Which sign on CXR represents decreased pulmonary vascularity associated with pulmonary embolism?

Westermark's sign is characterized by a focal area of decreased vascularity in the lung fields on a chest X-ray (CXR), which can be indicative of a pulmonary embolism (PE). This sign arises when a large pulmonary artery occlusion leads to diminished blood flow to the distal lung parenchyma, resulting in less vascular marking in the corresponding area of the lung. When viewing the CXR, this sign usually presents as an area that appears more lucent due to the lack of vascularity compared to the surrounding lung tissue.

In the context of a pulmonary embolism, recognizing Westermark's sign aids clinicians in assessing the likelihood of PE when paired with other clinical findings and imaging results. It’s essential to note that while other signs may indicate pulmonary pathology or other conditions, Westermark's sign specifically points to the decreased pulmonary vascularity that occurs when a significant embolus obstructs blood flow to a segment of the lung. This makes it a critical sign in the evaluation and diagnosis of pulmonary embolism.