How Do CT Scans Detect Pulmonary Embolism?

A PE is a blood clot that blocks blood flow within the lungs. Typically, this blood clot breaks off from another area of the body (such as the legs) and lodges in a blood vessel near the lungs.

A PE can be life threatening. If the lungs don’t get adequate blood flow, they can’t produce enough oxygen for you to live.

An estimated 250,000 people in the United States are hospitalized each year for a PE. The condition doesn’t always cause symptoms, but when it does, symptoms can include:

• chest pain
• cough, including coughing up blood
• low blood pressure (hypotension)
• rapid heart rate (tachycardia)
• shortness of breath (dyspnea)

If doctors suspect a PE, they will likely order a CT scan to confirm the diagnosis.

If doctors suspect a pulmonary embolism, they may order a CT scan. This scan uses radiation to obtain images of body parts, such as the lungs.

Specifically, CT with pulmonary angiography shows the blood vessels in the lungs. It can help a doctor identify an embolus or emboli.

There are several reasons CT scans may be the best option for diagnosing a PE. First, the scan is highly accurate in helping doctors identify a PE. A CT scan takes imaging “slices” of the body that allow doctors to see clots, even in very small blood vessels.

Also, CT scanners tend to be widely available, and the results can be received quickly.

However, CT scans aren’t completely risk-free. Some potential complications include:

• Allergic reaction. It’s possible to have an allergic reaction to the IV contrast dye given to you for a CT scan.
• Contrast-induced nephropathy. Contrast given through an IV can sometimes temporarily affect your kidney function. This could be especially harmful to a person who already has kidney damage.
• Radiation exposure. CT scans use radiation to produce images, which could potentially increase radiation risks over time.

For these reasons, a doctor should only order a CT scan if they think it’s highly likely that you have a PE.

Sometimes, a PE will display a “Polo mint sign” on a CT. Doctors named this sign after a famous mint in the United Kingdom that has a hole in the center, like Life Saver mints in the United States. The CT image will appear as a very white, circular structure around a blood vessel.

However, this sign isn’t always present. Other changes related to a PE on a CT scan may include:

• a blood vessel that doesn’t appear to fill with blood because of an obstruction (clot)
• a blood vessel that appears to bulge
• contrast dye that appears to light up parallel to the clot

Some people have chronic pulmonary emboli. These usually look different than an acute PE that causes symptoms.

Once you’ve had a PE, there’s a significant risk of having another one.

A doctor may prescribe anticoagulants, or blood thinners, for you to take on a regular basis to try to prevent future blood clots. However, taking anticoagulants can increase your risk of bleeding, so a doctor will weigh the risks against benefits in your situation.

You won’t always have another CT scan to check for a PE. If your symptoms were very severe or your doctor is worried about future blood clots, they may order another CT scan. However, with each additional CT scan, you’re exposed to more radiation.

A doctor may use other imaging tests to help them diagnose or confirm a PE. These include:

• Chest X-ray. A chest X-ray can help doctors eliminate other potential chest pain causes, such as pneumonia, collapsed lung (pneumothorax), or fluid in the lungs (pulmonary edema).
• MRI scan. An MRI scan uses magnetic fields to detect differences in body tissues. While an MRI may help a doctor accurately detect a PE, MRIs are not available in all settings. They also tend to take longer than a CT scan. And some people who have metal in their bodies cannot undergo an MRI.
• Transthoracic echocardiogram. This type of echocardiogram uses ultrasound waves to measure the heart’s functioning. Because the blood vessels in the lungs deliver blood to the right side of the heart, changes in the right heart’s functioning may indicate a PE.
• Ultrasound. An ultrasound is a painless, radiation-free test that allows a doctor to get images of veins in the legs to identify blood clots. While this test doesn’t diagnose a PE specifically, it does show whether you’re at increased risk of PE due to clots elsewhere in the body.

If you have a history of an allergy to contrast dye, are pregnant, or have kidney problems that may keep you from receiving contrast dye, your doctor may recommend one of the above imaging studies instead of a CT scan.

Laboratory tests

A doctor may also order laboratory testing to determine the likelihood that you have a PE.

One example is a D-dimer test. When a blood clot starts to dissolve, the body naturally breaks down the clot and releases proteins. The D-dimer test measures the levels of specific fragments that result from clot breakdown.

D-dimer test results that are below 500 nanograms per milliliter (ng/mL) usually indicate a low PE risk.

If your D-dimer levels are high, they could indicate that you have a blood clot. Sometimes, a doctor will order this test before they order a CT scan to determine whether it’s likely that you have a clot.

A CT scan is the most common diagnostic test doctors use to detect and diagnose a PE. If a doctor does identify a PE, they may decide to prescribe treatments, such as anticoagulants, or to simply monitor the clot over time.