What's the difference between a brain CT scan and a brain MRI?

Brain computerized tomography (CT) scan and brain magnetic resonance imaging (MRI) are imaging tests that create pictures of the brain. However, each uses different technology.

• A brain CT scan uses X-rays and a computer to create detailed images of the brain and skull. Brain CT scans show cross-sections of brain, bone, blood vessels and other soft tissues. They provide more detail than standard X-rays do.
• A brain MRI also may be called a head MRI or a cranial MRI. It uses a computer, radio waves and a very strong magnetic field to create clear, detailed images of the brain's structure. The images show thin slices of the brain, similar to slices of bread.

A brain CT scan and brain MRI are both useful imaging tests. Healthcare professionals choose between them based on how quickly results are needed and how much detail is required.

Brain CT scans are often the first imaging test used when speed is critical — such as after a head injury or when a stroke is suspected.

• Emergency situations. In emergency situations, CT is usually the first test performed because it's fast, widely available and less expensive than MRI. It can quickly find serious problems in the brain such as bleeding, swelling, fractures or irregular growths.
• Head injuries. CT scans also are commonly used after a head injury or suspected concussion to check for bleeding or swelling. An MRI may be used later because it can find injuries that CT scans miss, such as small areas of bleeding or bruising.
• Stroke. Brain CT scans are often the first test when stroke is suspected. They can quickly detect bleeding in the brain and help determine the type of stroke. They also can quickly look at the blood vessels that bring blood to the brain to determine if a clot is present.

A brain MRI is typically used when more-detailed images of brain tissue are needed, such as for diagnosing tumors, stroke, dementia or other neurological conditions.

• Brain tumors. Because MRI provides more-detailed images of brain tissue than CT, it can help detect tumors, clearly show their location and show how they affect nearby brain areas. Brain MRI also can identify when cancer cells from other parts of the body spread into the brain, known as brain metastases.
• Stroke. Both CT and MRI are used to assess stroke. But a brain MRI is usually better at identifying a certain kind of stroke called an acute ischemic stroke, especially in the first few hours after symptoms begin.
• Dementia. Brain MRI can show detailed changes in the brain's structure, including shrinkage linked to conditions such as Alzheimer's disease and other types of dementia. While a brain CT scan can rule out other causes of memory or thinking issues, such as tumors or large strokes, a brain MRI provides more detail when diagnosing dementia.

Both brain CT scans and brain MRI scans are important tools for diagnosing brain conditions. A CT scan is often used when the care team needs quick answers, especially in emergencies. An MRI may be used when more-detailed images of brain tissue are needed.

A new generation of CT scanning, known as photon‑counting CT, is improving care for people with neurological conditions. Photon-counting CT produces very detailed images that show structures as thin as a human hair while often using less radiation than standard CT scans.

Your healthcare professional will help choose the imaging test that's best for your symptoms and situation.