MRI's Magnetic resonance imaging, or MRI, is based on the fact that atoms contain both positive and negative charges. MRI's use magnetism to use the electrical charges of atoms to create images of materials. The most common use for MRI's is in medical diagnosis. MRI's were available for patients starting in 1984 (Nordenberg, 1999).

One of the MRI's greatest advantages is its relative safety compared to some other imaging techniques. The first method for imaging the body, x-rays, which use radiation to create its images. Another advantage is that MRI's can image less dense tissues than x-rays can (Nordenberg, 1999). But where the MRI has a tremendous advantage over x-rays is in its ability to create 3-dimensional images. It also does a better job of showing contrast between dense parts of the body, such as bones, and softer tissue, than other imaging techniques (Nordenberg, 1999).

How They Work

In medical use, MRI's focus on hydrogen atoms. The magnetic atmosphere the patient enters is a lop-sided one: the magnetic field generated will be stronger one side than the other, resulting in variances in resonance frequency, or how rapidly the hydrogen molecules vibrate in response to the magnetic field (Tro, 2006). That vibration, or resonance, is artificially created and controlled by the very powerful magnets pulling on hydrogen molecules in the body. These patterns are translated by a computer into a detailed image of the body part being imaged. An MRI creates such clear images between dense tissue and softer tissue because softer tissue contains more water, and hence, more hydrogen (Gould, DATE).

MRI's actually use four different types of magnets. The first is a resistive magnet. Resistive magnets are made by wrapping coils of wires around a center. An electric current then runs through the center, creating a magnetic field (Gould, DATE). These magnets draw a lot of electricity and are impractical for creating extremely powerful magnets....

Resistive magnets above the .03 tesla level are generally too expensive to operate.
The second kind is a permanent magnet. Permanent magnets must be very large, and can weigh many tons to generate a 0.4 tesla level strength.

Superconducting magnets are resistive magnets in a modified environment: they are surrounded by liquid helium at a temperature of -452.4F (Gould, DATE). The very low temperature reduces the resistance in the electrical wire, allowing for markedly more efficient use of the electricity used to create the magnetic field. While still expensive to operate, superconducting magnets can generate magnetic fields of up to 2.0 teslas (Gould, DATE).

MRI machines also use gradient magnets. These magnets are much weaker than the others, and their job is to cause the magnetic field to vary, and cause the MRI machine to focus on the selected part of the body.

Once the patient is in the MRI's magnetic field, nearly all of the billions of protons will line up with either the person's feet or head, canceling each other out. However, a few will not, and will spin in response to the magnetic pulse. The actual resonant frequency set for the machine will vary according to the type of tissue to be imaged, allowing the MRI to focus very precisely on specific spots on the body (Gould, DATE). The actual signal picked up by the MRI apparatus occurs when the magnets are turned off and the nonaligned hydrogen protons, which have been spinning, begin to slow down. That is the signal that is actually converted into a visual image (Gould, DATE).

Risks

Older MRI machines are solid tubes of relatively small diameter, with the patient lying on what amounts to a tray that slides in and out of the tube. In these enclosed tubes, some patients become severely claustrophobic (Nordenberg, 1999). More modern machines have open spaces on each side which help some people with that claustrophobic feeling. However, some people…