As their name implies, magnets are a critical component of magnetic resonance imaging technology. High-quality imaging requires a high-quality magnetic field. In an MRI machine, a main magnet immerses the patient in an unchanging, intense magnetic field while three other magnets create varying fields.
The human body consists of many billions of atoms, each with a nucleus that spins on an axis, like a spinning top, or the Earth on its axis. MRI focuses on hydrogen atoms, which have a single proton and tend to line up in the direction of a magnetic field when one is applied. In an MRI machine, the magnetic field runs straight down the center of the tube in which the patient is placed, so that when the machine is activated, the hydrogen atoms in his or her body will align with the magnetic field either toward the feet or the head.
After the patient has been scanned, the hydrogen protons begin to return to their natural alignment within the magnetic field, and to release their stored energy. This release comes in the form of a radio signal that is picked up by the coil, which transmits the data to a computer system, which is then converted into an image.
The magnets used in modern MRI machines range from 5,000 to 20,000 gauss. In contrast, the Earth's magnetic field is a wimpy 0.5 gauss. This means that any patient or support staff member must be thoroughly screened for metal objects before being allowed into the scan room. Patients with pacemakers, aneurysm clips in the brain, and occasionally dental implants can't be safely scanned with MRI, and must use other imaging technologies.