If you or a loved one has ever received a magnetic resonance imaging (MRI) scan, you may have wondered exactly how it works. Here’s an in-depth look at how a full-body MRI scan produces images of the inside of the body.
How an MRI scan works
MRI scanners use both magnets and radiofrequency signals to create images of organs and tissues in your body.
The MRI scanner first acts like a giant magnet, producing a powerful magnetic field. The strength of this magnet is directly related to the quality of the images it produces. Most hospital MRI scanners use a magnet that measures between .5 and 3 Tesla (T), thousands of times stronger than the earth’s natural magnetic field.
MRI scans operate the way they do because approximately 60 percent of the human body is made up of water. In a way, the MRI scanner measures the content and behavior of the water in different tissues within the body. These water particles contain hydrogen atoms, which are magnetic because of their subatomic particles called protons, that have a positive charge.
When an MRI scanner produces a magnetic field and directs it toward your body, the hydrogen atoms align with this field. This is similar to the way a compass points toward the North Pole.
Next, the MRI scanner sends out radio waves that disrupt the alignment of these hydrogen atoms. The protons in the hydrogen atoms produce signals. Since hydrogen protons act a little differently depending on the tissue, MRI scanners can use different radiofrequency pulses to examine and identify specific areas of the body.
Then, a receiver in the MRI scanner picks up those signals. A sensor in the machine detects and measures the intensity of the signals as the atoms return to their original alignment.
Essentially, the MRI scanner uses the signals that hydrogen atoms send out to map the location of the water molecules and develop a detailed image. A powerful computer processes all this information to build cross-sectional, detailed, black-and-white images.
What to expect from a full-body MRI scan
While it can be helpful to know how an MRI scan works, this doesn’t necessarily tell you what to expect when you receive one.
During an MRI scan, you will lie on a table that moves through the scanner’s round opening. You cannot feel either the magnet or the radio waves of an MRI scan, so the process is painless.
As the machine operates, you may hear noises such as, knocking or tapping. You may also hear the technician give you instructions to lie still or hold your breath for certain parts of the scan.
Because the magnet in an MRI scanner is so strong, you cannot wear metal accessories or implants during an MRI scan. This includes pacemakers, insulin pumps, and even some types of jewelry.
Sometimes a person receiving an MRI scan will also receive an injection of a contrast agent. This agent makes your protons realign faster with the magnetic field, creating a brighter image.
Overall, the MRI scan is fast and painless. There is no recovery time, so you can resume all normal activities right away.
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