MRIs are medical imaging systems used to diagnose health conditions.
Credit: MRI scan via Shutterstock
Magnetic resonance imaging (MRI), also known as nuclear magnetic resonance imaging, is a technique for creating detailed images of the human body.
The technique uses a very powerful magnet to align the nuclei of atoms inside the body, and variable magnetic field that causes the atoms to resonate, a phenomenon called nuclear magnetic resonance. The nuclei produce their own rotating magnetic fields that a scanner detects and uses to create an image.
MRI is used to diagnose a variety of disorders and measure brain structure and function, among other things.
How it works
The human body is mostly water. Water molecules (H20) contain hydrogen nuclei (protons), which become aligned in a magnetic field. An MRI scanner applies a very strong magnetic field (about 0.2 to 3 teslas, or roughly a thousand times the strength of a typical fridge magnet), which aligns the proton "spins."
The scanner also produces a radio frequency current that creates a varying magnetic field. The protons absorb the energy from the variable field and flip their spins. When the field is turned off, the protons gradually return to their normal spin, a process called precession. The return process produces a radio signal that can be measured by receivers in the scanner.
Protons in different body tissues return to their normal spins at different rates, so the scanner can distinguish among tissues. The scanner settings can be adjusted to produce contrasts between different body tissues. Additional magnetic fields are used to localize body structures in 3D. There are many forms of MRI, but diffusion MRI and functional MRI (fMRI) are two of the most common.
This form of MRI measures how water molecules diffuse through body tissues. It is used to diagnose conditions like stroke or disorders like multiple sclerosis. In neurons, molecules tend to diffuse along neural fibers, so the direction of diffusion parallels the fibers themselves. A recent method called diffusion tensor imaging (DTI) allows researchers to measure diffusion in multiple directions, and can be used, for example, to map connectivity between brain areas.
In addition to structural imaging, MRI can also be used to visualize functional activity in the brain. fMRI measures changes in blood flow to different parts of the brain, which is linked to the activity of neurons. The main form of fMRI involved blood-oxygen-level dependent (BOLD) contrast. BOLD signals are widely used as a proxy for brain activity.
Unlike other imaging forms like X-rays or CT scans, MRI doesn't use ionizing radiation. Still, the procedure can have risks, and medical societies don't recommend using MRI as the first stage of diagnosis.
Because MRI uses strong magnets, any kind of magnetic metal implant poses a hazard. The implant can move or heat up in the field. Several patients with pacemakers who underwent MRI scans have died, patients should always be asked about any implants before getting scanned. Many "MR-safe" implants are now being made.
The constant flipping of magnetic fields can stimulate peripheral nerves, and some volunteers report feeling a twitching sensation in their extremities during the switching. The flipping also produces clicking or beeping noises, so ear protection is necessary in the scanner room.
The radio frequency energy transmitted by the scanner can be absorbed by the body, causing heating. To prevent this from happening, there are limits on the transmitter rates than can be used.