Magnetic Resonance Imaging (MRI)—how does it work

MRI picture

“You need an MRI of your foot,” my doctor explained to me. “We need to find out what’s going on before we decide on surgery.” Oh yeah, what a good idea! But, why an MRI (which is quite expensive) instead of the good old fashion x-ray (which is more cost effective)?

An MRI uses a magnetic field and pulses of radio wave energy to make pictures of organs and structures inside the body. It is able to scan the body with the use of magnets and may also show additional problems that are not seen with other imaging tests. Another advantage of an MRI is that it is able to image in any plane unlike Computer Tomography (CT) which is limited to one plane. Although, there are open machines in some facilities, most MRIs are enclosed and provide better pictures.

The magnet is the most important component of the MRI system. The horizontal tube—the bore, where the patient enters, runs through the magnet from front to back. The magnet creates a very stable magnetic field which is created by a current of electricity passing through the many coils in the superconducting magnet. Large amount of energy such as superconductivity (reducing the resistance in the wires to almost zero) is used to maintain the large magnetic field. To proceed with such a process, the wires are continually immersed in liquid helium. The MRI system has different coils for the various parts of the body that transmit radiofrequency into the patient’s body. During the exam, these coils conform to the body part being imaged or reside very close. The loud hammering sound during the scan is due to the increased electrical current in the wires of the gradient magnets being opposed by the main magnetic field. Additional parts of an MRI include a powerful computer and a patient table which slides into the bore.

So what goes on during the scan?

The human body is made up of billions of atoms. The hydrogen atom, abundant in the body (made up mostly of water and fat), is what is important for the MRI scan.

  • The atoms go in various directions randomly
  • Atoms light up north and south, when placed in a magnetic field, (they line up in the direction of the field). About half of the atoms go each way, but there are a few unmatched atoms
  • When radio frequency pulse, specific to hydrogen, is applied the unmatched atoms spin the other way
  • When the radio frequency is turned off, the extra atoms return to their natural alignment. and releases energy absorbed from the radio frequency pulse
  • Energy is sent to a computer which uses a mathematical formula to convert the signal into an image

Okay, I’m ready for that MRI scan on my foot!

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