How Things Work: Magnetic Resonance Imaging
Advances in medical technology have changed not only the way that doctors treat patients, but also how doctors discover what, exactly, needs to be treated.
One of the most accurate forms of imaging available to doctors today is the MRI. An MRI, or magnetic resonance image, is a picture of the insides of a living organism.
Radio waves produce an MRI image, as opposed to the X-rays that produce images in CT scans.
When the process was first used in 1977, it could take up to five hours to obtain an image. However, due to technological advances, what was once a long process now takes only seconds.
The most recognizeable characteristic of an MRI machine is its magnet, which is cylindrical in shape with a hollow center. The magnets used in most MRI machines are called superconducting magnets, and are usually composed of many coils of wire.
When the MRI is turned on, an electric current passes through the coils, and a magnetic field is created.
The magnet’s shape serves a distinct purpose: The magnet surrounds the patient, and the MRI machine can take a picture of any part of a patient’s body without the patient having to move to different positions. This is especially important for patients suffering from injuries that make it difficult to move. Because of the MRI machine’s reliance upon magnets, metal objects in the vicinity of the machine can be very dangerous.
The magnet within an MRI machine creates a magnetic field whose power increases exponentially as an object comes into closer contact with the machine.
A physician should be careful about any metal object he or she is carrying at the time of the scan.
Paper clips, pens, and stethoscopes, among other objects, can be pulled out of pockets, without warning, and fly towards the opening of the magnet, where the patient is being scanned.
Patients with pacemakers, especially, are not allowed near the machine because the magnetic force can cause the pacemaker to malfunction.
Patients with artificial joints and metallic bone plates also should not have MRIs taken. The metal within their bodies can actually distort the image.
When the magnet is turned on, protons in hydrogen atoms present in the body act like tiny magnets with north and south poles.
The body’s atoms align in parallel columns that indicate the direction of the magnetic field.
Following their alignment, the MRI machine then emits radio waves.
The “magnets” present in the patient’s tissue atoms then absorb energy and begin to spin. As the atoms spin, they produce a signal.
A receiver connected to the MRI machine can analyze the signals produced from the atoms to produce an accurate image. The MRI can collect information about the atoms because their signal emissions are characteristic to each atom.
Most of the atoms in the body are hydrogen atoms, but MRIs can also detect areas with increased oxygen, especially in areas of increased nerve activity, as is the brain.
The MRI is useful in detecting blood clots, tumors, and other abnormalities. It is useful on the soft tissues of the abdomen. MRI machines are also able to gather information on individual blood cells.
Because the images produced are very detailed, the MRI is the scan of choice for diagnosing patients. This is especially the case for patients with possible head trauma, because an MRI can easily detect swelling or bleeding.
The process of having an MRI taken has also changed since 1977. Before, a patient would have to sit for up to five hours while the image was captured. This was especially problematic for patients suffering head trauma.
Today, any patient discomfort is minimized because the process takes a matter of seconds.
The scan can begin when the desired part of the body is in the exact center or isocenter of the magnet. The MRI essentially analyzes the type of tissue it is. The scan displays the image in varying shades of gray.
Different tissues are given a different shade of gray. For example, an MRI of the human brain displays the skull in a lighter shade than the actual brain.
Radiologists use the information obtained from an MRI to assess normal tissue as opposed to abnormal tissue.