What are the pitfalls of plain radiographic diagnosis of small bowel obstruction?
A small bowel loop is identified on plain abdominal radiography by intraluminal gas outlining valvulae conniventes. If a small bowel loop is completely filled with fluid, it cannot be identified on a plain abdominal radiograph. Plain abdominal radiography misses fluid-filled loops, underdiagnoses small bowel obstruction, and is unable to estimate the level of obstruction accurately. In contrast, CT can identify fluid-filled bowel loops just proximal to the site of obstruction that plain radiography cannot identify.
The clue to an obstruction is a transition between dilated small bowel proximal to an obstructing lesion and collapsed small bowel distal to the site of obstruction. False-negative diagnosis may be made with CT and plain abdominal radiography when bowel is not dilated owing to vomiting or nasogastric tube/long tube decompression or when an examination is performed before fluid can accumulate proximal to the obstruction. False-positive diagnosis of obstruction is frequent with CT and plain abdominal radiography and may be made in patients with an adynamic ileus predominantly involving the small bowel. Plain abdominal radiography detects about 80% of small bowel obstructions.
