A bowel obstruction is a blockage that fully or partially prevents the normal movement of contents through the intestines. It is a serious medical condition that can sometimes be missed on a CT scan. A CT (computed tomography) scan is an imaging test that uses X-rays and computers to create detailed images of the organs and tissues inside the body. CT scans are commonly used to diagnose bowel obstructions, but they are not foolproof. There are several reasons why a bowel obstruction may not be detected on a CT scan.
Overview of Bowel Obstructions
Bowel obstructions have several potential causes including:
- Adhesions – Scar tissue from previous abdominal surgeries can bind sections of bowel together and cause a blockage.
- Hernias – Sections of intestine can protrude through weak areas of muscle in the abdominal wall and become trapped or twisted.
- Tumors – Cancerous growths in the bowel, ovaries, or other pelvic organs can obstruct intestinal flow.
- Volvulus – A section of intestines can twist around itself and cause a closed-loop obstruction.
- Intussusception – One segment of bowel slides into an adjacent segment, blocking flow.
- Foreign bodies – Objects accidentally swallowed can become lodged in the intestinal tract.
The small intestine is the most frequent site of obstruction, but blockages can occur in both the small bowel and large bowel (colon). Complete obstructions totally cut off the flow of intestinal contents, while partial obstructions slow or restrict the movement of solids and liquids.
Symptoms of Bowel Obstruction
Common symptoms of a bowel obstruction include:
- Abdominal pain and cramping
- Bloating and abdominal swelling
- Vomiting, especially vomiting up stool if the blockage is in the lower intestines
- Inability to have a bowel movement or pass gas
- Watery diarrhea if the obstruction is partial
Severe or complete bowel obstructions are considered medical emergencies. If left untreated, they can lead to bowel perforation, peritonitis, sepsis, and even death. Prompt diagnosis is crucial.
Using CT Scans to Detect Bowel Obstructions
Computed tomography (CT) is one of the primary diagnostic imaging tests used when a bowel obstruction is suspected. CT provides clear and detailed images of the small intestine and colon that can reveal:
- The location and cause of the blockage
- Whether the obstruction is complete or partial
- Any twisting or abnormal dilation of bowel loops
- Effects of the obstruction like bowel wall thickening or fluid build-up
Contrast material is often used to enhance the CT images. You may receive an oral contrast solution to drink and an IV contrast dye injection. These fluids help highlight the intestinal tract and improve visualization.
CT is fast, painless, and non-invasive. It takes cross-sectional pictures of the abdomen and pelvis from many angles, allowing the radiologist to pinpoint areas of obstruction along the length of the bowel. CT is much more sensitive than plain abdominal X-rays.
Why CT Scans Can Miss a Bowel Obstruction
Although CT scanning is highly accurate for diagnosing bowel obstructions, estimated at 90-95% sensitivity, obstructions can sometimes still be missed. There are several reasons why this can occur:
Early Stage Obstructions
In the early stages of a developing bowel obstruction, there may only be mild, partial blockage. The resulting CT images can appear relatively normal if the obstruction is not yet severe. Repeat scanning in 6-24 hours will often detect progression as the obstruction worsens.
Intermittent Obstructions
Some obstructions come and go. Intestinal contents may only pass slowly or intermittently, then freely again once past the obstructed area. This can happen with internal hernias or intermittent twisting. A normal CT doesn’t definitively rule out an obstruction.
High Up in the Small Intestine
The first few feet of small bowel descending from the stomach are challenging to evaluate on CT. Obstructions here may not be obvious unless there is clear upstream bowel dilation.
Non-Obstructing Causes
Some bowel issues like ileus or pseudo-obstruction produce similar symptoms to a true mechanical obstruction but with no anatomically blocked section of intestine visible on CT.
Very Thin Bowel Adhesions
Even high-resolution CT may not be able to visualize extremely thin bands of adhesive scar tissue that are obstructing the bowel. Oral contrast reaching beyond the obstruction site could make it falsely appear unblocked on CT.
Metal Surgical Clips or Staples
Metal implant clips from prior bowel surgery can sometimes obscure or hide a developing obstruction on CT scans.
Suboptimal Bowel Distension
Lack of optimal intestinal filling and expansion with contrast material during the CT study can interfere with properly assessing the bowel. Collapsed loops of bowel may conceal an obstruction.
Operator Error
Mistakes in CT technique or inaccuracies in image interpretation by the radiologist are always possible. Complex cases of obstruction may be problematic to diagnose correctly.
Improving Detection of Bowel Obstruction on CT
There are ways to maximize the accuracy and reliability of CT scanning for bowel obstruction:
- Use of multi-phase contrast enhanced CT imaging
- Oral contrast agents and optimal luminal distension techniques
- Careful inspection of collapsed bowel segments
- Assessing for secondary signs like edema or ascites
- Correlation with clinical and laboratory findings
- Experienced radiologists familiar with bowel obstructions
- Consider repeat CT scanning when symptomatic despite normal initial CT results
With meticulous scanning protocols and interpretation, CT can reliably diagnose the majority of clinically significant bowel obstructions. However, its limitations and the possibility of false negative studies mean that clinical suspicion should always take precedence over an apparently normal CT report.
Other Imaging Tests for Bowel Obstruction
While CT is the first-line imaging test, there are other modalities that may be used as supplements when obstruction is suspected despite an unremarkable CT:
Plain Abdominal X-Ray
Abdominal X-rays may show dilated loops of bowel and air-fluid levels caused by an obstruction. But they are far less sensitive than CT and cannot pinpoint the location or cause.
Water-Soluble Contrast Study
Patients drink barium or water-soluble iodine contrast, then abdominal X-rays track its flow through the intestinal tract. Stalled progress of contrast can help confirm a total or partial obstruction.
Ultrasound
Abdominal ultrasound may be attempted but has limited utility beyond detecting very enlarged or fluid-filled dilated bowel loops. It cannot visualize most of the small intestine.
MR Enterography
MRI of the abdomen and pelvis with contrast material filling the bowel can diagnose obstruction similarly to CT in many cases. It lacks ionizing radiation but is more time-consuming and costly.
Small Bowel Follow-Through
In this fluoroscopic study, patients ingest barium contrast over time while X-rays track its passage through the intestines. It has largely been supplanted by CT enterography but can still be useful for subacute small bowel obstructions.
Takeaways
- CT scanning is the preferred first test for diagnosing bowel obstruction but is not 100% sensitive.
- Bowel obstructions may be missed or not visible on CT in the early stages, with intermittent or non-obstructing causes, and with technical limitations.
- Clinical suspicion should be given more weight than a normal CT result if symptoms persist.
- Careful CT technique and expertise, along with other imaging tests when needed, can improve detection of elusive bowel obstructions.
The Bottom Line
In summary, although CT imaging is highly accurate for diagnosing bowel obstructions, there are a number of reasons why a true obstruction may not show up on a CT scan. Clinical correlation is important. A normal CT does not definitively rule out an obstruction in a patient with persistent concerning symptoms, and repeat imaging or additional scan types may be needed for confirmation. With proper diligence and expertise, however, CT can detect the large majority of clinically significant obstructions.