and even though containing barium, it is considered a neutral contrast agent. Compared with water or meglumine diatrizoate, this contrast agent creates for better distension and the display of the bowel wall is very clear compared with these intraluminal contrast agents.

Milk has been used by some groups in order to distend the small bowel in patients undergoing CT angiography. Whole milk has a CT density similar to water, but has a slower small bowel transit time and should therefore result in better distention of the bowel.

Copyright ©Radiological Society of North America, 2006

Paulsen, S. R. et al. Radiographics 2006;26:641-657

Small bowel strictures in a 39-year-old man with Crohn disease and vomiting

Negative intraluminal contrast Negative contrast agents display a density below 10 HU in

MDCT and are normally fat based. Although they provide a good distension and can result in good visualization of the enhancing bowel wall they are not so widely used. Carbon dioxide could work as a negative contrast but patient tolerance is low. It is very difficult to apply as there is currently no easy way to non invasively distend the small intestine with air

Between 1,500 and 2,000 mL (or more) of contrast material is administered orally 45–90 minutes prior to the examination. To provide adequate and uniform distention of the bowel loops, patients are asked to steadily ingest the contrast material over a 20–60-minute period .

The contrast material may be administered through a nasojejunal catheter at a rate of 100–250 mL/min with the help of a roller pump and the technique is called CT enteroclysis. An increased rate of infusion or dual-phase intubation with an initial flow rate of 80–120 mL/min followed by a rate of 200 mL/min is recommended to achieve reflex intestinal atony and thereby minimize motion artifact. Use of a nasojejunal catheter allows better luminal distention but causes patient discomfort.

If necessary, 300– 1,000 mL of contrast agent can be administered transrectally.

CT scans are obtained from the dome of the liver to the level of the perineum to cover the entire course of the intestine. Imaging with the patient in the prone position is recommended to disperse the small bowel loops.

IV contrast The peroral contrast must be combined with IV contrast. Specially, the

combination of neutral oral contrast and IV contrast gives a good display of the bowel wall.

Inflammatory bowel disease and neoplasm are optimally displayed with the use of IV contrast. Non ionic iodinated contrast is most widely used today.

The ideal volume and injection rate are 125 mL and 3 to 4 mL/ s, respectively. For detailed display of the mesenteric vasculature, a higher volume (150 mL) and a higher flow rate 4-5 mL/ s are chosen. A 60 second scan delay is ideal to acquire the small bowel wall in its best enhancement phase

Copyright ©Radiological Society of North America, 2004

CT enterography differs from routine abdominopelvic CT in that it makes use of thin sections (2 to 2.5 mm section thickness and reconstruction intervall 1 to 1.5) and large volumes of enteric contrast material to better display the small bowel lumen and wall. Although CT enteroclysis profits from excellent distension of the entire small bowel and precise evaluation of the entire small bowel and precise evaluation of the extent of extraluminal disease, it has the major drawbacks of invasiveness and high radiation exposure.

Compared with the traditional small bowel follow-through examination, CT enterography has several advantages: (a) it displays the entire thickness of the bowel wall

(b) it allows examination of deep ileal loops in the pelvis without superimposition

(c) it permits evaluation of the surrounding mesentery and perienteric fat. CT enterography also allows assessment of solid organs and provides a global overview of the abdomen.

Copyright ©Radiological Society of North America, 2004

Furukawa, A. et al. Radiographics 2004;24:689-702

CT Enterography with the Use of neutral intraluminal contrast medium

The clinical efficacy of MR imaging has been investigated, and favorable results have been reported

High soft-tissue contrast, static and dynamic imaging capabilities, and the absence of ionizing radiation exposure represent advantages of MR imaging over CT.

On the other hand, MR imaging is more time consuming, less readily available, and more expensive.

Advantages of CT over MR imaging include greater availability, shorter examination times, flexibility in choosing imaging thickness and planes after data acquisition with multi–detector row CT, and higher spatial resolution.

Capsule endoscopy is a revolutionary new diagnostic tool for the detection of small bowel disease that makes use of a swallowable video capsule. Unlike conventional endoscopy, capsule endoscopy allows examination of the entire small bowel and does not require sedation.The main disadvantage of capsule endoscopy are:

(a) The inability to definitively localize or treat small bowel lesions. The method of roughly approximating capsule location described earlier is obviously prone to inaccuracy due to differences in small bowel transit time or variant anatomy.

(b) It does not allow treatment or biopsy sampling of abnormalities. (c) Battery failure in prolonged transit and also the false negative

results if there is rapid peristalsis at the lesion site or if there is bowel angulation at a lesion that impairs the camera view

Copyright ©Radiological Society of North America, 2005

Hara, A. K. et al. Radiographics 2005;25:697-711

Figure 1. Photograph shows the capsule (26 x 11 mm) in relation to a dime

Copyright ©Radiological Society of North America, 2005

Hara, A. K. et al. Radiographics 2005;25:697-711

Figure 2. Drawing illustrates sensors attached to the abdomen, along with the battery pack and recorder

Capsule endoscopy appears to be most useful in the difficult evaluation of obscure gastrointestinal bleeding when barium examinations and standard endoscopy are negative.

A known small bowel stricture or obstruction is a contraindication for capsule endoscopy, since capsules that are not excreted naturally will require surgical removal.

Disadvantages of conventional endoscopic techniques such as push enteroscopy and colonoscopy with ileoscopy include :

-limited endoscopic examination of the small bowel

-sedation requirements.

A complete endoscopic evaluation was previously possible only with intraoperative endoscopy.