What Is a Pulmonary Embolism?
A pulmonary embolism (PE) happens when a blood clot travels through the bloodstream and lodges in one of the arteries supplying the lungs. These clots usually originate in the deep veins of the legs or pelvis, a condition called deep vein thrombosis (DVT). Once a clot reaches the lungs, it blocks blood flow to a section of lung tissue. This can cause chest pain, shortness of breath, a rapid heartbeat, and in severe cases, it can be life-threatening.
Pulmonary embolism is more common than many people realize. It can affect anyone, though risk increases after surgery, during prolonged bed rest, in people with certain clotting disorders, or during long-distance travel. Early and accurate diagnosis is critical because treatment with blood thinners can prevent the clot from growing and allow the body to gradually dissolve it.
How the MAA Lung Perfusion Scan Works
A lung perfusion scan is a nuclear medicine imaging study that maps blood flow throughout your lungs. The scan uses tiny radioactive particles called macro-aggregated albumin (MAA), which are injected into a vein in your arm.
Here is what happens during the procedure: the MAA particles are slightly larger than the smallest blood vessels (capillaries) in the lungs. After injection, they travel through the bloodstream and temporarily lodge in the lung capillaries in proportion to blood flow. Areas receiving good blood flow will trap more particles, while areas blocked by a clot will trap very few or none at all.
A gamma camera then takes images of your lungs from multiple angles. The camera detects the radioactive signal from the MAA particles and creates a map showing where blood is flowing and where it is not. A region with absent or reduced blood flow, called a “perfusion defect,” raises suspicion for a blood clot blocking that vessel.
The entire procedure typically takes 30 to 45 minutes. The amount of radioactivity used is small and is cleared from the body within hours. The MAA particles themselves break down naturally and are absorbed by the body.
Perfusion Scan vs. CT Pulmonary Angiography
CT pulmonary angiography (CTPA) is the most commonly used test for detecting pulmonary embolism. It involves injecting iodine-based contrast dye into a vein and taking rapid CT images of the chest. CTPA is fast, widely available, and gives detailed pictures of the pulmonary arteries.
However, CTPA is not suitable for everyone. There are specific situations where a lung perfusion scan becomes the preferred choice:
Kidney Problems
The iodine contrast used in CT angiography can be harmful to the kidneys, especially in patients who already have reduced kidney function. A perfusion scan does not require any iodine contrast, making it much safer for these patients. The radioactive tracer used in a perfusion scan is processed differently and does not carry the same risk to the kidneys.
Contrast Allergy
Some patients have allergic reactions to iodine-based contrast agents. These reactions can range from mild hives to severe anaphylaxis. For patients with a known contrast allergy, a lung perfusion scan provides a reliable alternative that avoids the allergen entirely.
Pregnancy
Both CTPA and perfusion scans involve some radiation exposure. However, a perfusion scan delivers a lower radiation dose to the breast tissue compared to CTPA. Since pregnant and young women are more sensitive to breast radiation, a perfusion scan is often the preferred option. The radiation dose to the developing baby is very low with both tests, but the perfusion scan has a slight advantage in reducing overall maternal radiation exposure.
What the Results Mean
After the scan, a nuclear medicine physician analyzes the images. The results are typically reported in categories:
A normal perfusion scan shows even blood flow throughout both lungs. This makes pulmonary embolism very unlikely, and your doctor can often rule it out with confidence.
An abnormal scan with one or more perfusion defects may indicate a blood clot. The physician looks at the pattern, size, and number of defects. Sometimes, a ventilation scan (which maps airflow) is done alongside the perfusion scan. If blood flow is absent in an area where airflow is normal, the mismatch pattern is highly suggestive of a pulmonary embolism.
In some cases, results may be categorized as “intermediate” or “indeterminate,” meaning further testing or clinical correlation may be needed. Your treating doctor will combine the scan results with your symptoms, blood tests, and overall clinical picture to make a decision about treatment.
What to Expect on the Day
No special preparation is usually required for a lung perfusion scan. You can eat and drink normally before the test. The injection of MAA is given through a small needle in the arm, similar to a routine blood draw. You will then lie on a scanning table while the gamma camera captures images around your chest. The camera does not touch you and the process is painless.
After the scan, you can resume normal activities immediately. The radioactive tracer decays quickly and is eliminated from the body within a day. Drinking extra water after the test can help flush it out faster.
Talk to Your Doctor
If you are experiencing unexplained shortness of breath, chest pain, or if your doctor suspects a blood clot in your lungs, ask about the lung perfusion scan. The choice between this scan and CT angiography depends on your individual health profile, and your doctor is the right person to determine which test suits you. A nuclear medicine physician can provide further guidance on what to expect and how the results will be used in your care plan.