A 58-year-old woman presents to the emergency department with acute onset dyspnea and pleuritic chest pain. She returned from a 12-hour flight 3 days ago. Vital signs: BP 128/82 mmHg, HR 112 bpm, RR 24/min, SpO₂ 92% on room air. D-dimer is elevated at 2.8 µg/mL. CT pulmonary angiography (CTPA) is performed. Which of the following CT findings would be most specific for acute pulmonary embolism?

Explanation

## CT Findings in Acute Pulmonary Embolism **Key Point:** The most specific CT sign of acute PE is a **filling defect (thrombus) within the pulmonary artery lumen**, which appears as a low-attenuation (dark) material surrounded by the high-attenuation (bright) contrast-enhanced blood. ### Direct vs. Indirect Signs | Finding | Type | Specificity | Sensitivity | |---------|------|-------------|-------------| | **Filling defect in PA** | Direct | Very high (>95%) | Moderate–high | | **Wedge-shaped infarction** | Indirect | Moderate | Low (10–15%) | | **Hampton's hump** | Indirect (infarction) | Moderate | Low | | **Ground-glass opacities** | Indirect | Low | Non-specific | | **Atelectasis** | Indirect | Very low | Non-specific | **High-Yield:** On CTPA, the **central PE sign** (thrombus in the main, lobar, or segmental pulmonary artery) is the gold standard for diagnosis. The clot appears as a **low-attenuation (dark) defect** within the opacified vessel lumen. ### Why Wedge-Shaped Consolidation Is Not the Best Answer Wedge-shaped peripheral consolidation (Hampton's hump) is a **pulmonary infarction** — an indirect sign of PE. It occurs only when: - PE is massive enough to cause distal ischemia - There is concurrent right heart failure or reduced collateral circulation - It develops over **hours to days** after the embolic event Infarction is present in only 10–15% of PE cases, making it insensitive. The filling defect is present in >90% of CTPA-positive cases. **Clinical Pearl:** A patient with PE may have a normal chest X-ray initially. CTPA is the imaging modality of choice because it directly visualizes the thrombus, not secondary parenchymal changes. ### Technique Considerations **Mnemonic: CTPA Protocol — **C**ontrast timing, **T**hin slices, **P**ulmonary arteries, **A**rterial phase** 1. **Bolus tracking** or **test bolus** to time arterial phase acquisition 2. **Thin-slice acquisition** (≤1.5 mm) to detect subsegmental emboli 3. **Imaging during the arterial phase** (when PA is maximally opacified) 4. **Multiplanar reconstruction** (coronal, sagittal) to confirm filling defects **Warning:** Do not confuse **motion artifact** (pseudofilling defect) with true thrombus. Real thrombi are persistent across multiple slices; artifacts are transient and often at the PA bifurcation. [cite:Harrison 21e Ch 298]