What an MRI scan is

What MRI does

Magnetic resonance imaging uses a strong magnet and radio waves - not ionizing radiation - to build detailed pictures of soft tissues inside the body. It shows the brain, spinal cord, joints, organs and blood vessels in far more soft-tissue detail than a plain X-ray.

A computer assembles the signals into cross-sectional images that a radiologist reads. Some examinations use an injected contrast dye to make certain tissues or blood vessels stand out.

What happens during the scan

The person lies on a table that slides into the scanner, a tunnel-shaped machine. The scan is painless but loud - rhythmic knocking and tapping sounds - so earplugs or headphones are provided. Staying still matters, and a scan typically takes 20 to 60 minutes depending on the area being imaged.

People who feel closed-in in tight spaces can mention it beforehand; options range from reassurance and music to, in some centers, open scanners or mild sedation.

Safety and metal

Because the machine is a powerful magnet, metal is the central safety question. Staff ask carefully about pacemakers, implants, clips, shrapnel and some tattoos before anyone enters the scanner room. Many modern implants are MRI-safe or MRI-conditional, but each must be checked - never assumed.

MRI does not use X-rays, so it avoids the dose considerations of CT, which is one reason it is preferred for repeated imaging of the same condition.

What the images cannot say on their own

An MRI shows structure in great detail, but a finding on an image is not a diagnosis by itself. Incidental findings - changes that look unusual but cause no problem - are common. The formal report comes from a radiologist who interprets the images together with the symptoms, history and other tests.