What is an MRI

What is an MRI?

Many physicians today rely on advanced imaging techniques to assist in the diagnosis of athletic injuries. X-rays are the most commonly used diagnostic imaging technique in use today. The x-ray has one major limitation, it only visualizes bony problems. Soft tissue injuries, such as ligament or muscle tears, can not be seen with a plain film x-ray. To visualize these internal structures a physician may order a diagnostic magnetic resonance image or MRI.

The MRI allows a physician to see an image of internal structures. Prior to 1977, when the first human MRI was performed, surgery was the only way to visualize internal, soft tissue structures. The first MRI took 5 hours to produce one very fuzzy image. Since then improvements in technology have resulted in scans that take 60 to 90 minutes, and result in high quality images.

MRI’s are not totally safe for all individuals. The “magnetic” portion of the name is what allows the image to be produced. The MRI relies on a strong magnetic field to produce the image. This magnetic field is also the source of the danger. Any ferromagnetic object (a metal that contains iron) can be attracted to the magnet. Items that can be problematic or dangerous include jewelry, pacemakers, dental implants, paperclips, pens, keys, or other small objects. These objects can be pulled out of pockets, or any other hiding place, and travel at high rates of speed into the middle of the MRI unit. This poses a great risk for the patient being scanned. Due to this, the MRI technicians will be very diligent about determining if a patient has any metal in or on their bodies. Some people may be denied an MRI due to a possible threat from the high magnetic field.

The magnetic field used in an MRI is expressed in two relative values; Tesla and gauss. One Tesla is equivalent to 20,000 gauss. The magnetic field produced by the earth is 0.5 gauss. The most common MRI magnets used today are 0.5 Tesla to 2.0 Tesla (magnets greater than 2 Tesla have not been approved for human use but are used in research). This means that the magnetic field used by a typical MRI is approximately 40,000 times stronger than the gravitational field of the earth.

This large magnetic field is why MRI technicians carefully control who and what enter the room containing the scanner. The picture shows a pallet jacket that was “sucked” into the scanner by its magnetic field.

The magnets used in current scanners are called superconducting magnets. This is a typical resistive magnet, composed coils or windings of wire through which a current of electricity is passed. This type of magnet is typically made in high school physics classes. The MRI magnet has one significant twist; the coiled wire is bathed in a solution of liquid helium. Helium becomes a liquid at 452.4° below zero. This extremely low temperature lowers the electrical resistance of the wire to near zero, allowing relatively small amounts of power to create the large magnetic field.

Two types of magnets make up the scanner. The superconducting magnet produces a very large magnetic field of uniform strength and stability. Smaller gradient magnets are used to create small variations in the magnetic field. It is the interaction of these two magnetic fields that allows an image to be produced. A complex and powerful computer system interprets the interaction of these magnetic fields to produce the viewable image that is the inside of your body.

Physics Meets Kodak

The body is made up of billions of atoms. The nucleus of all atoms have a very specific spin. Think of 5 different shapes of tops, each have a unique variation to their spin. When a magnetic field is placed near an atom the spin can be controlled. Since hydrogen is the most abundant atom in the body, its spin characteristics within a magnetic field are measured to determine the image presented by the MRI.

Inside the MRI the magnetic field runs straight down. This causes the hydrogen atoms of the body to align themselves either pointing toward the head or feet. One “head pointer” cancels out one “foot pointer.” The number of “head pointers” and “foot pointers” are not equal; in other words, in a given space there may be more “heads” than “feet.” These odd balls are what we use to develop the image.

A radio frequency pulse is then added to the area scanned via a coil placed around the body part being scanned. This pulse is specific only to hydrogen atoms and causes the unpaired atoms to spin in a different direction than the other paired head-tail facing atoms. This produces the resonance portion of the MRI name. The spin of the unpaired atoms occurs at a specific frequency and in a specific direction.

At the same time the gradient magnets, mentioned earlier, become active. These smaller magnets produce a small change in a very specific area of the large magnetic field. These magnets, when rapidly turned on and off, create the ability to image the body in slices, much like slicing a loaf of bread. The best part is these slices can be made vertically, horizontally, or diagonally to image different areas of the same structure without having the patient turn.

The radio frequency pulse is then stopped, this allows the hydrogen atoms that were forced to spin a specific way to stop their spin. As the spin returns to “normal” the atoms release energy. It is this energy that is picked up by the coil and sent to the computer system. The computer then interprets as the MR image. This image is of a torn PCL in a knee.

Materials can be injected into joints that cause the magnetic field to react differently. These contrast media allow for some hard to see objects to be visualized. It can also help to reduce the distortion caused by implanted metal objects. (Implanted metal objects affect the magnetic field resulting in a blurred image). Contrast injections can be placed intravenously to help visualize blood flow and other internal structures.

MRI’s have proven to be amazing diagnostic tools, improving the physician’s ability to determine the extent of an injury. As technology improves the quality of the image will continue to improve.

Information on this site is not a substitute for physician directed care.

Please consult your personal physician for more detailed information

concerning specific injuries or illnesses.

Last Update for AthleticAdvisor.com: 10/24/2009 12:09:35 AM