How Does an MRI Machine Work?
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MRI Machines
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Magnetic resonance imaging, usually referred to by the acronym MRI, is a technique employed in medicine to create highly detailed images of the interior of the body. This technique involves the use of an MRI machine. MRI machines typically range in length from 5 to 8 feet. The patient is positioned with most or all of his body within the length of the machine. MRI machines come in two varieties, closed and open, both of which use essentially the same technology. In a closed MRI machine, the patient lies inside a tube that is centrally located within the machine. Generally, the tube of a closed MRI is about 2 feet wide. Open MRI machines tend to be less powerful than their closed counterparts, but they are the preferred option for patients who have difficulty with tight quarters or suffer from full claustrophobia. As the name implies, open MRI designs do not enclose the patient fully. Some models allow the patient to undergo a scan while sitting or standing.
How MRIs Function
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Inside the structure of an MRI machine are powerful magnetic coils that create an intense magnetic field where the patient is situated. This magnetic field causes the protons of some atoms in the patient's body---notably hydrogen atoms---to line up along the magnetic field. (Hydrogen atoms are ideal for medical imaging because the human body is predominately made up of water, which is composed of two hydrogen atoms and one oxygen atom.) The machine then sends a series of bursts of radio frequency (RF) that causes only the hydrogen atoms to become excited. These excited hydrogen atoms move out of alignment with the magnetic field the machine has produced. As the hydrogen atoms then attempt to return to alignment with the magnetic field, they shed the excess energy they picked up from the RF. The machine detects and records that energy. In some cases, the patient may swallow or receive an injection of a contrast agent (typically gadolidium) before undergoing an MRI scan. A contrast agent helps to create a MRI image with better resolution by making portions of the body where the contrast agent collects appear brighter in the image.
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The Images
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On popular medical shows you will sometimes see MRI images displayed on a computer screen. These types of images are not produced directly from the information an MRI machine records. What the machine is actually detecting is the energy being shed as a function of time. The images on the computer screen are a translation of those readings from a function of time to a function of frequency. In simplest terms, the machine is taking a lot of mathematical data about what the atoms are doing and processing that data through a complicated set of mathematical processes called a Fourier transformation. The Fourier transformation creates information that the system can read or display on a computer screen as a visual representation of the entire body or part of the body.
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