CTPA (CT Pulmonary Angiogram) Explained: Diagnosing Pulmonary Embolism
CTPA is the reference standard for pulmonary embolism. Learn indications, radiation dose, contrast risks, and pregnancy alternatives.
CTPA is the diagnostic reference standard for pulmonary embolism (PE) in patients with a positive gate (raised D-dimer or high pre-test probability). Radiation dose is ~5–10 mSv. Iodinated contrast is used, so kidney function (eGFR) and contrast allergy must be checked. V/Q scintigraphy is preferred in pregnancy or significant renal impairment.
CTPA has replaced pulmonary angiography as the gold standard for diagnosing PE. It combines helical CT with a bolus of iodinated contrast timed to the pulmonary arteries, resolving clots down to segmental and often subsegmental levels. Modern CTPA has sensitivity 83–100% and specificity 89–96%. Radiation dose 5–10 mSv (equivalent to ~2 years background) is significant, particularly for women of reproductive age (breast tissue dose). Kidney safety: check eGFR; use pre-hydration or contrast-sparing protocols if eGFR < 45 mL/min/1.73m². Alternatives: V/Q scintigraphy delivers lower breast dose and no iodinated contrast — preferred in pregnancy (ESC 2019 endorsement) and in known contrast allergy or renal impairment. Incidental findings occur in up to 25% of CTPAs — pulmonary nodules, thyroid nodules, lymphadenopathy — and have their own management pathways. Never CTPA a patient with low pre-test probability and negative D-dimer.
- Radiation dose
- ~5–10 mSv (~2 y background)
- Sensitivity
- 83–100%
- Alternative
- V/Q in pregnancy / renal impairment
- Kidney check
- eGFR before contrast
- Incidental findings
- ~25% of scans
CTPA vs V/Q for PE diagnosis
| Factor | CTPA | V/Q |
|---|---|---|
| Radiation | ~5–10 mSv | ~1–2 mSv |
| Breast dose | High | Low |
| Contrast | Iodinated | None |
| Speed | Fast | Slower |
| Pregnancy | Second-line | Preferred |
| Alternative diagnoses | Excellent | Poor |
Before requesting CTPA
Apply Wells score. Only CTPA if low/intermediate probability + positive D-dimer, or high probability.
Check eGFR and contrast allergy.
Pregnant patient? Consider bilateral leg ultrasound first; V/Q if leg USS negative.
- CTPA positive for massive/saddle PE + haemodynamic instability — thrombolysis pathway.
- CTPA incidentally shows suspicious pulmonary nodule — Fleischner Society follow-up.
- Contrast allergy history — premedicate or use alternative modality.
Suspected PE workup
- 1Wells score high?Straight to CTPA (skip D-dimer).
- 2Low/intermediate + positive D-dimer?CTPA (or V/Q if pregnancy or renal impairment).
- 3CTPA positive?Anticoagulation immediately.
- 4CTPA negative + clinical concern persists?Consider lower-limb Doppler ± repeat imaging.
Related questions people ask
Frequently asked questions
- CTPA is the reference standard for PE.
- Always gate with Wells + D-dimer to avoid unnecessary scans.
- Check eGFR and contrast allergy before request.
- V/Q preferred in pregnancy and severe renal impairment.
- 25% incidental findings — have a follow-up plan.
References
2 sources- ESC 2019 PE Guideline
European Society of Cardiology.
academic.oup.com
- NICE NG158VTE Diseases
UK national guideline.
nice.org.uk
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