A pulmonary embolism (PE) is a sudden blockage within one of the pulmonary arteries in the lungs. This obstruction most often occurs when a blood clot, typically formed in the deep veins of the legs, breaks off and travels through the bloodstream. PE is a medical emergency because it restricts blood flow, potentially causing permanent lung damage, low oxygen levels, and heart strain. Imaging is necessary to confirm the diagnosis, and computed tomography (CT) scanning is the most common method used to visualize this event.
The Role of CT Pulmonary Angiography (CTPA)
The standard procedure for diagnosing PE is Computed Tomography Pulmonary Angiography (CTPA). A standard CT scan cannot differentiate the soft tissue of a blood clot from surrounding blood vessels. CTPA overcomes this limitation by using a concentrated, iodine-based contrast material injected into a vein.
This contrast agent rapidly travels through the bloodstream and into the pulmonary arteries, making the blood flow appear bright white on the resulting images. The contrast acts as a highlighter for the interior of the vessels, allowing the CT scanner to capture high-resolution images of the lung’s vascular network.
Any obstruction, such as a blood clot, prevents the contrast from filling that area, creating a visible shadow against the bright background. The clot is visualized as a “filling defect,” which is the definitive sign of a pulmonary embolism. This method is the preferred tool for confirming a PE diagnosis in patients presenting with symptoms like acute shortness of breath or chest pain.
The Patient Experience During the Scan
The CTPA procedure requires precise timing to ensure the contrast material is concentrated in the pulmonary arteries during scanning. A small intravenous (IV) line is placed, usually in the arm, through which the contrast dye is injected by an automated pump. Patients commonly report a transient sensation of warmth or a metallic taste in the mouth as the contrast enters the bloodstream.
To capture the optimal image, the scan uses bolus tracking, which monitors the contrast in a target vessel, such as the main pulmonary artery. Once the contrast reaches a pre-set level of brightness, the CT scanner automatically triggers image acquisition. This timing is crucial because the window of peak concentration in the pulmonary arteries is very short.
The patient is positioned lying down and instructed to hold their breath for a brief period, often less than ten seconds, during the scan. This breath-hold minimizes motion artifacts that could blur the image and obscure smaller clots. The entire process, from injection to image capture, is rapid, which benefits patients experiencing respiratory distress.
Interpreting the Image: Visualizing the Clot
On a CTPA image, the pulmonary arteries appear uniformly bright white due to the contrast agent. A pulmonary embolism is identified as a dark area within this bright vessel, known as a filling defect, indicating where the clot has displaced the contrast-filled blood.
Acute emboli, which are newly formed clots, often demonstrate specific visual markers. If a clot is centered in a vessel, the contrast may flow around it, creating a thin, bright rim known as the “polo mint sign” in cross-section. If the clot is elongated, parallel lines of contrast along its sides can create the appearance of a “railway track sign.”
A large, non-occlusive clot that makes an acute angle with the vessel wall is also characteristic of an acute event. Clot location is important: central emboli are found in the main or lobar arteries, while peripheral emboli are located in smaller segmental or subsegmental branches.
One concerning finding is a “saddle embolus,” a single, large clot that straddles the bifurcation of the main pulmonary artery. This location indicates a significant burden of clot. In contrast, chronic emboli, which are older and organized, may present with features such as webs, bands, or intimal irregularities, often forming obtuse angles with the vessel wall.
How CT Scans Determine Embolism Severity
Beyond confirming the presence of a clot, the CT scan helps clinicians determine the severity and immediate impact of the PE on heart function. The acute blockage increases pressure in the pulmonary arteries, forcing the right ventricle (RV) to work harder. This strain can lead to right ventricular dysfunction, a major predictor of poor outcomes.
The size of the heart chambers can be measured on the CT images to assess this strain. The most commonly used indicator is the ratio of the Right Ventricle diameter to the Left Ventricle diameter (RV/LV ratio).
Normally, the RV is smaller than the LV, but an RV/LV ratio greater than 0.9 or 1.0 suggests the right ventricle is dilated due to pressure overload. Other signs of right ventricular dysfunction visible on the CT include the interventricular septum bowing or flattening into the left ventricle. These indicators allow for the rapid stratification of patients into high-risk categories, guiding decisions on aggressive therapies like clot-dissolving medications.