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MRI: The Dance of the Whirling Protons

Magnetic resonance imaging (MRI) (originally termed nuclear magnetic resonance or NMR) is a technology that provides structural information about soft tissues in the body. The MRI machine uses an incredibly powerful (superconducting) magnet to generate images of any part of the body placed inside the bore (center) of the magnet. MRI is especially useful to peer into the brain; as the person lies on a movable table inside the magnet precise models of brain structure down to less than a millimeter can be generated in very little time—it usually takes about 30 minutes. Take a closer look to see how this happens.

An MRI scanner consists of a large cube (with a tunnel in the middle) that contains a strong magnet 3 smaller magnets and an electromagnetic coil that delivers radio frequency pulses. A person lies on a table that slides inside the tunnel for the scanning. From: http://patientinfo.nimh.nih.gov/CC

Protons spin

The actual MR signal is produced by detecting the spin properties of hydrogen nuclei or protons when subjected to a magnetic field and to radio frequency waves (a type of electromagnetic radiation). What is so special about protons from hydrogen? Our bodies contain billions of billions of protons. In fact hydrogen makes up about 63% of the atoms in our bodies—hydrogen is found in water and fat (we are mostly water and fat). The proton has an interesting property—it can spin about an axis much like a spinning top. As each proton spins the movement of charge creates the proton’s own small magnetic field with its own “north and south” poles.

The hydrogen atom contains an electron cloud that surrounds a nucleus of one proton. Protons have an angular momentum (spin) that generates a magnetic field.
The hydrogen atom contains an electron cloud that surrounds a nucleus of one proton. Protons have an angular momentum (spin) that generates a magnetic field.

Spinning protons wobble

There are two basic steps to obtaining the MR signal. The first is the application of a strong magnetic field. The person is placed inside the scanner and is subjected to a very strong magnetic field that is oriented down the center of the magnet. This is not dangerous unless one is wearing jewelry or left keys in his/her pocket! When placed in this magnetic field the spinning protons in the tissue circle around the axis of the magnetic field (this is called precession) much like a spinning top wobbles in a circular pattern around a vertical axis as it falls down. The spinning protons precess around the applied magnetic field with a certain frequency that is dependent on the strength of the magnetic field coming from the magnet.

A spinning proton wobbles (precesses) about an externally applied magnetic field.
A spinning proton wobbles (precesses) about an externally applied magnetic field.

Not only do the protons precess, but they also align themselves with their magnetic poles in the direction of or against the magnetic field inside the scanner (i.e., toward the head or the feet). The protons that line up in the same direction as the MR’s magnetic field are in a low energy state; those in the opposite direction are in a high energy state. The opposing orientations cancel out each other’s magnetic fields. However, it is a lot easier for protons to line up in the low energy state (i.e., in the direction of the applied magnetic field) so there are a few extra protons without an opposing partner lined up in the opposite direction. These unbalanced protons are important for generating the MRI signal—they are the ones that get a “push” from some radio frequency waves to help create an image.

Resonance results

Now the second step. If you go into an MR scanner you will hear a series of annoying noises that click clang and buzz (listen to a few of them); the technician will give you earplugs to muffle the sound. The noises are produced by 3 smaller magnets that help “fine tune”or focus the magnetic field on specific body areas. At the same time the MR machine delivers radio frequency pulses to the body part to be imaged. The radio frequency matches the frequency generated by the precessing “unbalanced” protons. These protons absorb the energy (actually photons) from the radio frequency waves that is needed to make them spin in a different direction—now they move from their low energy state into a high energy state.

The synchronized frequency that causes the absorption of energy is called resonance the R in the MRI. When the radio frequency pulse is turned off the protons relax back to their low energy state or their normal alignment in the magnetic field. As they relax the protons release their excess stored energy in the form of light (photons) which is detected by the MR machine and converted by a computer into an image.

You can see a teen getting an MRI–go to the PBS Frontline report on the Teenage Brain (Chapter 2).