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MRI - Magnetic Resonance Imaging

Magnets Are the Difference


Updated January 16, 2008

MRI - Magnetic Resonance Imaging

MRI Image

David Job / Getty Images
A type of imaging used a great deal in the care of HIV-positive patients is magnetic resonance imaging, or MRI as it is more often called.

Like other types of imaging such as x-rays and CT scans, the MRI is used to look at specific areas inside the body to diagnose infections, tumors, cancers, or other illnesses. MRIs are so detailed and can produce such high resolution images that the only way to see the inside of the body better would be to cut it open.

What is an MRI?

The science of MRIs is very complex. Unlike x-rays and CT scans that use focused beams of x-rays to create images, the MRI uses magnetic fields and radio waves to create very detailed and high resolution images. Understanding exactly how MRI works would take pages of text, explaining the physics of magnets and their effect on the atoms in the body. Let's try to simplify things a bit.

Let's look at the main parts of the MRI in order to better understand the procedure.

  • "M" - The Magnet - The most important component of the MRI is the magnet. We all played with magnets as kids, but the magnet in an MRI is nothing to play with. The magnet is so strong that it can pull metal objects like tables, IV poles, and chairs into the machine. It's strong enough to affect the atoms of the human body.

  • "R" - Radio Waves - This is the "resonance" portion of the MRI. During an MRI, the area of the body being examined is subjected to strong magnetic fields. Hydrogen atoms in the area respond by emitting radio wave energy at a specific frequency. When this energy is released by the hydrogen atoms, the signal produced is detected and sent to the computer for interpretation.

  • "I" - The Computer - As the magnets and radio waves do their work, it is the computer that takes all the information obtained and creates the images your doctor will use to make a diagnosis.

Put It All Together

  • The person having the MRI is placed on a table that is moved into the center of the machine. Simply put, the person on the table is surrounded by the magnets contained in the MRI machine.
  • While the person is inside the machine, a magnetic pulse of radio waves is turned on and off in rapid succession. Remember, these magnetic pulses are absorbed by the hydrogen atoms, which in turn emit radio waves at specific frequencies.
  • While the hydrogen atoms respond to the magnetic pulses by aligning in the direction of the magnetic field, shutting of the pulse causes them to return to their natural state.
  • Returning to their natural state causes the atoms to release stored energy in the form of radio waves.
  • These waves are detected by the machine and sent to the computer for interpretation.
  • The computer assembles all the information gathered, noting the positions and locations of the hydrogen atoms, and creates high resolution images of the area being examined.

Much like the CT scan, the MRI makes a series of "slices" of the area in question. Think of it as a loaf of sliced bread. Put all the slices together and you have a complete loaf. Put all the slices of the MRI together and you have a high-resolution image.

Before the MRI

Depending on the facility, you may or may not have some dietary restrictions the night before your MRI. Unless told otherwise, you may eat, drink, and take your medicines as usual before your MRI. There are some other things to be aware of before you go into the MRI scanner:
  • Leave all your jewelry at home. Because the magnets used in MRIs are so powerful, any metal can become a dangerous projectile. Things to leave at home include rings, watches, earrings, necklaces, bracelets, piercings, and eye glasses (leave them in the car when you park if you can see without them).

  • Sometime prior to the MRI, you will have to answer a few questions. Most importantly, you will be asked if you have any metal implants, implanted devices, or any other types of metal in your body. Again, the magnets are so strong that metal objects in the body can be moved and shifted by the MRI magnet. In some cases, such as metal clips in the brain or implanted pacemakers and defibrillation devices, movement caused by the magnet can be extremely dangerous.

  • Some types of MRIs will require you to drink contrast material or have contrast material infused intravenously.

    Important Note!

  • Notify your MRI technologist if you know of any allergies to MRI contrast material or if you have ever had a reaction to MRI contrast in the past. Note that MRI contrast material is different that the contrast material used in CT scans.

  • Notify the MRI technician if you have any metal, implanted devices, or have ever had metal in your eyes. If you are a welder, let the technician know prior to your test.

During Your MRI

  • During the MRI you will have very little to do other than just laying on your back.
  • For some MRIs it is important to be absolutely still so you may have to wear an appliance that will keep motionless the body part being examined.
  • During the test you will hear frequent loud banging noises. These are a normal part of the test. Some facilities have music you can listen to or non-metallic ear plugs you can use if the noise bothers you.
  • The length of your MRI will depend on what part of the body is being examined. An MRI of the knee will take about 20 minutes while an MRI of the brain can take an hour or more.

    Important Note!
    Because the MRI table is inside the machine, it can feel very confining. If you suffer from claustrophobia, speak with your doctor a few days before your MRI. He or she can prescribe a sedative that will help you relax while in the MRI machine.

After The MRI

  • After the MRI, you will be able to resume your normal activity.
  • If you had an IV during the procedure, that will be removed and a bandage will be applied to the site. Monitor the site for signs of infection or bleeding.
  • Your test will be read by a radiologist and the results will be passed along to your doctor, who will discuss the results with you.


    Gould, T.; "How an MRI Works"; How Stuff Works; 12 Dec 2007.

    Radiological Society of North America, Inc. (RSNA); "MRI of the Body"; Radiology Info; 3 Jul 2007.

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