Which of the following is the most sensitive imaging modality for detecting pulmonary embolism in a haemodynamically stable patient with normal renal function?

Explanation

## Imaging Modality Selection in PE ### Gold Standard for PE Diagnosis **Key Point:** CTPA (computed tomography pulmonary angiography) is the most sensitive and widely used imaging modality for detecting pulmonary embolism in haemodynamically stable patients with preserved renal function. ### Sensitivity and Specificity Comparison | Modality | Sensitivity | Specificity | Clinical Use | |----------|-------------|-------------|---------------| | CTPA | 94–98% | 95–98% | First-line imaging | | V/Q scan | 80–90% | 90–95% | Contraindicated renal disease; pregnancy | | CXR | 10–15% | Variable | Rule out other diagnoses | | Doppler ultrasound | 50–60% (proximal DVT) | High | Adjunct if DVT suspected | ### Why CTPA is Superior 1. **Direct visualization** of pulmonary arterial tree and emboli 2. **High sensitivity** even for subsegmental PE 3. **Rapid acquisition** (seconds) — ideal for unstable patients 4. **Identifies alternative diagnoses** (pneumonia, aortic dissection, pneumothorax) 5. **Reproducible and operator-independent** (unlike V/Q scan) **High-Yield:** CTPA has become the investigation of choice in most centres and is recommended by major guidelines (ESC, ACCP) for initial PE imaging in haemodynamically stable patients. ### When to Consider Alternatives - **V/Q scan:** Pregnancy, severe contrast allergy, renal insufficiency (eGFR <30) - **Doppler ultrasound:** Adjunct to identify DVT source; not diagnostic for PE alone - **Chest X-ray:** Baseline imaging; rarely diagnostic but may show infarction, pleural effusion, or alternative pathology **Clinical Pearl:** In pregnancy, V/Q scan or MR angiography is preferred over CTPA due to radiation exposure, though low-dose CTPA protocols are increasingly used. [cite:Harrison 21e Ch 297]