How Does an MRI Work?
What is an MRI Scan?
An MRI scan, or Medical Resonance Imaging Scan, is one of the most common types of scans ordered for injuries and serious medical conditions. These scans are done for a variety of reasons, including muscular/skeletal problems, head injuries, and even cancer. The scans have been performed since the 1980s, yet many patients do not know what is happening when they are sent into the MRI tube.
An MRI is a giant, extremely strong magnet that uses magnetic fields and radio waves to produce images of the body. Magnets are measured in a unit known as the gauss. This is more understandable when you note that the earth has a 0.5-gauss magnetic field. Large magnets are measured in a unit called a tesla, which equals 10,000 gauss. An MRI has a magnet that creates magnetic field of .5 to 2.0 tesla. When a patient receives an MRI, he is sent into the bore of the magnet, which is the hole in the middle, on a special movable table. The area of the body being scanned is placed into the exact center of the magnetic field before the scan begins.
The Science Behind MRI Scans
The MRI scan takes advantage of the fact that the body is made of a lot of water. This water contains oxygen and hydrogen molecules. Since hydrogen molecules have a natural magnetic spin, the MRI can change the alignment of the atoms' nuclei using the magnetic field and a radio-frequency wave burst. Once they are aligned, another pulse of radio-frequency waves will cause them to resume their normal position. When the hydrogen molecules move in this manner, the atoms themselves send out radio waves, which are recorded and mapped by the scanner.
The time it takes for these molecules to regain their natural alignment varies depending on the type of tissue being scanned. The computer will record the amount of time the molecules take to realign themselves, and this allows the scan to detect different types of tissue as it makes a map of the body.
What MRI Scans Show
MRI scans are able to show most body tissues. Tissues, like bones, that do not have much water in them, and therefore do not have much hydrogen, will appear dark on the scan. Other tissues will appear brighter. The scan will deliver two-dimensional pictures of the body, allowing the doctor to look layer by layer at the area being scanned. This can be turned into three-dimensional models on the computer that can be manipulated, allowing the doctor to see even more detail about the tissue before planning treatment. Blood flow can also be measured using an MRI scan, without the need for a contrast injection as other imaging techniques require.
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