What CT scan is used to rule out Pulmonary Embolism?

CT Pulmonary Angiography (CTPA) is the Imaging Test of Choice to Rule Out Pulmonary Embolism

Multidetector CT pulmonary angiography (CTPA) is the primary imaging modality for evaluating patients suspected of having acute pulmonary embolism (PE). 1

Diagnostic Algorithm for Suspected PE

1. Initial Assessment:

   • Use validated clinical prediction rules (Wells score, Geneva score) to estimate pretest probability of PE
   • For low pretest probability patients who meet all Pulmonary Embolism Rule-Out Criteria (PERC), no further testing is needed 1

2. D-dimer Testing:

   • For low/intermediate pretest probability patients who don't meet PERC criteria: obtain high-sensitivity D-dimer
   • For patients >50 years, use age-adjusted D-dimer thresholds (age × 10 ng/mL) 1
   • If D-dimer is negative, no imaging is needed

3. Imaging:

   • If D-dimer is positive OR patient has high pretest probability: CTPA is indicated 1
   • CTPA has high sensitivity and specificity for diagnosing PE 1

Why CTPA is Preferred

• CTPA has largely replaced conventional pulmonary angiography as the gold standard 1, 2
• High diagnostic accuracy with sensitivity of 83% and specificity of 96% 1
• Low false-negative rate with negative predictive value of 98.8% 3
• Fewer "nondiagnostic" studies compared to ventilation-perfusion (V/Q) scans 1
• Can identify signs of right ventricular dysfunction that have prognostic significance 1
• Can detect alternative diagnoses that may explain the patient's symptoms 1

Special Considerations

• Pregnancy: V/Q scanning may be preferred over CTPA due to lower maternal breast radiation exposure, though both have acceptable fetal radiation doses 1
• Renal Impairment: Use caution with contrast administration; consider risk-benefit ratio 4
• Technical Factors: Meticulous attention to technique (timing of contrast) is necessary to achieve optimal results 1

Common Pitfalls to Avoid

• Overuse of CTPA: Indiscriminate use results in unnecessary radiation and contrast exposure 1, 5
• Relying solely on CTPA in very high-risk patients: In patients with Wells score >6 points, the 3-month incidence of VTE after negative CTPA is higher (6.3%), suggesting closer follow-up may be needed 6
• Misinterpreting subsegmental PE: Small peripheral emboli detected on CTPA may be of indeterminate clinical significance 1
• Technical inadequacy: A small proportion of CTPA examinations may be technically unsatisfactory; ensure proper protocols are followed 1

CTPA has revolutionized the diagnostic approach to PE and is now firmly established as the imaging test of choice for ruling out PE in most clinical scenarios, with V/Q scanning reserved for patients with contraindications to CTPA or when CTPA is not available 1.