Diffusion MRI uses an MRI sequence that allows the doctor or scientist to follow the movement of water molecules. A simple way to look at it is by imagining that the diffusion sequence 'tags' a given water molecule, then acquires a first image, waits a moment and then acquires a second image. The movement the water molecule underwent in that time frame can then be estimated.
There are many aspects of the human body that can be investigated using diffusion MRI. However, one application in particular benefits from the fact that sometimes the molecule's movement is hindered by "walls". One example can be found in the brain, where we can investigate the movement of water within axons (see The developing brain - White matter
The movement of water can be described in general by Brownian motion, or simply speaking a random walk. This happens due to collisions with other water molecules over time. Once a "wall", such as the axonal membranes, is introduced, the motion cannot take place freely, leading to a preferential diffusion direction.
Mathematically, this preferential diffusion direction can be expressed through measurements of anisotropy. The amount of anisotropy (directionality) has been used to study patients and healthy subjects, where low levels of anisotropy can identify white matter damage. This is of particular interest, as it has been shown that the level of anisotropy is related to how well a task is performed.
The principle of diffusion MRI was introduced by Stejskal and Tanner in 1965. After the spins are tipped away from their preferred direction (see MRI overview
) a bipolar gradient is applied. That means that the same gradient with opposite signs is used. In simple terms, this means that something is done (a change in phase) to the spins in a first step and then "undone" in the second. If the water molecule stays on average at the same position, there will be no effect and therefore no detectable signal. This is the case, for example, for unrestricted Brownian motion.
If the water molecule changes its position between the two steps, however, we will be able to see a net effect. This is due to the fact that the Larmor frequency (see MR Physics
) is dependent on the amplitude of the magnetic field, which changes depending on the location due to the gradient. This net effect can then be related to the movement of the water molecules and the structure can be mapped.