The Small Bowel
There are now a number of approaches to investigating the small bowel, both endoscopically with developments in optical as well as capsule endoscopy, and in imaging where MRI is leading the development of small bowel imaging.
The small bowel is seen well during routine abdomino-pelvic CT, particularly if oral contrast has been given. This may be either positive (very dilute barium or Gastrografin) or more often negative (water) as then the bowel wall may be highlighted with intravenous contrast enhancement to give more specific information about bowel wall vascularity and changes. Small bowel abnormalities are often picked up during CT, but begs the question as to whether this is the examination of choice for targeted investigation.
In acute small bowel obstruction, CT is the examination of choice. The problem with the plain film is that one can only see the small bowel when it is gas filled, and there is seldom enough gas to determine the exact site and nature of the obstruction. However, fluid in the small bowel provides excellent contrast for CT, which can not only confirm obstruction, but also show the site and often the nature of the obstruction as well as assessing bowel viability(1).
Often the cause and presence of the obstruction are known, and the clinical problem is whether to operate early or wait. CT is still indicated if there is concern as to bowel viability, but if not then an alternative approach, giving 100mls of Gastrografin orally and taking a film after 4hrs, may be used to divide those in whom the contrast is in the colon and it is safe to leave, and those in whom the contrast is held up in the small bowel who probably require urgent surgery. US can be used in acute small bowel obstruction to determine bowel distension, but is not as accurate as CT in localising the point of obstruction.
Endoscopy, whether by ileoscopy or capsule endoscopy, is obviously the most sensitive test to show early Crohn's disease. However, compression studies in standard barium follow through (BaFT) can also show superficial ulcers, but is difficult to perform in all parts of the small bowel, and it has been suggested that double contrast enteroclysis is the most accurate radiological examination for superficial disease(2)
However, this is technically demanding and not a routine study. The standard small bowel enema may not show early mucosal disease, as it is better at showing early fold deformity than superficial ulcers that may occur without bowel wall distortion and in this respect is inferior to good compression during a BaFT. Long segment advanced Crohn's disease will be readily visible on any examination, but short segment disease and narrow strictures may be missed more easily.
CT and MRI enterography are gaining in usage; particularly as oral techniques have become more successful, obviating the problem of intubation. Both show bowel vascularity, mural and transmural changes, which may help in assessing Crohn's disease activity(3) and especially in diagnosing tumours with an extramural component such as GIST or carcinoid lesions. US with compression yields surprisingly good images of Crohn's disease, but is not as accurate as CT or MRI and has poor localisation.
Adhesions are often ignored in small bowel studies, though these are an important and common problem. These may be suggested on CT, but confirmation often requires moving bowel loops to show that they do not move freely on the mesentery and are adhesed to other loops or the abdominal wall. This is possible only with fluoroscopic palpation and compression.
A well performed BaFT remains a highly accurate examination and still gives the best roadmap of disease. There are some clinical situations, such as obstruction, where CT is the examination of choice. US and MRI have important roles in inflammatory bowel disease as no ionizing radiation is involved. Please do feel free to discuss with the radiologist the best approach for any particular problem.
Reference List
(1) Maglinte DD, Heitkamp DE, Howard TJ, Kelvin FM, Lappas JC. Current concepts in imaging of small bowel obstruction. Radiol Clin North Am 2003; 41(2):263-83, vi.
(2) Maglinte DD. Capsule imaging and the role of radiology in the investigation of diseases of the small bowel. Radiology 2005; 236(3):763-767.
(3) Colombel JF, Solem CA, Sandborn WJ, Booya F, Loftus EV, Jr., Harmsen WS et al. Quantitative measurement and visual assessment of ileal Crohn's disease activity by computed tomography enterography: correlation with endoscopic severity and C reactive protein. Gut 2006; 55(11):1561-1567