This page discusses the diagnosis of pulmonary embolism on various imaging modalities.
CT Angiography
CT angiography is the mainstay modality for suspicion of pulmonary embolism due to wide availability, high diagnostic accuracy, and ability to identify other diagnoses that commonly cause chest pain/dyspnea.
Acute pulmonary embolus manifests as a low attenuation filling defect in the center of a pulmonary artery.
Chronic pulmonary embolus manifests as eccentric/peripheral low attenuation defects, linear webs, or calcified emboli.
Serial axial CT images above show an acute embolus in the anterobasal segmental branch of the right lower lobe pulmonary artery.
VQ Scan
The hallmark of pulmonary embolism on a nuclear medicine lung ventilation/perfusion scan (VQ scan) is a wedge shaped perfusion defect without a corresponding ventilation abnormality.
See Pulmonology Case 2 for an example of other VQ findings in pulmonary embolism.
Several interpretation criteria have been published for diagnosis of pulmonary embolism with VQ scan.
Revised PIOPED Criteria
High probability scan (risk of pulmonary embolism >80%):
- Two or more large mismatched segmental defects or equivalent moderate/large defects
Intermediate probability scan (risk of pulmonary embolism 30-35%):
- Matched perfusion defect with associated x-ray opacities (“triple match”)
- Greater than 25% of a segment and less than 2 mismatched segmental perfusion defects
- Difficult to characterize as high or low probability
Low probability scan (risk of pulmonary embolism <20%):
- Non-segmental defects – small effusion blunting costophrenic angle, cardiomegaly, elevated diaphragm, ectatic aorta
- Any perfusion defect with substantially larger radiographic abnormality
- Matched ventilation and perfusion defect with normal chest radiograph
- Small sub-segmental perfusion defects
Normal scan (risk of pulmonary embolism 1-2%):
- No perfusion defects
Very Low Probability with PIOPED (Risk of pulmonary embolism <10%)
- Nonsegmental lesion (prominent hilum, elevated diaphragm, cardiomegaly, etc.) with no other perfusion defects
- Perfusion defect smaller than radiographic lesion
- Stripe sign (preserved perfusion between a defect and the pleural surface)
- Solitary triple-matched defect in upper or mid lung segment
- 2 or more matched perfusion/ventilation defects with regionally normal CXR and some areas of normal perfusion elsewhere in the lung
- 1-3 small segmental perfusion defects
- Pleural effusion at least 1/3 of pleural cavity, with no other perfusion defects
Modified PIOPED II Criteria
PE Present = High Probability
- Two or more large mismatched segmental defects or equivalent
PE Absent = Normal or Very Low Probability
- No perfusion defect
- Nonsegmental lesion (prominent hilum, elevated diaphragm, cardiomegaly, etc.) with no other perfusion defects
- Perfusion defect smaller than radiographic lesion
- Stripe sign (preserved perfusion between a defect and the pleural surface)
- Solitary triple-matched defect in upper or mid lung segment
- 2 or more matched perfusion/ventilation defects with regionally normal CXR and some areas of normal perfusion elsewhere in the lung
- 1-3 small segmental perfusion defects
- Pleural effusion at least 1/3 of pleural cavity, with no other perfusion defects
Nondiagnostic = Low or Intermediate Probability
- All other findings
Perfusion-Only Modified PIOPED II Criteria
PE Present
- Two or more large segmental defects without radiographic abnormality
PE Absent
- No perfusion defect
- Nonsegmental lesion (prominent hilum, elevated diaphragm, cardiomegaly, etc.) with no other perfusion defects
- Perfusion defect smaller than radiographic lesion
- Stripe sign (preserved perfusion between a defect and the pleural surface)
- Solitary matched perfusion/radiographic abormality in upper or mid lung segment
- 1-3 small segmental perfusion defects
- Pleural effusion at least 1/3 of pleural cavity, with no other perfusion defects
Nondiagnostic
- All other findings
Perfusion-Only PISAPED Criteria
PE Present
- One or more wedge-shaped perfusion defect
PE Absent
- No perfusion defect
- Non wedge-shaped perfusion defect
- Nonsegmental lesion (prominent hilum, elevated diaphragm, cardiomegaly, etc.) with no other perfusion defects
Nondiagnostic
- All other findings
Below is a VQ segmental anatomy reference, adapted from Kamal Singh (NucRadShare) and other references:
Pulmonary Angiography
Long considered the gold standard for diagnosis of pulmonary embolism, invasive pulmonary angiography is now rarely performed for diagnosis.
Pulmonary embolism is seen as an abrupt cutoff or filling defect within a pulmonary artery after contrast injection, and absence of distal branching vessels. This results in a wedge shaped area of decreased/absent perfusion extending to the lung periphery (similar to a VQ scan).
Signs of PE on Unenhanced Imaging
On chest radiograph the most common findings in pulmonary embolism are a normal chest or nonspecific atelectasis. More specific but less sensitive radiographic findings associated with pulmonary embolus include peripheral round or wedge-shaped opacity (Hampton’s hump), decreased peripheral vascularity (Westermark sign), pleural effusion (usually small with PE), and prominent central artery (Fleischner sign). See Pulmonology Case 4
Unenhanced CT is not adequate for the evaluation of pulmonary embolism. On occasion, however, pulmonary embolus may be seen as as slightly hyperdense pulmonary artery with or without enlargement of the artery.
References
VQ interpretation:
Gottschalk A, et al. J Nucl Med 1993;34:1119-1126.
Sostman HD, et al. Radiology 1994;193:103-107.
Stein PD, et al. Radiographics 2000;20:99-105.
Sostman HD, et al. J Nucl Med 2008;1741-1748.
Parker JA, et al. SNM Practice Guideline for Lung Scintigraphy 4.0 http://snmmi.files.cms-plus.com/docs/Lung_Scintigraphy_V4_Final.pdf
CTA interpretation:
Wittram C, et al. Radiographics 2004;24:1219-1238.
Castaner E, et al. Radiographics 2009;29:31-53.
Radiographic findings:
Worsley DF, et al. Radiology 1993;189:133-136.
Stein PD, et al. Chest 1991;100:598-603.