A pulmonary embolism (PE) is a sudden, potentially life-threatening blockage in one of the pulmonary arteries, the main blood vessels that carry deoxygenated blood from the heart to the lungs. Accurate diagnosis is crucial, and medical imaging plays a central role. The current gold-standard procedure is Computed Tomography Pulmonary Angiography, or CTPA. This advanced scan provides detailed cross-sectional images, allowing clinicians to visualize the pulmonary vasculature and definitively confirm or rule out a blockage.
Understanding Pulmonary Embolism
A pulmonary embolism is almost always caused by a blood clot, known as a thrombus, that forms elsewhere in the body and travels to the lungs. The vast majority of these clots originate in the deep veins of the legs, a condition called deep vein thrombosis (DVT). Once the clot breaks free, it travels through the bloodstream, passes through the right side of the heart, and becomes lodged in one of the progressively narrowing pulmonary arteries.
This obstruction impedes blood flow to the affected area of the lung, preventing normal oxygen uptake and exchange. The resulting lack of blood flow (perfusion) can lead to lung tissue damage. Furthermore, the sudden increase in pressure against the right side of the heart can strain the heart muscle. The seriousness of a PE is related to the size and number of vessels blocked, making rapid diagnosis imperative.
Various factors increase a person’s risk of developing DVT and subsequent PE. These include:
- Extended periods of immobility, such as during long flights, prolonged bed rest after surgery, or paralysis, which cause blood to pool and clot in the deep leg veins.
- Major surgery, particularly orthopedic procedures involving the hip or knee, due to immobility and vessel injury.
- Certain medical conditions, including cancer, heart failure, and inherited blood clotting disorders.
- Hormone-related therapies, such as oral contraceptives or estrogen replacement therapy, which alter the blood’s clotting properties.
- Pregnancy, due to hormonal changes and the mechanical compression of pelvic veins by the enlarging uterus.
Recognizing these risk factors helps clinicians prioritize diagnostic testing when a patient presents with symptoms like sudden shortness of breath or chest pain.
The CT Pulmonary Angiogram Procedure
The CT Pulmonary Angiogram (CTPA) is the specific imaging test designed to visualize the pulmonary arteries. The procedure requires the intravenous injection of a special iodine-containing contrast agent, often referred to as dye, which is crucial for making the blood vessels visible on the scan. The patient is positioned on the CT table, and an IV line is established, typically in the arm or hand, to deliver the contrast at a high rate using an injector pump.
The timing of the scan is a precise and carefully managed part of the procedure. The goal is to capture images at the exact moment the contrast material reaches its maximum concentration within the pulmonary arteries. Technologists often use a technique called “bolus tracking,” where a monitoring slice is set over the pulmonary artery, and the scan is triggered automatically once the contrast enhancement reaches a predetermined threshold.
This precise timing ensures that the arteries appear bright white against the surrounding dark lung tissue on the CT images. If the scan is performed too early or too late, the concentration of the contrast in the pulmonary arteries may be insufficient, making an accurate diagnosis difficult or impossible.
Direct Visual Confirmation of the Embolus
The definitive sign of a pulmonary embolism on a CTPA is the presence of a “filling defect” within the pulmonary artery lumen. The contrast dye, which is radiopaque, fills the normal, unobstructed parts of the vessel and appears bright white. The blood clot, however, does not absorb the contrast and appears as an area of lower density or darkness, effectively creating a defect in the contrast-filled vessel.
This defect can take on different characteristic appearances depending on the clot’s location and age. A particularly dangerous presentation is the “saddle embolus,” which is a large clot that physically straddles the bifurcation of the main pulmonary artery, blocking blood flow to both the right and left lungs simultaneously. Smaller clots appear as dark, linear, or rounded defects within the segmental or subsegmental branches of the pulmonary vasculature.
In cases of acute, or recently formed, emboli, the clot may be seen suspended within the contrast, sometimes surrounded by a thin rim of contrast material, which is known as the “polo mint sign” when viewed in cross-section. This sign indicates a freely floating thrombus that has not yet adhered to the vessel wall. Older, or chronic, emboli may look slightly different, often appearing retracted, irregular, or adhered to the vessel wall, and sometimes showing signs of calcification. The presence and distribution of these filling defects confirms the diagnosis of pulmonary embolism.
Ancillary Findings on CT
While the filling defect is the direct evidence of a PE, the CTPA often reveals several indirect, or ancillary, findings that support the diagnosis and indicate the severity of the obstruction. One of the most important secondary signs is evidence of right heart strain, which occurs when the right ventricle struggles to pump blood against the increased pressure caused by the blocked arteries. The right ventricle (RV) may become visibly enlarged compared to the left ventricle (LV), with a measured ratio of the RV-to-LV diameter often exceeding 0.9 or 1.0.
The severe pressure can also cause the interventricular septum (the wall separating the two ventricles) to flatten or bow inward toward the left ventricle. Other signs of strain include the reflux of contrast material backward into the inferior vena cava (IVC) or the hepatic veins, indicating high pressure backing up the venous system. These signs of right heart strain correlate with a higher risk of adverse outcomes.
Beyond the heart, the lung tissue itself may show secondary changes. Damage distal to the blocked vessel can manifest as wedge-shaped areas of opacity, known as pulmonary infarcts, though this occurs in a minority of patients. Additionally, a small collection of fluid, or pleural effusion, may be noted in the space surrounding the lung in about half of PE cases. These ancillary signs provide context regarding the physiological impact and severity of the pulmonary embolism.