Relaxation Dependent Magnetic Resonance Imaging (e.g. BOLD)

Three factors determine the intensity of a spectroscopic signal (fractional decrease, or increase, in the amount of light at a given frequency):

1) The amount of material interacting with light of that frequency (e.g. the number of protons in a given local magnetic field).

2) The strength of the interaction (how strongly the electromagnetic radiation "couples" to the motion of electrons, atoms, or molecules at that frequency - how good a handle the matter presents for the light). For proton magnetic resonance all protons present the same kind of handle (although other nuclei have different magnetic moments).

3) The difference in population between low- and high-energy state species.

Consider the third factor in more detail.

Interaction with light tends to equalize (not equilibrate) the populations of states that differ by energy corresponding to the light frequency. Thus if there are initially more protons in the low-energy state than in the high-energy state, light energy will be mostly absorbed. If for some reason there are initially more protons in the high-energy state (an unusual situation), light energy will be mostly emitted, as in a laser.

For protons at Boltzmann equilibrium in typical magnetic fields the energy difference is so small that the excess population of the low-energy state is very modest, less that 0.01%, and the signal is correspondingly weak.

As light is absorbed (or emitted), the populations equalize, so no further net absorbtion (or emission) is possible. The signal is then said to be "saturated", unless high-energy protons can be converted to the low-energy state WITHOUT emitting light. This non-radiative leakage is called "relaxation".

Under steady illumination net energy absorption from light cannot occur faster than relaxation.
One type of relaxation involves interaction of the nuclear magnets with the electronic magnetism of oxygen molecules in the environment. This interaction converts the "extra" magnetic energy of the protons into atomic or molecular motion as heat. Thus NMR signals of the protons in water can be strengthened by the presence of molecular oxygen, which acts as a relaxation agent.

Oxygen relaxation is the basis of BOLD (Blood Oxygen Level Dependent) Imaging. When a portion of the brain becomes "activated" and fresh blood replaces oxygen-poor blood, the proton signal of water becomes stronger because of increased relaxation by the oxygen. Such an increase in signal strength can be detected very rapidly (a few seconds) and very locally (a few millimeters) so as to show which areas of the brain are being activated in response to various stimuli.

The BOLD image on the right is a difference map showing brain response to visual stimuli. It presents the intensity of absorption by the protons of water in different regions of the brain while the subject is being shown images of faces minus the intensity of absorption while the subject is being shown images of houses. The green (positive) area is where the blood becomes more oxygenated in response to images of faces than to images of houses. The magenta (negative) area next to it is where the response to houses is greater than the response to faces.

If you're curious, you can see a better rendition of this image and other examples in a report from the Kirby Research Center in Baltimore (click here for PDF file)

by permission of S. Yantis, Kirby Research Center

Return to Chem 125 homepage

text copyright 2003 J. M. McBride