What is the best initial imaging study to order for a post-operative adult patient with suspected post-op ileus, recent abdominal surgery, and symptoms such as abdominal pain, nausea, and vomiting?

Imaging for Suspected Post-Operative Ileus

CT abdomen and pelvis with IV contrast is the imaging study of choice for post-operative patients with suspected ileus, as it achieves >90% diagnostic accuracy in distinguishing true ileus from mechanical obstruction and can identify life-threatening complications requiring urgent intervention. 1, 2, 3

Why CT with IV Contrast is Superior

CT provides critical information that plain radiographs cannot deliver:

• Distinguishes ileus from mechanical obstruction with 100% sensitivity and specificity, whereas clinical examination combined with plain films achieves only 19% sensitivity 3

• Identifies the transition point and underlying cause when mechanical obstruction is present, which fundamentally changes management 1, 2

• Detects life-threatening complications including bowel ischemia, strangulation, closed-loop obstruction, perforation, and abscesses that require immediate surgical intervention 4, 1

• Characterizes postoperative fluid collections with IV contrast helping to distinguish simple fluid from infected collections or abscesses 4

Plain Radiographs Have Severely Limited Value

Abdominal X-rays should not be the primary imaging modality:

• Plain films demonstrate only 50-60% sensitivity for detecting obstruction and have minimal utility for diagnosing ileus 1

• Radiographs have low sensitivity for detecting abscesses, fluid collections, and sources of fever—the very complications you need to rule out 4

• The ACR states that abdominal radiographs after CT add no diagnostic value and should not be routinely performed 1

• Plain films may be useful only if there is specific concern for retained surgical sponge (which has characteristic markers on X-ray) 4

CT Protocol Specifications

Optimal CT technique for post-operative ileus evaluation:

• IV contrast is strongly preferred to evaluate bowel wall perfusion and identify potential ischemia 2

• Oral contrast is generally not required for suspected high-grade obstruction, as the nonopacified fluid already present in dilated bowel provides adequate intrinsic contrast 1, 2

• Positive oral contrast can obscure subtle findings and is not needed for diagnostic accuracy 1

Water-Soluble Contrast Challenge (Gastrografin)

After initial CT diagnosis of ileus or partial obstruction, water-soluble contrast serves both diagnostic and therapeutic purposes:

• Administer 50-150 mL of water-soluble contrast (Gastrografin) orally or via NG tube only after adequate gastric decompression 1

• This achieves 96% sensitivity and 98% specificity for predicting resolution with conservative therapy 1

• If contrast reaches the colon within 24 hours on follow-up abdominal X-ray, this predicts successful non-operative management 1, 2

• Water-soluble contrast also has therapeutic benefit, reducing operative rates and hospital stay 1

Critical Safety Considerations

Do NOT administer water-soluble contrast until:

• The stomach is adequately decompressed via NG tube (to prevent aspiration pneumonia and pulmonary edema) 1

• The patient is adequately rehydrated with IV fluids (to prevent hypovolemic shock) 1

• You have ruled out complete high-grade obstruction, perforation, or peritonitis on CT 1

When to Obtain Urgent Surgical Consultation

High-risk CT findings mandating immediate surgical evaluation:

• Closed-loop obstruction 1

• Signs of bowel ischemia: reduced/absent bowel wall enhancement, pneumatosis, mesenteric venous gas 1, 2

• Mesenteric edema with ascites and absence of small-bowel feces sign 1

• Intraperitoneal free air suggesting perforation 1

• Small bowel dilation >3.9 cm 1

Follow-Up Imaging Strategy

If conservative management fails to show improvement:

• Repeat CT at 48-72 hours, as this represents the safe cutoff for non-operative management 1

• Do not delay repeat CT if clinical deterioration occurs, as ischemia can develop rapidly with 25% mortality 2

• For pregnant women and children requiring repeat imaging, consider MRI instead of repeat CT to minimize radiation exposure (MRI demonstrates 95% sensitivity and 100% specificity for bowel obstruction) 1

Common Pitfalls to Avoid

• Do not rely on plain radiographs as your primary imaging modality—they miss critical diagnoses in 30-50% of cases 1, 2

• Do not order abdominal X-rays after CT has been performed—they add no diagnostic value and can be misleading in 20-40% of patients 1

• Do not give water-soluble contrast before adequate gastric decompression and IV hydration—this risks aspiration and hypovolemic shock 1

• Do not delay repeat CT if the clinical picture worsens—CT sensitivity for ischemia is limited, and clinical correlation is essential 1

Areas Without Strong Consensus

The international Delphi consensus study of 35 colorectal surgery experts could not reach agreement on the optimal imaging modality for diagnosing post-operative ileus 5. However, this lack of consensus in the literature should not prevent you from ordering CT with IV contrast in clinical practice, as it remains the highest-yield study with the best evidence for distinguishing ileus from mechanical obstruction and identifying complications 1, 2, 3.