What is an MRI (Magnetic Resonance Imaging)?
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 a 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, such as strokes, tumors, aneurysms, spinal cord injuries, multiple sclerosis and eye or inner ear problems, according to the Mayo Clinic. It is also widely used in research to measure brain structure and function, among other things.

"What makes MRI so powerful is, you have really exquisite soft tissue, and anatomic, detail," said Dr. Christopher Filippi, a diagnostic radiologistat North Shore University Hospital, Manhasset, New York. The biggest benefit of MRI compared with other imaging techniques (such as CT scans) is, there's no risk of ionizing radiation, Filippi told Live Science.

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 and made into an image, Filippi explained.

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.

An MRI scan reveals the gross anatomical structure of the human brain.
An MRI scan reveals the gross anatomical structure of the human brain.
Credit: Courtesy FONAR Corporation

This form of MRI measures how water molecules diffuse through body tissues. Certain disease processes — such as a stroke or tumor — can restrict this diffusion, so this method is often used to diagnose them, Filippi said. Diffusion MRI has only been around for about 15 to 20 years, he added.

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. Functional MRI, or fMRI, measures changes in blood flow to different parts of the brain. The main form of fMRI involves blood-oxygen-level dependent (BOLD) contrast. BOLD signals are widely used as a proxy for brain activity, because neurons use more oxygen when they're active. This technique has been especially useful in neuroscience — "It has really revolutionized how we study the brain," Filippi said.

Increasingly, researchers are using fMRI and DTI together, Filippi said. They use fMRI to map brain activity, and use DTI to map tracts of white matter — the connective cables of the brain.

Unlike other imaging forms like X-rays or CT scans, MRI doesn't use ionizing radiation. The imaging method is increasingly being used to image fetuses during pregnancy, and no adverse effects on the fetus have been demonstrated, Filippi said.

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 implants today are "MR-safe," however, Filippi said.

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.

MRI methods are constantly improving. "There are always new [magnetic] pulse sequences and different ways to image different body parts," Filippi said. Techniques are becoming much more quantitative, and smaller and smaller regions can be imaged — scientists can now image brain areas down to 1 millimeter thickness, he said.

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