How is radio frequency used to get an MRI image?

The magnet in an MRI makes hydrogen atoms line up. For every atom that is lined up toward the feet, another atom that is lined up toward the head will cancel it out. Only a very few atoms are not canceled out.

The MRI machine then applies a radio frequency pulse that is specific only to hydrogen. The machine directs the pulse toward the area a doctor wants to examine. The pulse causes protons in that area to absorb the energy that is required to make them spin in a different direction. This is where the "resonance" part of MRI comes from. The radio frequency pulse forces the few atoms that are not canceled out to spin at a particular frequency and in a particular direction.

Now, three gradient magnets can be activated. These magnets are arranged inside the main magnet so when they are turned on and off rapidly and in a specific manner, they alter the primary magnetic field on a very local level. This means the doctor can use the MRI to pick a very specific area and take a picture.

MRI operators speak about "slices." Kind of like a loaf of bread, the MRI's pictures are like slices that make up the whole - although, with the MRI, the slices are as thin as a few millimeters. Doctors can "slice" any part of the body in whatever direction he or she wants to. This provides a huge advantage over any other type of imaging. It also means that patients don't have to move for the MRI to get an image from a different direction - the MRI itself can manipulate everything with the gradient magnets.

When the radio frequency pulse is turned off, the hydrogen protons slowly return to their natural alignment within the magnetic field. They also release their excess stored energy. When this happens, the protons give off a signal that the MRI picks up and sends to a computer system. The system receives mathematical data that can be converted, into a picture and put on film. That is the "imaging" part of an MRI.

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