A CT scan of the heart produces detailed cross-sectional images that reveal the coronary arteries, heart chambers, valves, the surrounding pericardium, and major blood vessels like the aorta. Depending on the type of scan ordered, it can detect calcium buildup in artery walls, plaque that could trigger a heart attack, narrowing of the arteries, structural problems with the heart muscle, and abnormalities in nearby structures. There are two main types: a coronary calcium score scan and a coronary CT angiography (CTA), and each one shows different things.
Calcium Score: A Quick Snapshot of Risk
A coronary calcium score scan is the simpler of the two. It doesn’t use contrast dye and takes only a few minutes inside the scanner. What it measures is the amount of calcified plaque sitting in your coronary artery walls. The result is a single number called an Agatston score.
The score breaks down like this:
- Zero: No calcium detected. This suggests a low chance of heart attack in the coming years.
- 1 to 99: Mild plaque deposits. Some buildup is present, but risk remains relatively low.
- 100 to 300: Moderate plaque deposits, associated with a notably higher risk of heart attack or other heart disease over the next three to five years.
- Over 300: More extensive disease and a higher heart attack risk.
The calcium score is useful as a screening tool, but it has a significant blind spot. It only detects hard, calcified plaque. Softer, non-calcified plaque, which can be just as dangerous, won’t show up. In fact, among people who score zero on a calcium scan, about 17% still have blockages caused by non-calcified plaque. So a zero score is reassuring but not a guarantee.
CT Angiography: A Detailed Look Inside the Arteries
Coronary CT angiography goes much further. It uses an iodine-based contrast dye injected into a vein, which lights up the blood vessels and heart chambers on the scan. This lets the radiologist see not just calcium but the full range of plaque types, the degree of artery narrowing, and the condition of the heart’s internal structures.
CTA can distinguish between different kinds of plaque based on how dense they appear on the scan. Dense, bright areas indicate heavy calcium. Intermediate-density material suggests fibrous plaque. Lower-density regions point to fatty or “soft” plaque, which is the type most likely to rupture and cause a heart attack. The scan can even identify specific high-risk features: tiny specks of calcium scattered within soft plaque, or a pattern where a dark fatty core sits right against the artery’s inner wall surrounded by a brighter rim, a hallmark of vulnerable plaque.
Beyond plaque composition, CTA shows how much an artery has narrowed. Blockages in the range of 30 to 69% are considered moderate, and the scan helps determine whether a narrowing is severe enough to restrict blood flow or whether it can be managed with medication and lifestyle changes. Overall diagnostic accuracy for CTA is about 81%, compared to roughly 69% for the calcium score alone, making it the stronger test when actual blockages need to be confirmed or ruled out.
Heart Structures Beyond the Arteries
A cardiac CT doesn’t stop at the coronary arteries. The images capture the heart’s four chambers, the thickness and motion of the heart muscle walls, and the valves that control blood flow between chambers. The mitral, tricuspid, aortic, and pulmonary valves are all visible, anchored to a ring of fibrous tissue that forms the structural skeleton of the heart. Doctors can spot valve calcification, abnormal thickening, or structural defects.
The pericardium, the thin sac surrounding the heart, also shows clearly. Fluid buildup around the heart (pericardial effusion), thickening of the pericardium, or scarring from previous inflammation can all be identified. In rare cases, CTA reveals an outpouching of the heart wall where muscle has ruptured but been contained by the pericardium, a dangerous condition called a pseudoaneurysm.
What It Reveals About the Aorta
Because the scan captures a wide field of view, the thoracic aorta is visible in detail. Radiologists measure the aorta at multiple points along its length, perpendicular to its flow, to detect aneurysms, areas where the vessel wall has ballooned outward. The scan can also identify dissections, where the layers of the aortic wall separate, as well as blood clots along the wall, signs of inflammation (aortitis), and whether disease in the aorta extends into branch vessels that supply major organs. For people who have had previous aortic surgery, CT can evaluate grafts and stents for complications.
What to Expect Before and During the Scan
The entire appointment usually takes about 30 minutes, though the actual scanning portion lasts only a few minutes on modern machines. Most of the visit is preparation.
For a CTA, your heart rate matters. The target is below 65 beats per minute, because a slower heart produces sharper images with less motion blur. If your resting heart rate is higher, you’ll likely be given a medication to bring it down before the scan. You may also receive a spray or tablet that temporarily widens the coronary arteries, making them easier to see.
The contrast dye is injected through an IV, typically in your arm. You might feel a warm flush or a metallic taste for a few seconds as it circulates. Kidney function is checked beforehand because the kidneys filter the dye out of your blood. People with reduced kidney function face a higher risk of contrast-related kidney stress. About 8% of people with mildly reduced kidney filtration and up to 27% of those with severely reduced function develop some degree of kidney irritation after contrast exposure. If your kidney function is borderline, your doctor may adjust the amount of contrast used or choose a different type of imaging.
Radiation Exposure in Perspective
Cardiac CT does involve radiation, and the dose varies depending on the technique. Older methods that image the heart continuously throughout the heartbeat deliver higher doses, averaging around 10 to 13 millisieverts (mSv). Modern protocols that limit the X-ray beam to a narrow window of the heart cycle cut that dramatically, down to 1 to 5 mSv. The newest photon-counting scanners can bring the dose below 1.5 mSv, which is comparable to a standard mammogram.
A simple calcium score scan uses even less radiation since it doesn’t require contrast timing or as many images. For context, the average person absorbs about 3 mSv per year from natural background radiation. The risk from a single cardiac CT is very small, though it’s slightly more relevant for younger patients, where cumulative lifetime exposure matters more.