A pulmonary embolism (PE) is a serious medical condition where an artery in the lungs becomes blocked, typically by a blood clot that has traveled from a vein elsewhere in the body. This blockage restricts blood flow, leading to low oxygen levels and strain on the heart. Although a chest X-ray is often the first imaging test performed when PE is suspected, it is generally not the definitive diagnostic tool because the clot itself is not visible. The X-ray serves a different, highly important function in the initial assessment.
The Primary Purpose of a Chest X-Ray in PE Evaluation
A chest X-ray (CXR) is performed early in the evaluation process to quickly rule out other conditions that mimic PE symptoms, such as sudden shortness of breath and chest pain. These non-specific symptoms can be caused by serious illnesses, including pneumonia, a collapsed lung (pneumothorax), or heart failure. The utility of the CXR lies in its ability to immediately visualize these alternative diagnoses, which require different and immediate treatment.
The X-ray can show signs of fluid buildup (pulmonary edema) associated with heart failure, or the characteristic air pocket of a pneumothorax. If the X-ray points to one of these diseases, the diagnostic path shifts away from PE, saving time. Conversely, a normal chest X-ray in a patient who is acutely breathless and has low oxygen levels actually raises suspicion for PE, as it removes the most common alternative explanations.
The X-ray acts as a triage tool, helping professionals narrow down possibilities rather than confirming the blood clot itself. Even when PE is confirmed, the initial chest X-ray is often reported as normal. This quick, non-invasive imaging provides context for subsequent, more specialized tests, such as the ventilation/perfusion (V/Q) scan.
Specific X-Ray Findings That May Suggest Pulmonary Embolism
Although a chest X-ray cannot directly show the blood clot, a few specific, uncommon findings can appear as a result of the PE’s effect on the lung tissue. These subtle signs can increase suspicion for PE. One such sign is the Westermark sign, which appears as a localized area of decreased blood flow (oligemia) in the lung. This occurs because the clot has blocked the artery, reducing vascularity to that specific region.
Another finding is Hampton’s hump, a wedge-shaped opacity—an area of increased whiteness—located at the periphery of the lung. This sign represents pulmonary infarction (lung tissue death caused by lack of blood supply) and occurs in approximately 23% of PE cases. The Palla’s sign refers to an enlargement of the right descending pulmonary artery, the vessel just before the blockage, due to increased pressure behind the clot.
The presence of these signs—Westermark, Hampton’s hump, or Palla’s sign—is highly suggestive of PE. However, their absence does not exclude the diagnosis, as most chest X-rays in PE patients are normal. For example, the Westermark sign is found in only 8% to 14% of confirmed PE cases. When noticed, these findings prompt the clinician to move immediately to more definitive imaging techniques.
Definitive Imaging Techniques for Diagnosing PE
Since the chest X-ray is not a reliable diagnostic tool for PE, physicians rely on specialized tests to confirm or rule out the condition. The most commonly used and definitive imaging method is the Computed Tomography Pulmonary Angiography (CTPA). This test involves injecting a contrast dye into a vein, which travels through the bloodstream to the pulmonary arteries.
A CT scanner takes detailed cross-sectional images. If a PE is present, the blood clot appears as a dark filling defect against the bright, contrast-filled artery. CTPA has largely replaced older, more invasive procedures due to its high diagnostic accuracy, with a sensitivity often cited between 83% and 97% and specificity up to 96%. The resulting images also allow for the assessment of the heart’s right ventricle, providing additional information about the clot’s severity and the patient’s prognosis.
An alternative imaging test is the Ventilation/Perfusion (V/Q) scan, which uses a radioactive tracer to compare how air flows into the lungs (ventilation) with how blood flows through them (perfusion). In a patient with PE, the V/Q scan typically shows a mismatch: air enters a lung segment, but blood flow is blocked. This test is often preferred for patients who have kidney impairment or an allergy to the contrast used in the CTPA, or for younger patients, especially pregnant women, because it involves a lower radiation dose.
Before imaging, the highly sensitive D-dimer assay blood test is often used as an initial screening tool for patients with low or intermediate clinical suspicion. D-dimer is a protein fragment released into the bloodstream when a clot is being broken down. A negative D-dimer result has a high negative predictive value, meaning it can safely rule out PE in low-risk patients, avoiding the need for radiation-based imaging.