Residency · Residency · Diagnostic Radiology

Pulmonary Embolism: CT Pulmonary Angiography Interpretation

CTPA Technique

Protocol Optimization

A high-quality CT pulmonary angiography study requires careful attention to technique. Contrast injection typically uses 60 to 80 mL of non-ionic iodinated contrast injected at 4 to 5 mL/s through an antecubital vein via an 18 to 20 gauge IV. Bolus tracking, with a region of interest placed in the main pulmonary artery and a trigger threshold of 100 to 150 HU, ensures optimal timing. The scan can be acquired caudocranially or craniocaudally depending on institutional preference; caudocranial acquisition may reduce breathing motion artifact at the lung bases. ECG gating is not routine but can reduce cardiac motion artifact, which is particularly helpful for evaluating the right middle lobe artery. Reducing the tube voltage to 100 kVp (or 80 kVp in thin patients) increases iodine contrast enhancement while reducing radiation dose. Thin-section reconstruction at 0.625 to 1.25 mm is necessary for evaluating subsegmental arteries.

Common Causes of Suboptimal Studies

Several factors can compromise study quality. Poor contrast opacification may result from a slow injection, a small IV gauge, a delayed trigger, or severe right heart failure causing slow pulmonary transit time. The transient contrast interruption artifact occurs when dense unopacified blood from the IVC mixes with contrast-opacified blood from the SVC during a Valsalva maneuver, creating a transient filling defect in the pulmonary arteries. Respiratory motion blurs vessels and creates pseudofilling defects. Beam hardening streaks from dense contrast in the SVC can obscure the right pulmonary artery. When a study is technically inadequate, it should not be reported as negative for PE; instead, repeat CTPA, V/Q scan, or lower extremity ultrasound should be recommended.

Direct Signs of Acute PE

Intraluminal Filling Defect

The hallmark of acute PE on CTPA is a central or eccentric filling defect that partially or completely occludes the vessel lumen. Several specific signs describe the appearance of thrombus. The polo mint sign (or doughnut sign) describes contrast surrounding a central clot on a cross-sectional (axial) view. The railway track sign shows contrast outlining a central linear thrombus on a longitudinal view of the vessel. Complete occlusion produces a vessel segment filled with low-attenuation thrombus that appears distended compared to adjacent patent vessels. In acute PE, the thrombus forms an acute angle with the vessel wall because the clot is not yet adherent to the endothelium.

Location Classification

PE is classified by the most proximal extent of thrombus. A central or saddle PE straddles the main pulmonary artery bifurcation, extending into both left and right main pulmonary arteries. Lobar PE involves the right or left main pulmonary artery. Segmental PE involves segmental branches, and subsegmental PE involves fourth-order and smaller branches. For severity assessment, the RV/LV ratio is more commonly used than clot burden scoring systems such as the modified Miller index or Qanadli score.

Indirect Signs of Acute PE

Parenchymal and Pleural Findings

The Hampton hump is a wedge-shaped peripheral consolidation representing pulmonary infarction, with its apex pointing toward the hilum. The Westermark sign describes focal oligemia (reduced vascularity) distal to an occluded vessel. Mosaic perfusion refers to geographic areas of differing lung attenuation resulting from heterogeneous blood flow. Subsegmental linear or plate-like atelectasis, often basilar, is common. Small pleural effusions, unilateral or bilateral, are frequently present and may be hemorrhagic if infarction has occurred.

Right Heart Strain on CTPA

An RV/LV diameter ratio greater than 1.0, measured in the axial plane at the widest short-axis dimension of each ventricle, indicates RV dilation and is associated with worse outcomes. Interventricular septal bowing or flattening, with leftward displacement, indicates RV pressure overload. Reflux of contrast into the IVC and hepatic veins suggests tricuspid regurgitation and elevated right-sided pressures. A main pulmonary artery diameter exceeding 29 mm suggests pulmonary hypertension, and SVC distension provides corroborative evidence of elevated right-sided pressures. RV strain on CTPA is an independent predictor of short-term mortality and helps identify patients with intermediate-risk (submassive) PE who may benefit from more aggressive therapy.

Acute vs. Chronic PE Comparison

Chronic PE

Imaging Features Distinguishing Chronic from Acute PE

Chronic PE has a distinctly different appearance from acute PE. Eccentric mural thrombus is adherent to the vessel wall, forming obtuse angles rather than the acute angles seen with fresh thrombus. Webs and bands, which are linear filling defects traversing the vessel lumen, represent organized and recanalized thrombus. The affected vessels may show stenosis or complete occlusion, sometimes with calcified thrombus. Mosaic perfusion reflects chronic heterogeneous blood flow. Bronchial artery enlargement indicates collateral arterial supply from the systemic circulation. Dilated main pulmonary artery and right heart chambers indicate chronic pulmonary hypertension, and RV wall thickness exceeding 6 mm on CT indicates hypertrophy.

Chronic Thromboembolic Pulmonary Hypertension (CTEPH)

CTEPH is defined as a mean pulmonary arterial pressure of 25 mmHg or greater persisting more than 3 months after acute PE despite adequate anticoagulation. It occurs in 2 to 4% of acute PE survivors. Identification is critical because CTEPH is potentially curable with pulmonary thromboendarterectomy (PTE) or balloon pulmonary angioplasty. CTPA and V/Q scan are complementary for diagnosis, with V/Q scan being more sensitive for detecting CTEPH.

Subsegmental PE Controversy

The Debate

Isolated subsegmental PE (ISSPE) is detected in approximately 5 to 15% of CTPA-positive studies, and its clinical significance remains uncertain. It may represent true pathologic emboli, but false positives are also common at this level due to small vessel size, partial volume averaging, and respiratory motion. Interobserver agreement for subsegmental PE is poor compared to central or lobar PE.

Management Considerations

Some experts advocate observation without anticoagulation for ISSPE in patients who do not have DVT on lower extremity ultrasound, cancer, reduced cardiopulmonary reserve, hospitalized or immobilized status, or prior venous thromboembolism. The ACR Appropriateness Criteria and Fleischner Society statements support selective use of anticoagulation for ISSPE. When in doubt, lower extremity ultrasound is recommended; if DVT is present, anticoagulation is indicated regardless of the subsegmental location.

Alternative and Incidental Diagnoses on CTPA

Common Diagnoses Found on CTPA When PE is Absent

Up to 25% of CTPA studies performed for suspected PE reveal an alternative clinically significant diagnosis. Common findings include pneumonia or lung abscess, pleural effusion, pericardial effusion, aortic dissection or aneurysm, lung mass or nodule, lymphadenopathy, coronary artery calcifications, and incidental findings in the upper abdomen such as liver lesions and adrenal masses. The entire study should always be systematically evaluated, not just the pulmonary arteries.

Reporting Standards

Essential Elements in the PE-Positive Report

A complete PE-positive report should document the presence of PE, its location (central/saddle, lobar, segmental, subsegmental), laterality and number of involved vessels, the RV/LV ratio and any signs of right heart strain, the presence of pulmonary infarction, pleural effusion, incidental findings, and comparison with prior studies when available.

<image>A series of axial CTPA images demonstrating direct signs of acute PE. Image 1 shows a saddle embolus at the main pulmonary artery bifurcation with low-attenuation thrombus extending into both left and right main pulmonary arteries. Image 2 shows the polo mint sign (doughnut sign): a cross-sectional view of a segmental artery with central low-attenuation thrombus completely surrounded by a ring of contrast. Image 3 shows the railway track sign: a longitudinal view of a lobar artery with linear thrombus outlined by contrast on either side. Each sign is clearly labeled with arrows and annotations.</image>

<image>An axial CT image diagram at the level of the cardiac ventricles comparing normal RV/LV ratio with right heart strain from PE. The left panel shows a normal heart with the RV smaller than the LV (RV/LV ratio less than 1.0), with measurement lines drawn across the short-axis diameters of each ventricle. The right panel shows an enlarged RV with the RV/LV ratio greater than 1.0, flattened interventricular septum bowing toward the left ventricle, and contrast refluxing into the IVC and hepatic veins. Each finding is labeled with arrows. A caption states that RV/LV ratio greater than 1.0 on CTPA indicates right heart strain and is associated with increased short-term mortality in acute PE.</image>

<image>A comparison diagram showing acute versus chronic PE features on CTPA. The acute PE panel shows a central filling defect making an acute angle with the vessel wall, with a distended vessel lumen. The chronic PE panel shows an eccentric mural thrombus making an obtuse angle with the vessel wall, vessel stenosis, intraluminal webs/bands traversing the lumen, and a small inset showing calcification within the adherent thrombus. Both panels are labeled with arrows and descriptive captions highlighting the key distinguishing features.</image>

Clinical Pearls

An RV/LV ratio greater than 1.0 on CTPA is the most important prognostic indicator to report in acute PE, as it identifies patients with right heart strain who may benefit from more aggressive therapy in the setting of submassive PE. A saddle embolus does not automatically indicate massive PE; hemodynamic status (blood pressure, need for vasopressors) determines the severity classification, not clot location alone. Chronic PE features such as eccentric mural thrombus, webs and bands, and vessel stenosis should be specifically reported because CTEPH is a treatable cause of pulmonary hypertension. When reporting isolated subsegmental PE, lower extremity Doppler ultrasound should be recommended to assess for DVT, which would solidify the indication for anticoagulation. A technically inadequate CTPA (poor opacification, motion) should not be reported as negative for PE; alternative evaluation should be recommended. Up to one-quarter of CTPA studies reveal an alternative diagnosis, reinforcing the importance of systematically evaluating the entire study.

References

• ACR Committee on Practice Parameters. "ACR Practice Parameter for the Performance and Interpretation of CT Pulmonary Angiography," 2019
• Raja AS, et al. "Evaluation of Patients with Suspected Acute Pulmonary Embolism: Best Practice Advice from the Clinical Guidelines Committee of the ACP." Annals of Internal Medicine, 2015
• Konstantinides SV, et al. "2019 ESC Guidelines for the Diagnosis and Management of Acute Pulmonary Embolism." European Heart Journal, 2020
• Moore AJE, et al. "Imaging of Acute Pulmonary Embolism: An Update." Cardiovascular Diagnosis and Therapy, 2018
• Qanadli SD, et al. "New CT Index to Quantify Arterial Obstruction in Pulmonary Embolism." AJR, 2001
• Bariteau A, et al. "Systematic Approach to CT Pulmonary Angiography Interpretation." RadioGraphics, 2021

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