In MRI, a patient lies inside a cylindrical magnet and is exposed to a powerful magnetic field. Protons in water molecules line up when exposed to this field, then vibrate out of alignment when zapped with radio waves. As the protons realign, they produce a signal that can be picked up and processed into an image by a computer. Water-rich tissues send stronger signals and show up bright in the resulting images; bones appear relatively dark.
The technique is used here to depict arteries in the brain and neck. After an injection of contrast fluid, the scan is repeated as the dye moves through blood vessels, allowing a radiologist to see blockages that could cause a stroke, brain aneurysms, or even trauma injuries.