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What Is the Difference between an MRI and PET Scan?

By Erik J.J. Goserud
Updated Mar 03, 2024
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The primary difference between an MRI and PET scan is the way in which each method conjures up the image that health providers rely on. These scans are incredibly valuable in confirming and exploring possible diagnoses, without which physicians and other professionals would be less able to effectively treat disease. Functionally, the both of these imaging studies act to penetrate peripheral structures in obtaining similar goals; however, their methods of doing so differ. Simply stated, an MRI uses magnetic technology, while a PET scan detects a substance in the body to produce an active image.

Both the MRI and PET scan are acronyms. MRI is an abbreviation for magnetic resonance imaging, while PET stands for positron emission tomography. Although both terms may be intimidating to the lay person, they truly are very descriptive of what each scan accomplishes and how it does so.

Magnetic resonance imaging uses multiple magnetic fields, each very strong, to physically align specific atoms in the internal structures of the body. These magnetic fields cause a rotational movement of the nuclei in the center of the atoms, and these variations are due to locational differences as well the type of atom. These disparities in rotational speed are recorded and translated to form a magnetic resonance image. This image generally reflects an internal structure of the body. One strength of MRIs is their ability to illustrate discrete differences in soft tissues, notably the brain, heart, muscles, and growths.

The MRI and PET scan differ mostly in modality. While the MRI uses magnetic fields, an active ingredient is the key for successful PET scanning. This substance is usually injected into a specific region of the body or ingested, and as this radio tracer decays, it emits positively charged particles, which are picked up up with specialized equipment. The differences in emission show how the body's systems are functioning. Functional differences may be expressed with differing brightnesses and colors on the computerized image.

It is important to realize that an MRI and PET scan differ fundamentally in the way they acquire their images as well as the circumstances under which they may be utilized. Taking and interpreting such images requires a great deal of knowledge, which is why only trained professionals should administer or draw conclusions from these tests. These tests are tremendously complicated procedures whose mechanical processes are only completely understood by those trained to do so.

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Discussion Comments

By Lostnfound — On Sep 28, 2014

My dad had an MRI and said it was like being in a sealed bathtub while a freight train was bearing down on you. He was not a fan of the procedure.

I know that people who take certain kinds of diabetes medication cannot have some tests that have contrast, nor can people who are allergic to iodine have some procedures with contrast.

I've heard some fairly scary stories about people having bad reactions when they have an MRI, so I might think twice about having one if another test would do the same thing.

By Grivusangel — On Sep 27, 2014

One big difference is that someone who has an artificial joint, like a knee or hip, can have a PET scan, but not an MRI. The magnets in the MRI machine would pull the artificial joint out of the body!

My mom has an artificial hip and needed a lung scan. She had to have a PET scan for that reason.

She said the procedure was odd, though. She had to lie down about an hour before the test while the contrast dye was given, but could not sit up. She had to lie down. She said the whole test made her feel a little weird.

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