Ordering a CT (or CAT) scan and wondering whether contrast is needed? And if you do need contrast, should it be IV or oral? Let’s make this simple for you.

In this post, we’ll break down when to order non-contrast CT, contrast CT, and the different types of contrast to consider (oral, IV, or both). You’ll have a clear framework to guide your decisions by the end.

Types of Contrast

Contrast can be administered in three ways:

• Oral contrast: Helps visualize the gastrointestinal (GI) tract.
• Rectal contrast: Occasionally used for imaging the lower GI tract.
• IV contrast: Highlights blood vessels, organs, and certain tissues.

1. When to Use Oral or Rectal Contrast

The only times you’ll need oral or rectal contrast are:

• GI perforation or anastomosis leak: Use water-soluble iodinated contrast (e.g., Gastrografin) instead of barium-based contrast.
• Distal colonic or rectal perforation/leak: Consider rectal contrast.

Important Note: Oral contrast is unnecessary for diagnosing small bowel obstruction (SBO). Use IV contrast with CT abdomen/pelvis for SBO.

2. When to Use IV Contrast

Use IV contrast for:

• Cancer: Evaluating suspicious lesions or staging.
  • Exceptions: Low-dose lung cancer screening, colon cancer screening (CT colonography), pulmonary nodule follow-up, and lymphoma assessment.
• Infections: Abscesses, empyema, osteomyelitis, septic arthritis, or infected hardware.
• Trauma with penetrating or vascular injuries. if there are no penetrating or vascular injuries then there is no need to give IV contrast
• Inflammatory conditions: Such as IBD, pancreatitis, diverticulitis, and appendicitis.
  • Exception: Interstitial lung disease (ILD) assessment with high-resolution CT (HRCT)—no IV contrast needed.
• Infarction/Ischemia: This includes kidney infarcts and ischemic bowel.
  • Exception: For brain or lung infarctions, non-contrast CT is preferred.
• Vascular conditions: Thrombosis, dissection, aneurysms, or pseudoaneurysms. Use CT angiography (requires IV contrast).

3. When to Order Non-Contrast CT

Non-contrast CT is sufficient for:

• Initial stroke evaluation.
• Hematomas: Retroperitoneal hematomas (although IV contrast can also be used).
• Kidney stones.
• Musculoskeletal issues: Fractures and spinal diseases (if infection isn’t suspected).
• Non-penetrating trauma.
• Interstitial lung disease (HRCT).
• Screenings:
  • Low-dose lung cancer screening (LDCT).
  • Coronary calcium scoring.
  • CT colonography.

4. Real-Life Clinical Scenarios

Here are practical examples to solidify your understanding:

Scenario 1: Pulmonary Embolism (PE)

A 40-year-old woman on birth control pills presents with sudden shortness of breath. PE is suspected. PE is a vascular condition.

Order: CT chest angiography with IV contrast.

Scenario 2: Diverticulitis

A 65-year-old patient with LLQ abdominal pain. Acute diverticulitis is suspected.
Acute diverticulitis is an inflammatory process.

Order: CT abdomen/pelvis with IV contrast.

Scenario 3: Trauma and Anticoagulation

An 80-year-old man on warfarin falls and sustains a head laceration. CT of head, neck, chest, abdomen, and pelvis ordered. On exam, he only had some head lacerations and no signs of other injuries.

The patient had a trauma without penetrating or vascular injuries, hematoma also needs to be ruled out as he was on anticoagulation.

Order: Non-contrast CT.

Scenario 4: Kidney Stones

A 30-year-old woman with recurrent flank pain and a history of kidney stones, and renal colic was suspected to be the culprit.

Order: Non-contrast CT abdomen/pelvis.

Scenario 5: Severe Abdominal Pain

A 45-year-old patient with severe abdominal pain and vomiting, on physical exam, his abdomen remained soft but with diffuse tenderness, The ED physician ordered CT abd/pelvis.

Order: CT abdomen/pelvis with IV contrast (oral contrast is unnecessary, particularly with vomiting).

Scenario 6: Suspected Stroke

80-year-old man with sudden aphasia. Stroke suspected.
Order: Non-contrast CT head.

Scenario 7: Infection with Hardware

58-year-old woman s/p back surgery involving hardware placement presented with back pain and surgical wound discharge. MRI is unavailable.
Order: CT lumbar spine with IV contrast (infection suspected).

Final Tips

• Always assess for contrast allergies and ensure normal kidney function before ordering contrast studies.
• When in doubt, consider the clinical scenario—contrast should add diagnostic value and improve outcomes.