Magnetic Resonance Imaging (MRI)

The following was adapted from: The Basics of MRI by Dr. Joseph P. Hornak. The full text, which is a very nice tutorial on MRI, can be found at:
http://www.cis.rit.edu/htbooks/mri/

    Magnetic resonance imaging (MRI) is a procedure used to produce high resolution images of the inside of the body, including the brain. MRI is a type of nuclear magnetic resonance (NMR), which is used by chemists to study the properties of molecules. The technique was called magnetic resonance imaging rather than nuclear magnetic resonance imaging because of the public's negative associations with the word nuclear in the late 1970's (remember 3-Mile Island?).

    MRI, like other imaging techniques such as CAT and PET, is primarily a tomographic imaging technique ("tomo" means slice). This means that it produces an image in a thin slice through the body.  This form of imaging is, in some respects, very much like a real anatomical slice through the body. In fact, to the untrained eye, an MRI image may be indistinguishable from a slice taken from a post-mortem body.  However, unlike a post-mortem slice, the MRI is formed by an MRI camera or scanner and the slices are composed of groups of picture elements or pixels displayed on a computer monitor.

    How does it work? The human body is mostly made up of  fat and water. These substances have many hydrogen atoms (the body is approximately 63% hydrogen!). Hydrogen nuclei have an NMR signal due to the spin of particles in their nuclei (spin is a property of nature like electrical charge). Particles with opposite sign spin can pair up to eliminate the observable manifestations of spin (e.g.,. helium), so, in MRI it is the unpaired nuclear spins of hydrogen protons that are of importance. When protons are placed in a magnetic field they behave like a magnet (i.e., they have a north and south pole) and become aligned with the external field. There is a low energy state where the poles of the proton are aligned in one direction and a high energy state where they are aligned in the opposite direction. Protons can move from the low to the high state by absorbing a photon. However, in order for this to happen the energy of the photon must match the energy difference between the two states. The frequency of the photon in MRI is in the radio frequency (RF) range -- typically between 15 and 80 MHz for hydrogen.

    In MRI the subject or patient is placed inside the scanner which is made up of a moveable bed-like structure and a large hollow tube. The tube contains a thick coil of wire that generates a very intense magnetic field (between 1.5 and 4 Tesla) which is strong enough to accelerate a paper clip to near lethal velocities across the distance of a small room! In order to generate a magnetic field of this strength the coil is cooled to near absolute zero with liquid helium. This very strong magnetic field is then used to align the hydrogen nuclei of the tissue to be imaged. Also placed near the tissue to be imaged is another coil of wire called an RF coil (RF because it generates/reads radio frequency fields). The RF coil is used to both change the energy state of the hydrogen nuclei and to record the RF output of these perturbations. The latter are the raw data of an MRI which are recorded by the computer, transformed and then displayed. Here is a schematic of an MRI machine.