For a patient suspected of having a pulmonary embolism (PE), do you need a computed tomography (CT) scan with or without contrast?

CT With Contrast is Required for Pulmonary Embolism Diagnosis

For suspected pulmonary embolism, you need CT pulmonary angiography (CTPA) WITH intravenous contrast—CT without contrast cannot diagnose PE and should not be performed for this indication. 1

Why Contrast is Essential

• Contrast is mandatory because PE diagnosis requires visualization of filling defects (clots) within the pulmonary arteries, which are only visible when the vessels are opacified with IV contrast material 1
• The timing of contrast administration is specifically optimized to opacify the pulmonary arteries during the CT acquisition 1
• CT without contrast has no role in PE evaluation and is explicitly not supported by ACR guidelines 1
• CT without and with contrast (dual-phase) is also not indicated for suspected PE 1

The Correct Diagnostic Algorithm

Step 1: Clinical Risk Stratification First

• Assess pretest probability using Wells criteria or Geneva score before ordering any imaging 1, 2
• Evaluate for hemodynamic stability, symptoms (chest pain, dyspnea, tachycardia), and risk factors 2

Step 2: D-dimer Testing (When Appropriate)

• Low or intermediate pretest probability: Order D-dimer first 1, 2, 3
• If D-dimer is negative (<500 ng/mL, or age-adjusted cutoff of age × 10 ng/mL for patients >50 years), no imaging is needed—PE is safely excluded 1, 3
• This approach avoids unnecessary radiation and contrast exposure in 30-50% of patients 2

Step 3: CTPA With IV Contrast (When Indicated)

• Order CTPA with IV contrast if: 2, 4
  • D-dimer is positive
  • High pretest probability (skip D-dimer and go directly to imaging)
  • Patient is hemodynamically unstable
• CTPA has >95% sensitivity for segmental or larger emboli and is the gold standard 3, 5, 6

Alternative Imaging (Only When CTPA Contraindicated)

• Ventilation-perfusion (V/Q) scan is the first-line alternative when CTPA cannot be performed due to: 2, 3
  • Severe contrast allergy
  • Renal failure/contraindication to IV contrast
  • Pregnancy concerns
• Lower extremity venous ultrasound can be considered as a second alternative—if DVT is found, it may preclude need for pulmonary imaging 4

Critical Pitfalls to Avoid

• Never order CT chest without contrast for suspected PE—it provides no diagnostic information about pulmonary arteries 1
• Do not skip D-dimer testing in low-to-intermediate probability patients and proceed directly to CTPA—this exposes patients to unnecessary radiation, contrast nephropathy risk, and incidental findings 2
• Do not use standard CT chest with contrast (non-angiographic protocol)—the contrast timing is different and may not adequately opacify pulmonary arteries 1

For Recurrent or Residual PE

• CTPA with IV contrast remains the appropriate test for suspected recurrent or residual PE in patients with known prior PE 1
• Recurrent clots appear as filling defects; residual chronic disease shows webs, stenosis, or poorly opacified vessels 1