MRI works because of interactions among multiple magnetic fields. The human body consists mainly of water, and the hydrogen atoms in each water molecule each have a magnetic dipole, a measure of the strength and direction of each atom's magnetic field. The scanner uses a standing magnetic field to align these dipoles. It then pulses a radio frequency magnetic field to alter the alignment, and the rotating dipoles generate a signal that the scanner can interpret as a 2D or 3D image of the interior of the human body.
Water isn't the only magnetically resonant material in the human body. Blood cells contain hemoglobin, an iron-rich protein that binds to oxygen. When hemoglobin is deoxygenated, its dipoles align with magnetic fields, but when it is deoxygenated, its dipoles oppose magnetic fields. As neurons become more active, they require an increased volume of oxygenated hemoglobin, and the resulting variation in magnetic dipoles is called the blood-oxygen-level dependent (BOLD) signal. Tracking the BOLD signal to indirectly track neural activity is called functional MRI, or fMRI.
Traditional structural MRI uses multiple pulses of the radio frequency magnetic field to produce high resolution images of the soft tissues in the brain. It can precisely identify tissue damage when a patient presents an unexplained behavioral disorder, and the correlation of the two can lead to a more rigorous demonstration of a causal connection.
fMRI images are taken every two to three seconds and so are somewhat lower resolution. The faster images allow researchers to track the brain's oxygen consumption while volunteers perform tasks. These data can demonstrate a stronger relationship between the brain's structure and function than can be inferred from structural MRI.
The changes in oxygenated blood flow that underly the BOLD signal measure the energy needs of large pools of neurons. This means the BOLD signal is an indirect measure of neural activity, and can only be used to infer correlation and not causation. fMRI also has low temporal resolution because vascular response occurs on a longer time scale than neural activity, and the two to three seconds for each fMRI image are slower still. As a consequence, fMRI is best suited to researching questions about localization of sustained brain activity.
The standing magnetic field used for fMRI can be 60,000 times the strength of the Earth's magnetic field, and can be very dangerous. Ferromagnetic objects like oxygen tanks and fire extinguishers can become deadly projectiles when brought too close to the scanner. Even implants like aneurysm clips or metal shavings that accidentally enter the body are affected and can cause significant bodily harm. MRI scanners are usually supported by safety certified technicians, and it is important to adhere strictly to safety protocols during the course of research.