Tuesday, January 20, 2009

A, Sort of, Basic Explanation of fMRI

The question of why white matter is displayed in brain images when it is only neurons, gray matter, that are active came up in a discussion today with one of my colleagues. Here is the answer:

The Very Basic Answer:

Measurements are made of protons in the water molecules in the body (or brain or region of interest). One frequency, or specific magnetic field, is induced within the scanner to align the direction of all the protons (with the direction of the magnetic field inside the scanner). A second, different, frequency is then transmitted which alters this orientation and the time it takes to return to the original position is measured.

The More Complicated Answer:

Protons, as well as some other nuclei, have a spin of ½ and this spin has two different orientations – up and down (so the two different spin values can be thought of as +1/2 and -1/2). The organisation of spins in a molecule is such that the resulting energy of the molecule will be the lowest possible. At lower energy levels (you can think of the energy levels as in the Bohr model of an atom. Though it is technically wrong, it is good for illustrative purposes.), this is achieved by orienting each additional spin ½ nuclei in the opposite direction of the previous nuclei so that the total spin, say in H2 which has two protons, is S= ½ + (-1/2) = 0. Because Hydrogen is only one proton, it always has a spin value of ½. It is this non-zero spin value (magnetic moment) that allows the magnetic field to change the orientation of the proton (hydrogen).

Once the protons are aligned with the magnetic field within the MRI machine, the second frequency is transmitted causing the protons to alter their orientation. But once this frequency is turned off, the protons will re-align with the magnetic field. The time it takes for this re-alignment to occur is called the relaxation time and varies in different tissues. (Other magnetic fields are also transmitted once the second frequency is turned off to determine the location of the protons). The energy released during the realignment is what is measured by the MRI scanner and the differences in energy/relaxation time create the contrasts of white matter, gray matter, and cerebrospinal fluid in the black and white image produced.

The process of neuroimaging for research purposes is called functional MRI (fMRI) and measures differences in neuron activity. This is what produces the bright red and blue spots (active areas) you see in pictures. When a neural network is activated, the brain demands a higher oxygen: blood ratio in that area and this ratio can be measured over time (each scan in fMRI is about 2-3 seconds), so the BOLD (Blood Oxygen Level Dependent) signal can be measured over, typically, 20-30 seconds displaying the level of neural activity in different areas of the brain. Exactly how this relationship between blood-oxygen and neural activity works is still being investigated.

So, the reason you see white matter activation in fMRI images even though it is only the gray matter neurons that are firing, is because it is the blood-oxygen level in the vascular system supplying these areas that is being measured, not the neurons themselves.

And new research indicates that white matter, typically ignored in relation to gray matter, may actually be important in cognition and memory.

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