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How Do MRI Machines Work?

A MRI scan of the brain.
MRI technicians often observe the procedure from an adjoining room.
An MRI machine uses a magnetic field and radio waves to create pictures of internal organs and bone structures.
An MRI machine.
Medical professionals use MRI machines to diagnose internal maladies.
Brain MRIs are important to both researchers and medical doctors.
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  • Written By: Matt Brady
  • Edited By: Heather Bailey
  • Last Modified Date: 10 November 2014
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MRI — short for magnetic resonance imaging — machines use high-powered magnets to create incredibly detailed images of the body. A powerful primary magnet creates a magnetic field that's much stronger than even the magnetic field given off by the earth. The intense magnetic field causes the abundant hydrogen atoms in our bodies to arrange uniformly along the edge of the magnetic field. Then, smaller gradient magnets pulsate magnetic fields with surgical precision, which scatter the hydrogen atoms and cause them to spin in different directions. As the primary magnetic field pulls the hydrogen atoms back to their uniform formation, their movement and alternate spinning directions give off energy, called resonance, which can be translated into images with the help of radio frequencies.

MRI machines are tubular, with an opening just large enough to allow a person to fit inside. Images interpreted by magnetic fields are incredibly susceptible to distortion caused by movement. As a result, patients must remain as close to perfectly still as possible while scanning is under way. For some people, this can be quite difficult and uncomfortable, as it can take up to an hour or more to complete the scanning process. The process is also quite loud, due to the rotation of various magnets. To help patients pass the time without listening to awful clunking sounds, doctors often allow patients to have a headset for listening to music.

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MRI scans can be achieved using a variety of primary magnets to generate a large magnetic field. A superconducting magnet, consisting of coiling, electrified wire, is one of the most powerful primary magnets in use. As electricity is passed through wires, they create superconductivity, which results in a sizable magnetic field. A superconducting magnet only works, however, if the wires are kept at extremely cool levels — below zero — using liquid helium.

Some MRI scanners use the same set of electrified coils and wires as are used for superconducting magnets, but without the liquid helium to keep them cool. Used that way, the coils and wires create a resistive magnet, rather than a superconducting magnet. Without the cooling effect of the liquid helium, superconductivity isn’t accomplished; instead, much heavier currents of electricity are used to create a somewhat weaker, but still effective magnetic field. The other kind of primary magnet that may be used for MRI scanning is a permanent magnet. Permanent magnets are literally giant magnets that constantly give off a magnetic field. Due to their size and crushing weight, they’re not the most favored type of magnet for use in MRI machines.

Gradient magnets are able to rotate completely around a person's body. The smaller magnetic fields given off by gradient magnets are able to pinpoint with stunning precision and clarity what part of the body needs to be scanned. These magnets work in conjunction with coils and wires that emit radio frequencies, which also affect hydrogen atoms in such a way as to be able to gather detailed readings of various parts of the body. This combination of magnetic fields and radio frequencies allows experts to scan “slices” of a person’s body from any angle, providing an unparalleled look into what’s going on inside of the body.

Although MRI scanning is in many ways superior to other scanning methods, the tedium of operating MRI machines isn’t really necessary to detect most injuries. Broken bones, for example, often show up quite clearly on X-rays, which are far less laborious and expensive to operate. What X-rays can't pick up on so well, however, is soft-tissue images. For those, MRI machines are one of the most preferred methods of image scanning.

MRI machines are capable of giving detailed images of soft tissue anywhere in the body. This makes them ideal for detecting soft-tissue conditions such as brain hemorrhages, breast cancer and ligament injuries. Another upside to MRI machines is that they don't give off any radiation. Although radiation from scanning methods such as X-rays isn't proven to be harmful, it often provides some peace of mind to patients to know that they won't be exposed to any radiation.

Due to the powerful magnetic fields created by MRI machines, they must be carefully operated under close supervision, and certain precautions must be taken to prevent injury. Patients undergoing MRI scans must not have any metallic objects on their person, and they have to reveal whether they’ve ever had any metallic objects surgically inserted into their body. Even rooms that house MRI machines must be devoid of loose metallic objects while the machine is in use, as magnetic fields have been known to pull in objects from a considerable radius.

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