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CT Pulmonary Angiography

CT pulmonary angiography is the first-line imaging test for suspected acute pulmonary embolism, directly demonstrating intraluminal thrombus within the pulmonary arteries down to the segmental and often subsegmental level. Optimal timing places peak contrast enhancement in the pulmonary arterial tree, and the study simultaneously assesses right heart strain and provides an alternative diagnosis when PE is absent. Clinical pretest probability and D-dimer should guide appropriate use.

Indications

  • Suspected acute pulmonary embolism with intermediate/high pretest probability or positive D-dimer
  • Assessment of clot burden and right heart strain in confirmed PE to risk-stratify
  • Evaluation of chronic thromboembolic pulmonary disease/hypertension
  • Hemoptysis workup to assess pulmonary and bronchial vasculature and parenchymal cause
  • Suspected pulmonary artery anomaly, tumor, or arteriovenous malformation
  • Pulmonary vascular assessment prior to or after intervention

Contraindications & Cautions

  • Iodinated contrast allergy: premedicate or consider V/Q scanning as an alternative
  • Renal impairment: assess eGFR and weigh contrast-associated nephropathy risk; hydrate and consider V/Q
  • Pregnancy: consider V/Q or perfusion scintigraphy to reduce maternal breast dose; if CTPA performed, use dose-optimized protocol
  • Inability to hold breath or maintain the required contrast bolus can degrade diagnostic quality
  • Hemodynamic instability requiring resuscitation before or during scanning; monitor closely

Patient Preparation

  • Obtain large-bore IV access (18-20 G, antecubital) suitable for high-rate power injection
  • Screen renal function, contrast allergy, and pregnancy; document Wells/Geneva score and D-dimer
  • Fasting is not strictly required; ensure resuscitation readiness for contrast reactions
  • Coach a brief breath-hold at end-inspiration (or shallow suspended respiration to limit transient interruption of contrast)
  • Position supine with arms elevated; attach ECG if cardiac gating is used
  • Rehearse breathing instructions to avoid respiratory motion and contrast dilution

Technique & Parameters

  • Thin-section helical acquisition, 1-1.5 mm collimation, from above the aortic arch (or lung apices) to below the diaphragm
  • IV contrast 50-80 mL of high-concentration iodinated agent at 4-5 mL/s, followed by a saline chaser
  • Bolus tracking with ROI in the main pulmonary artery (trigger ~100-150 HU) or empiric ~15-20 s delay
  • 120 kVp standard; lower kVp (80-100) enhances iodine attenuation and reduces dose/contrast in appropriate patients
  • Caudocranial scanning can reduce respiratory and superior vena cava streak artifact
  • Reconstruct axial and multiplanar (including oblique) images; measure RV:LV diameter on axial images

Systematic Review

  • Confirm adequate pulmonary arterial opacification before excluding PE
  • Trace the main, lobar, segmental, and subsegmental arteries systematically for filling defects
  • Assess RV:LV short-axis diameter ratio and interventricular septal bowing for right heart strain
  • Look for reflux of contrast into the IVC/hepatic veins as a strain sign
  • Evaluate lung parenchyma for infarcts, wedge opacities, mosaic perfusion, and alternative diagnoses
  • Review the pleura, mediastinum, and any incidental pulmonary nodules
  • Check the aorta and cardiac structures within coverage for alternative causes of symptoms

Key Findings & Significance

  • Intraluminal filling defect (partial or complete) forming acute angles with the vessel wall: acute PE
  • Eccentric, mural, calcified, or webbed defects with vessel narrowing: chronic thromboembolic disease
  • RV:LV ratio >1.0, septal bowing, and IVC contrast reflux: right heart strain / higher-risk PE
  • Peripheral wedge-shaped opacity (Hampton hump) or linear band: pulmonary infarct
  • Mosaic attenuation with pruned vessels and enlarged pulmonary artery: pulmonary hypertension
  • Bronchial artery hypertrophy and parenchymal cause on hemoptysis studies
  • Alternative diagnoses: pneumonia, edema, pneumothorax, aortic pathology, or malignancy

Differential Considerations

  • Central filling defect: acute PE vs flow-related mixing artifact vs pulmonary artery sarcoma
  • Perfusion mosaicism: chronic PE vs small-airways disease vs pulmonary vascular disease
  • Right heart strain: acute PE vs pre-existing pulmonary hypertension vs chronic cor pulmonale
  • Peripheral opacity: infarct vs pneumonia vs hemorrhage vs atelectasis
  • Enlarged pulmonary artery: pulmonary hypertension (any cause) vs idiopathic dilatation
  • Apparent subsegmental defect: true clot vs partial volume/motion artifact vs poor opacification

Pearls & Pitfalls

  • A suboptimally opacified study is nondiagnostic for PE, not negative; consider repeat or alternative imaging
  • Transient interruption of contrast from deep inspiration dilutes pulmonary arterial enhancement and mimics defects
  • Beam-hardening and flow artifacts commonly simulate defects; correlate on multiple planes and windows
  • Report right heart strain because it changes risk stratification and management
  • Isolated subsegmental PE has uncertain clinical significance; correlate with clinical risk and DVT status
  • Do not miss an alternative diagnosis (e.g., dissection, pneumonia) that explains the presentation

Structured Report

  • State protocol, contrast timing, and adequacy of pulmonary arterial opacification
  • Report presence, location (most proximal level), and extent of emboli, or explicitly state their absence
  • Include RV:LV ratio and other signs of right heart strain
  • Describe pulmonary infarcts, parenchymal, pleural, and mediastinal findings
  • State any alternative or incidental diagnosis explaining the clinical presentation
  • Impression: PE present/absent with clot burden and strain, plus recommendations for management or follow-up

References

  • ACR Appropriateness Criteria: Suspected Pulmonary Embolism
  • ESC Guidelines for the Diagnosis and Management of Acute Pulmonary Embolism
  • PIOPED II Investigators (Stein et al.), NEJM: Multidetector CT for Acute PE
  • Fleischner Society statement on subsegmental pulmonary embolism

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.