How Magnetic Resonance Imaging (MRI) Works
Physics, NMR, and Diagnostic Medical Images of the Human Body
Oct 11, 2009
One of the many modern diagnostic tools available to medical doctors is magnetic resonance imaging (MRI). MRI scans provide physicians a view of the interior of the patient's body without harming or invading the patient's body in any way. Magnetic resonance imaging is based on the fundamental physics of nuclear magnetic resonance (NMR). How does MRI work?
Nuclear Magnetic Resonance
Nuclear magnetic resonance is an effect that occurs when the nucleus of an atom is placed in a magnetic field. The spinning nucleus in a constant magnetic field wobbles just like a spinning top.
If in addition to the constant magnetic field, there is another magnetic field that varies at the same frequency as the nucleus wobbles, the nucleus will flip back and forth so that the nucleus effectively alternates the direction in which it spins. As the nucleus flips its spin direction, it either absorbs or emits low energy radio waves. Studying these radio waves allows physicists to deduce various properties of the atomic nuclei undergoing NMR.
Safety of NMR and MRI
Nuclear magnetic resonance uses the word nuclear because it involves the nucleus of the atom. It does not however in any way involve any dangerous radiation as people expect from nuclear weapons or other nuclear reactions. The only radiation patients are exposed to by nuclear magnetic resonance imaging is very low energy radio waves. Nuclear magnetic resonance imaging is therefore very safe. The word nuclear was dropped however to allay patient fears. To the average person magnetic resonance imaging sounds less dangerous than nuclear magnetic resonance, and MRI is very safe.
Magnetic Resonance Imaging
Magnetic resonance imaging is a very useful application of NMR. Medical MRI machines are designed to image the nuclei of the hydrogen atoms in the human body. Human bodies contain a high percentage of water, so there are a large number of hydrogen atoms in all human tissue. X-rays image bones well, but image other tissues very poorly. Magnetic resonance imaging therefore provides medical personnel with much better images of the soft tissue in the patient's body than X-rays can provide.
A patient getting an MRI is placed inside a hollow cylinder, which is basically a solenoid. A solenoid is a cylindrical coil of wire, carrying an electric current, that produces a strong magnetic field inside the solenoid. The magnetic field inside the MRI machine is not perfectly uniform, by design.
In NMR the exact frequency of the radio waves a hydrogen nucleus absorbs or emits depends on the magnetic field strength. Hence the hydrogen nuclei in different portions of the patient's body, at different magnetic field strengths, emit different frequency radio waves. This way the nonuniform magnetic field allows the MRI machine to image slices of different portions of the patient's body.
The MRI machine transmits radio waves into the patient's body. The hydrogen nuclei absorb these radio waves and flip the direction of their spin. The nuclei then flip their spin back to the original direction and emit radio waves. The MRI machine receives these radio waves. Analyzing the origin of these radio waves from within the patient's body allows a computer program to construct an image of the soft tissues within the patient's body.
The resulting MRI image of the patient's body gives physicians a look inside the human body that helps them diagnose medical problems.
MRI is a very safe diagnostic technique because it is noninvasive and patients are exposed only to low energy radio waves rather than high energy X-rays.
Further Reading
Serway, R.A., Moses, C.J., and Moyer, C.A., Modern Physics 3rd ed., Thomson, 2005.
Bloomfield, L.A., How Things Work 3rd ed., Wiley, 2006.
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