What is the difference between a CT scan and an MRI?

Both a CT scan and an MRI make detailed pictures of the inside of your body, but they get there in different ways. A CT scan uses an X-ray machine linked to a computer. An MRI uses a powerful magnet and radio waves, with no radiation at all.

In a CT scanner, a motorized X-ray tube spins around a donut-shaped ring called a gantry while the bed slides slowly through it. One rotation takes a second or less. A computer stitches the angles together into cross-sections and 3-D images.

An MRI works on your own body. The magnet lines up protons, tiny particles inside you, in one direction. A pulse of radio energy knocks them out of line, and the scanner listens as they settle back. Different tissues settle at different speeds, and that difference is what forms the picture. MRI scans take much longer than CT scans, and you have to lie still throughout.

What each one is better at

CT is fast and shows dense structures well. It is the usual choice for complex bone fractures, badly worn joints, bone tumors, and for scanning the chest, abdomen, and pelvis at once. Its speed makes it the practical option in an emergency.

MRI is better for soft tissue. It is well suited to the brain, spinal cord, nerves, muscles, ligaments, and tendons. For a tumor in or near the brain or spine, MRI usually gives the clearer answer.

The two are not rivals. Many people with cancer get both, because each answers a different question. Your doctor picks the scan that fits the question at hand.

Radiation, and why it matters more for some people

CT uses ionizing radiation. The FDA notes that this exposure "may cause a small increase in a person's lifetime risk of developing cancer." Risk per unit of dose is higher in younger patients, who also have more years ahead for any effect to appear. The same agency is clear about the balance: "When used appropriately, the benefits of a CT scan far exceed the risks."

MRI carries no such exposure. That is one reason it is often chosen for children, for young adults, and when the same area must be checked again and again.

Dye, and who should be careful

Either scan may involve contrast, a dye that highlights certain areas so the images are easier to read.

CT contrast is usually iodine-based and given through a vein. Barium-based drinks are used for the digestive tract. MRI contrast uses gadolinium, which can make tumors show up brighter. In people on dialysis, some gadolinium agents have been linked to a rare but serious illness called nephrogenic systemic fibrosis, a hardening of the skin and other tissue. Tell the staff about kidney disease before either scan.

What to say before an MRI

Some metal is dangerous near the magnet. People with implants that contain iron should not enter the scanner. That includes pacemakers, implantable cardioverter-defibrillators, and vagus nerve stimulators. Tell the technologist about any implant, surgical clip, or metal fragment in your body, and about pregnancy.

Expect noise. Some scanners reach 120 decibels of clicking and beeping, so ask for ear protection. If small spaces frighten you, say so ahead of time rather than on the table. Sedation and open scanners are options at many centers.

Before either test, ask two things: what question is this scan meant to answer, and are my old scans available for comparison. If earlier imaging was done elsewhere, bring the disc or have the report sent. A change over time often tells your team more than a single image can.