How magnetic resonance imaging creates detailed body images

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).

Specialized MRI's

Several variants of the MRI have been developed since the invention of the first machine. One is called a Diffusion MRI. The diffusion MRI relies on the fact that water within cells tends to be anisotropic and unlikely to move out of the cell. The diffusion MRI allows the machine to image such things as neurons, because the water in the neuron tends to not cross the myelin membrane covering the neuron. Capitalizing on the predicted action of water molecules within tissue has allowed the development of MRI for use in diagnosis of strokes, Alzheimer's and multiple sclerosis (Wikipedia, 2005).

Other MRI's have been developed that can accurately image arteries, helping with the diagnosis of stenosis, or narrowing of arteries, as well as aneurisms, or balloon-like stretches in artery walls (Wikipedia, 2005).

Another dramatic development has been the ƒMRI, or functional MRI. The functional MRI can make images of the brain as it works by measuring neuron activity (Wikipedia, 2005). It can do this because the parts of the brain that are more active demand more oxygen. The ƒMRI uses magnetic fields to measure increases and decreases of oxygen levels in the hemoglobin of the patient's blood, thus mapping brain activity through time (Wikipedia, 2005).

One of the most recent developments in MRI technology has been that of weight-bearing images. Until this approach was developed, all MRI's were done with the patient lying flat on a table. Now, MRI's can be performed with the patient in a variety of other positions, including standing or bent in various ways (Wendling, 2005). The new positions can reveal problems that were hidden when doing a prone MRI.

MRI's have particular value in helping manage some dynamic illnesses. For instance, the MRI can be adjusted to precisely pinpoint tumors before x-ray therapy is begun, so the x-rays can be aimed as precisely as possible at the cancerous tissue while minimizing both damage and exposure to healthy tissue (Wikipedia, 2005).

SOURCES:

Gould, Todd A., RT-(R)(MR)(ARRT) "How MRI Works," in How Stuff Works. Accessed via the Internet 10/11/05.

Nordenberg, Tamar. 1999. "The picture of health." FDA Consumer 33:1, p. 8.

Tro, Nivaldo J. Introductory Chemistry Essentials, Sec. Ed. Upper Saddle Creek, NJ: Pearson Prentice Hall: 2006.

Wendling, Patrice. 2005. "Stand-up MRI can make pathology stand out: applications range from the musculoskeletal to the cardiovascular, but interpreting images is tough." Internal Medicine News, February.

Wikipedia. 2005. "Magnetic resonance imaging." Wikipedia Online Encyclopedia. Accessed via the Internet 10/11/05.