Will a Pulmonary Embolism Show Up on a CT Scan Without Contrast?
No, a pulmonary embolism will not reliably show up on a CT scan without intravenous contrast, and non-contrast CT chest has no role in diagnosing pulmonary embolism. 1
Why Non-Contrast CT Cannot Diagnose PE
The American College of Radiology explicitly states there is no relevant literature to support the use of CT chest without IV contrast for suspected pulmonary embolism. 1 This is because:
- Pulmonary emboli appear as filling defects within contrast-enhanced pulmonary arteries - without contrast material, there is no way to visualize the clot against the blood in the vessel 1
- The diagnosis of PE depends entirely on seeing the thrombus as a dark area within the bright, contrast-filled pulmonary artery 1
- Non-contrast CT may show indirect signs like pulmonary infarction or pleural effusion, but these findings are neither sensitive nor specific for PE 1
The Gold Standard: CT Pulmonary Angiography (CTPA)
CTPA with intravenous contrast is the first-line diagnostic imaging tool and is routinely performed for nearly all patients with suspected pulmonary embolism. 1 The evidence supporting this is robust:
- CTPA is highly sensitive (74-81%) and specific (89-98%) for detecting PE 2, 3
- The negative predictive value of CTPA is 99.1%, meaning a negative scan effectively rules out clinically significant PE 4
- CTPA can detect both central and peripheral emboli equally well 5
- The clinical validity of using CTPA to rule out PE is similar to that of conventional pulmonary angiography 4
Contrast Timing Matters
When IV contrast is administered during CT acquisition, the study must be performed as a CTPA with contrast timing specifically optimized for the pulmonary arteries - not as a generic "CT chest with contrast." 1, 6 This distinction is critical because:
- CTPA protocols time the contrast bolus to maximize opacification of the pulmonary arterial tree 6
- Standard CT chest angiography is timed for the aorta and systemic circulation, not the pulmonary arteries 6
- Improper contrast timing can result in suboptimal pulmonary artery opacification and missed PE 1
Common Pitfall to Avoid
Do not order a "CT chest without contrast" when you suspect PE - this will not answer your clinical question and exposes the patient to radiation without diagnostic benefit. 1 Always order CTPA (CT pulmonary angiography) with IV contrast when pulmonary embolism is in your differential diagnosis. 1
Alternative Imaging When Contrast is Contraindicated
If the patient has a contraindication to iodinated contrast (severe contrast allergy or renal failure), consider: 2
- Ventilation-perfusion (V/Q) scan - has high negative predictive value when normal, though produces nondiagnostic results in many patients 1, 7
- MR angiography of the pulmonary arteries - can detect central/lobar PE with 78% sensitivity and 99% specificity, though technically inadequate in 25% of cases 1