MRI Magic
Not an X-Ray
Medical x-rays provide images of the body but utilize radiation that in large doses can damage cells. A completely different technology, magnetic resonance imaging (MRI), emerged in the late 1970s. It produces highly detailed images of soft tissues and requires no x-radiation. MRI is based on nuclear magnetic resonance, a technique developed to probe molecular structure. Ironically, public anxiety about radioactivity and nuclear energy kept the word “nuclear” out of the name of this new kind of imaging, which otherwise might have been called NMRI (nuclear magnetic resonance imaging).
Precessing Protons
Proton in a magnetic field: The arrow from the proton shows the proton’s spin axis; the tip of this arrow moves in a circle perpendicular to the magnetic field.
To understand the MRI, let’s think about a molecule that contains hydrogen, like a molecule of water or fat. The nuclei of these hydrogen atoms are protons. Each proton possesses an intrinsic spin, so that it acts much like a spinning top. Now suppose we put the molecule between the pole-pieces of a magnet, so the protons are in a magnetic field. Each proton now turns around the field direction like a little gyroscope. (See diagram.) This motion is called precession.
As they precess, the protons, according to quantum mechanics, can have only two orientations. Because of the magnetic field, these two orientations correspond to slightly different energies. If we shine radio waves on these protons, and the radio waves have just the right frequency, the lower energy protons can absorb photons of radiation and flip over. (Quantum mechanics tells us that the photon’s energy is proportional to its frequency.) By observing the details of this energy absorption, which depend on the environment of the protons, we can figure out, for example, if the protons are in water or in fat.
If we analyze many such measurements, we can get an image of a “slice” of the body, as shown. Notice the detail can be seen in the soft tissues. MRI images can show features as small as 1 mm.
Left: MRI image showing a vertical section of the brain (© 2000 Joseph Hornak, The Basics of MRI)
Right: MRI image showing a horizontal section of the brain (© 2000 Joseph Hornak, The Basics of MRI)
Resonance
Magnetic force on belt buckle from MRI magnetic field levitates belt. (© 2000 Joseph Hornak, The Basics of MRI)
Resonance is a general phenomenon that is quite important. A periodic system, like a playground swing, repeats its motion or pattern regularly as time passes. The frequency of the system is the number of repeats per second. If someone pushes a swinging child in synch with the swing’s motion, pushing when the swing is already moving away, the push adds energy to the system and the child swings higher. If the frequency of the push equals the frequency of the swing, the maximum amount of energy is added to the system per push. The swinging and the pushing are in resonance with each other. Similarly, in MRI, if the photon has just the right frequency, its energy equals the difference between the energies of the protons’ two orientations, and the collection of photons will absorb the maximum amount of energy.
Your MRI
If you have an MRI, you will recline with part or all of your body inside a large housing, about two feet in diameter. This housing contains a superconducting magnet, which will create a large magnetic field (See Physics in Action Archives, Superconductors, Superconducting Magnet Applications, MRI Scanners). You will wear a special gown to ensure that no magnetic materials are brought into the magnet. The photo of the levitating belt-buckle shows the strength of this magnet.
The exam lasts between a half an hour and an hour. You will have to maintain the same position for minutes at a time. A radiologist, a medical doctor trained to read images, will interpret the results.
