A chest CT scan is an imaging test that uses X-rays to create detailed cross-sectional pictures of your chest and upper abdomen. Unlike a standard chest X-ray, which produces a single flat image, a CT scanner rotates an X-ray beam around your body and assembles hundreds of individual slices into a highly detailed, three-dimensional view of your lungs, heart, blood vessels, airways, ribs, and surrounding tissues. The entire scan typically takes about 15 seconds of actual imaging time, though your total appointment will be longer.
What a Chest CT Can Detect
Doctors order chest CTs when they need more detail than a standard X-ray can provide. In emergency departments, the most common reason is to check for a pulmonary embolism, a blood clot in the lungs, which accounts for roughly 80% of emergency chest CT orders. Aortic problems (such as tears or aneurysms in the large artery leaving your heart) make up about 8%, and trauma evaluation about 4%.
Beyond emergencies, chest CTs are used to investigate lung nodules, diagnose infections like pneumonia when X-ray results are unclear, evaluate chronic lung diseases, stage cancers, and assess abnormalities in the lymph nodes or structures between the lungs. About 5% of chest CT scans detect new lung nodules that weren’t previously known, which may need follow-up imaging to monitor for changes over time.
With Contrast vs. Without Contrast
Some chest CTs are done “with contrast,” meaning you receive an iodine-based dye through an IV before or during the scan. This dye makes blood vessels and certain tissues show up more clearly, which is essential when doctors are looking for blood clots, vascular problems, or tumors. You may feel a warm sensation or a metallic taste in your mouth when the dye is injected. Both are normal and pass quickly.
If you’re getting a contrast scan, you’ll typically be asked not to eat for three hours beforehand, though clear liquids are fine. If you have diabetes, the guideline is slightly different: eat a light breakfast or lunch at least three hours before your appointment. For scans without contrast, no fasting is required, and you can eat, drink, and take your medications as usual.
The main concern with contrast dye has historically been its potential effect on the kidneys, a condition called contrast-induced nephropathy. However, recent research suggests this risk was likely overstated for most people. Current evidence shows that IV contrast rarely causes kidney injury in patients whose kidney function is above a certain baseline (an eGFR above 30, a measure your doctor can check with a simple blood test). If you have known kidney problems, your care team will weigh the benefits against the risks before using contrast.
How to Prepare
Preparation is minimal. You’ll likely change into a hospital gown and remove any jewelry, piercings, or metal objects that could interfere with the images. Leave valuables at home if possible. Beyond the fasting rules for contrast scans described above, there’s nothing special you need to do beforehand.
Let your technologist know if you’re pregnant, have allergies to iodine or contrast dye, or have kidney disease. These factors can change how (or whether) the scan is performed.
What Happens During the Scan
You’ll lie on a narrow table that slides into a large, doughnut-shaped machine. The opening is wide and shallow, so most people don’t feel claustrophobic the way they might in an MRI tunnel. The technologist will step into an adjacent room but can see and hear you the entire time.
When the scan starts, the X-ray tube rotates around you while the table moves slowly through the machine. You’ll be asked to hold your breath for about 10 to 15 seconds so that the movement of your lungs and diaphragm doesn’t blur the images. Some facilities ask you to hold your breath after breathing out partway, while others simply ask you to breathe quietly. Either approach produces good image quality. The key is to stay as still as possible during those few seconds.
The scan itself is painless. If you received contrast through an IV, the needle site may be slightly sore afterward, but that’s typically the only physical sensation to note. Most people are in and out of the imaging room within 15 to 30 minutes, including setup time.
Radiation Exposure
A standard chest CT delivers about 8 millisieverts (mSv) of radiation, which is roughly equivalent to 100 to 800 standard chest X-rays. That sounds like a lot, but for context, the average American receives about 3 mSv per year from natural background radiation alone. The clinical benefit of a chest CT, such as catching a blood clot or identifying a tumor, almost always outweighs the small statistical increase in cancer risk from a single scan. Still, doctors try to order CTs only when the information they provide will genuinely change your care.
Low-Dose CT for Lung Cancer Screening
A special version called a low-dose CT (LDCT) uses significantly less radiation than a standard chest CT and is specifically designed to screen for lung cancer in people at high risk. The U.S. Preventive Services Task Force recommends annual LDCT screening for adults aged 50 to 80 who have a 20 pack-year smoking history and either still smoke or quit within the past 15 years. A pack-year equals smoking one pack (20 cigarettes) per day for one year, so someone who smoked a pack a day for 20 years, or two packs a day for 10 years, meets the threshold.
Screening stops once you’ve been smoke-free for 15 years or if a health condition makes lung surgery no longer a realistic option. If you think you might qualify, this is worth bringing up with your primary care provider, as early detection through LDCT screening has been shown to reduce lung cancer deaths.