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

Both make detailed pictures of the inside of your body, but they get there in completely different ways. A CT scan uses X-rays. An MRI uses a strong magnet and radio waves. Nearly every other difference between them follows from that one.

The National Cancer Institute describes a CT as an "X-ray machine linked to a computer" that takes "a series of pictures of your organs from different angles." It describes an MRI as using "a powerful magnet and radio waves" to take "pictures of your body in slices."

How each one builds the picture

A CT scanner spins. The National Institute of Biomedical Imaging and Bioengineering explains that "a motorized x-ray source rotates around the circular opening of a donut-shaped structure called a gantry." Your bed moves slowly through it. A computer then turns those signals into slice-by-slice images.

An MRI has no X-ray source at all. NIBIB describes a magnet that lines up protons in your body's water. A radio pulse then knocks them out of line. When it stops, "the MRI sensors are able to detect the energy released as the protons realign." Different tissues realign at different speeds, and that gap is what creates the detail.

The difference that matters most

Radiation dose. The Food and Drug Administration states that "MR images are made without using any ionizing radiation, so patients are not exposed to the harmful effects of ionizing radiation." A CT does use it.

That shapes real decisions. For someone young, or someone facing years of repeat scans, dose is a genuine factor in choosing between them.

What each one is better at

Neither is simply superior. NIBIB notes that CT produces excellent detail for bone, including "complex bone fractures, severely eroded joints, or bone tumors." It is also the standard choice for the chest and lungs.

MRI is stronger on soft tissue. It reads small tissue differences finely and uses no ionizing radiation. NIBIB describes it as particularly suitable for imaging the brain. NIBIB adds that the spinal cord, nerves, muscles, ligaments, and tendons are seen much more clearly with MRI than with CT.

Doctors often order both, for different questions about the same cancer.

Practical differences on the day

The contrast dyes are not the same. NIBIB notes that CT uses "intravenous (IV) contrast agent based on iodine." For the digestive tract it uses "oral contrast agents, such as barium-based compounds." MRI contrast is gadolinium based instead. NIBIB warns that it should go to people on dialysis only when essential. The concern is a rare condition called nephrogenic systemic fibrosis.

The MRI also has safety rules a CT does not. The magnet is never switched off, so metal matters. FDA sorts devices into three labels. MR Safe means it "poses no known hazards in all MR environments." MR Conditional means safe only under set conditions. MR Unsafe means keep it out. Pacemakers, defibrillators, nerve stimulators, and metal fragments in the eye all need checking first.

And an MRI is loud, reaching up to 120 decibels, so you will be given ear protection. The tube is narrow and the scan takes longer, which is harder for anyone claustrophobic or in pain.

Before either scan, tell the team about implants, past injuries involving metal, kidney problems, allergies to contrast, and any chance of pregnancy. And if you want to know why one was chosen over the other, ask. The answer is usually tied to one specific question your doctor is trying to settle.