What medical imaging technique uses strong magnetic fields and radio waves?

Magnetic resonance imaging (MRI) is the medical imaging technique that uses strong magnetic fields and radio waves. The magnetic field aligns hydrogen nuclei, mostly the protons in water molecules, and radio-frequency pulses disturb that alignment. As the protons relax, they emit signals that a computer converts into detailed images.

MRI does not use X-rays or other ionizing radiation, which distinguishes it from radiography and computed tomography (CT). Its ability to show soft tissues makes it especially useful for imaging the brain, spinal cord, muscles, ligaments, joints, and many organs. Different pulse sequences emphasize different tissue properties, allowing radiologists to identify abnormalities that may be difficult to see on an X-ray.

MRI is not simply a stronger version of an X-ray. The scanner’s powerful magnet means metal objects and some implanted devices require careful screening. The loud knocking sounds come from rapidly switching magnetic field gradients. Some examinations use gadolinium-based contrast agents, which are chemically different from the magnetic fields and radio waves that create the basic image.