The radiologic assessment of the small bowel has been compared to separating out and identifying a large number of writhing snakes in a crowded reptile tank at the zoo [1].
Assisting the clinician in this Herculean task is an array of imaging modalities and techniques each with their own set of advantages and limitations and each with varying levels of sensitivity and specificity (Table 1).
Modality
|
Advantages
|
Disadvantages
|
Barium
follow-through
|
Spatial resolution
|
Radiation dose
Poor assessment of extra-luminal disease
|
Small bowel enteroclysis
|
Spatial resolution
Detects early mucosal disease
|
Radiation dose
Discomfort of naso-jejunal intubation
|
CT Enterography
|
Spatial resolution
|
Radiation dose
|
CT Enteroclysis
|
Spatial resolution
|
Radiation dose
|
Ultrasound
|
No radiation dose
Can asses extra-enteric disease
|
Operator dependent
|
MR Enterography
|
Contrast resolution
No radiation dose
|
Spatial resolution
Poor proximal small bowel distention
|
MR Enteroclysis
|
Contrast resolution
Low radiation dose
|
Spatial resolution
NJ intubation
|
Double balloon Enteroscopy
|
Direct visualization of bowel mucosa
Ability to biopsy lesions
|
No assessment of extraluminal disease
|
Wireless Capsule Endoscopy
|
Direct visualization of bowel mucosa
Patient convenience
|
No assessment of extraluminal disease
Expense and time
Problems with stenosis
|
Table1: Modalities for imaging the small bowel.
Traditionally the small bowel has been imaged with fluoroscopic barium studies (small bowel follow through and small bowel enteroclysis).
With its superior luminal distention and double contrast effect fluoroscopic enteroclysis can provide excellent views of the small bowel,
detailing subtle mucosal changes in early inflammatory bowel disease [2].
Both techniques are limited however by their lack of three-dimensional view of the overlapping bowel loops and provide only indirect information about extra-enteric disease.
CT enterography and CT enteroclysis give the added advantage of being able to assess extra-luminal disease as well as the inherent benefits of short imaging time and excellent spatial resolution. The main disadvantage as with traditional fluoroscopic studies is the radiation burden [3].
Recently there has been increasing awareness of the risks of ionizing radiation with an emphasis on reducing patient radiation exposure by using alternative imaging investigations [4].
This is especially pertinent in young patients with Crohn’s disease or polyposis syndromes who during their follow-up will be subjected to multiple imaging investigations over their lifetime leading to a significant amount of radiation exposure.
This subgroup of patients were highlighted in a study by Desmond et al [5].
CT will however continue to play an important role in the evaluation of patients who are acutely unwell,
particularly if there is clinical suspicion of high grade obstruction,
perforation or intra-abdominal sepsis.
With the increasing importance of keeping the radiation dose as low as reasonably possible,
radiologists and clinicians are turning to other imaging modalities which do not involve ionizing radiation.
Ultrasonography is a radiation free imaging modality which can be used to identify focal small bowel thickening in inflammatory bowel disease.
Is limitation lies in its dependence on operator skill and experience.
Other methods of investigating small bowel pathology include endoscopy.
Current techniques used are ileoscopy,
double balloon enteroscopy and capsule endoscopy.
The obvious advantage of endoscopy is that the mucosal surface can be directly visualized and with ileoscopy or double balloon enteroscopy tissue diagnosis can be achieved.
However this method of small bowel investigation can be limited by operator dependence and cost.
Also there is no scope for assessing extra-luminal disease and capsule endoscopic assessment is contraindicated in patients with small bowel strictures and suspected small bowel obstruction [6].
Advantages and Limitations of MR Imaging of the Small bowel
Advantages:
- No ionizing radiation.
- Superior soft tissue contrast.
- Evaluation of extra-enteric disease and disease penetration in inflammatory bowel pathology.
- MR Fluoroscopy can monitor the filling of the small bowel,
transit time of contrast and assess dynamic function of diseased segments of small bowel.
- Can be used in patients with contra-indication to contrast enhanced CT (pregnancy and contrast allergy).
Disadvantages:
- Time,
expense and availability.
- Variability of examination quality.
- Patients with poor breath-hold capability.
- Movement artifacts from bowel peristalsis.
- Lower temporal and spatial resolution than CT [7].
MR Enteroclysis versus MR Enterography
The two main techniques of imaging the small bowel with MR are enteroclysis (luminal contrast administered by a naso-jejunal tube) and enterography (contrast administered orally).
There remains controversy as to which one these techniques is the more superior,
with little evidence available in the literature.
Some studies have reported a similar sensitivity of enterography and enteroclysis in detecting active inflammation in Crohn’s disease [8,9,10].
However one study by Masselli et al.
suggests that the superior bowel distention provided by enteroclysis demonstrates early small bowel mucosal disease more clearly than MR enterography [11].
These findings are echoed in a recent Australian study comparing the small bowel distention in MR enterography and in enteroclysis.
The results showed that the techniques were equivalent for distal small bowel distention but that the proximal small bowel distention was frequently less optimal in MR enterography than in MR enteroclysis [12]. Others have argued that although jejunal distention is frequently suboptimal in enterography,
the ileum,
which is the most common site of small bowel involvement in Crohn’s disease,
is usually well demonstrated [13].
The major drawback of MR enteroclysis is the placement of the naso-jejunal tube.
Firstly there is the technical and logistical difficulty of tube placement.
Then there is the matter of patient acceptance.
Lastly there is the problem of radiation exposure for naso-jejunal catheter placement,
all of which however can be kept to a minimum level with operator experience.
We use MR enteroclysis with naso-jejunal intubation at our institution which we feel gives a more consistent and superior bowel distention with acceptable levels of patient comfort through safe and judicious use of sedation and analgesics.