Pulmonary Embolism
In this episode of The Radiology Review Podcast, we cover pulmonary embolism (PE), an essential diagnosis to master for the radiology boards and for clinical practice. This high-yield review highlights imaging findings, pitfalls, and pearls across modalities, with a focus on CT pulmonary angiography (CTPA). Download the free study guide on this episode by clicking here..
Show Notes/Study Guide:
I’ve already covered VQ scans on prior podcast episodes so VQ scans will be only briefly covered in this episode. Please refer to those prior episodes for a deeper dive into VQ scans, including VQ scans for pulmonary embolism.
What is the primary and most common imaging method for diagnosing pulmonary embolism (PE)?
CT pulmonary angiography (CTPA) is the most common and standard imaging method for diagnosing PE. Nuclear medicine VQ scans are an alternative when CTPA cannot be performed.
How does an acute pulmonary embolus typically appear on CTPA?
An acute PE typically appears as a central intraluminal filling defect within a pulmonary artery.
Where do pulmonary emboli commonly lodge?
Pulmonary emboli often lodge at vessel bifurcations.
What CTPA finding might suggest chronic thromboembolic disease?
Eccentric, circumferential filling defects on CTPA may suggest chronic thromboembolic disease.
Are associated pulmonary abnormalities seen on CTPA specific for PE?
No, associated pulmonary abnormalities seen on CTPA are generally nonspecific. Examples include wedge-shaped consolidation, pleural effusion, and linear bands of subsegmental atelectasis.
What is a crucial aspect to evaluate on CTPA in the context of PE?
It is crucial to evaluate the heart on CTPA for signs of acute right heart strain, which can be caused by PE.
What are some CTPA findings indicative of acute right heart strain due to PE?
Characteristic findings of acute right heart strain include abnormal position, flattening, or paradoxical bowing of the interventricular septum toward the left ventricle, right ventricular enlargement greater than the left ventricle (elevated RV:LV ratio), and a pulmonary trunk larger than the aorta. Additional signs are features of right heart failure such as inferior vena cava contrast reflux, dilation of the azygos system, and dilated hepatic veins with or without reflux.
What is the significance of an elevated RV:LV ratio on CTPA in patients with massive PE?
An elevated RV:LV ratio is correlated with increased mortality in massive PE.
What are some potential pitfalls on CTPA that can mimic pulmonary embolism?
Potential pitfalls on CTPA that can mimic PE include hilar lymph nodes, cardiac motion artifact, respiratory motion, mucus-impacted bronchi, and transient disruption of the contrast bolus. Unopacified pulmonary veins may also simulate peripheral emboli on a single CT slice. Remember to interrogate coronal and sagittal reformats when equivocal findings are seen on axial images.
While CTPA is the standard, can plain film radiography suggest PE?
Yes, while CTPA is the standard, plain film findings can suggest PE, especially when the diagnosis is not clinically suspected. Potential radiograph findings of PE include the Fleischner sign, Hampton’s hump, and the Westermark sign. Other signs also exist, but these are perhaps the most commonly tested on board exams.
What is the Fleischner sign and what does it suggest in the context of PE?
Fleischner sign describes widening of the pulmonary arteries due to clot and is specifically seen as an enlarged central pulmonary artery on a chest radiograph. This is best appreciated as a new finding compared to prior radiographs. In the context of PE, it suggests massive pulmonary embolism. Note that the Chang sign refers to the dilatation and abrupt change in caliber of the main pulmonary artery.
What is Hampton's hump and what does it represent?
Hampton’s hump is a peripheral wedge-shaped opacity on a chest radiograph due to a pulmonary infarct.
What is the Westermark sign and what does it indicate?
Westermark sign refers to regional oligemia (decreased blood flow) in the lung distal to the pulmonary artery thrombus. Look for regional decreased vascularity peripherally on a chest radiograph. Fleischner sign of an enlarged pulmonary artery may also be seen on this side.
Is pleural effusion a common finding in PE on a plain film?
Pleural effusion is a nonspecific finding seen in some PE cases (approximately 30%).
In what specific patient population is ventilation-perfusion (VQ) scanning often recommended for suspected PE?
VQ scanning is often recommended in pregnant patients with suspected PE and a normal chest radiograph.
What is the key finding for interpreting a VQ scan as positive for PE?
The interpretation of a VQ scan as positive for PE relies on identifying mismatched defects, where there is normal ventilation but reduced perfusion. Typically you are looking for wedge-shaped segmental mismatched perfusion defects.
What VQ scan finding essentially rules out PE?
A normal perfusion scan essentially excludes PE.
According to criteria like PIOPED II, how is a high probability for PE often defined on a VQ scan?
Based on criteria like PIOPED II, a high probability for PE is often defined by two or more large (>75% of a segment) mismatched segmental defects without an associated radiographic abnormality, or equivalent combinations of medium and large defects (e.g., four medium mismatched defects).
What constitutes a triple match on a VQ scan and what is its probability for PE in the lower lung?
A triple match is a matched defect on perfusion and ventilation images with a corresponding opacity on a chest radiograph. A triple match in the lower lung is considered intermediate probability for PE.
What are some general imaging characteristics that might help differentiate acute PE from chronic PE?
Acute PE tends to be more central, often with venous dilation and perivenous soft tissue edema.
Chronic PE may be more peripheral, with shrunken veins and the development of collateral vessels. Calcifications may also be present within the thrombi or the venous walls in chronic cases.