Magnetic Resonance Imaging

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 must take some kind of sedation in order to cope with the MRI environment, which in addition to being a very confining space, is quite noisy. The noise is also unsettling to some patients. A more serious risk is that the patient's body may contain some undisclosed metal object that responds to magnetism (Gould, DATE). The magnetic field of an operating MRI machine is quite intense and can force heavy metal objects out of a person's hands if standing near the machine.

For instance, a metal fragment in the eye from an old injury can be moved by the magnetic field and cause serious damage to the person's eye. People with pacemakers must stay far away from MRI equipment, because the equipment can completely disrupt the pacemaker or even cause the person to die (Gould, DATE). Because of this risk, patients must be screened very carefully regarding metal that might be present in their bodies for some reason.

However, there are no known hazards to people from the process of having an MRI performed. In spite of that, erring on the side of caution, MRI's are rarely performed on pregnant women (Gould, DATE) although some recent developments in the use of MRI's may provide important new ways to image high-risk fetuses (Wikipedia, 2005). 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.