In a just published paper in PNAS by Le Bihan and colleagues, a technique called diffusion MRI is used to measure the activation of the brain. This is rather unusual. Diffusion MRI is normally used to measure the diffusion, or movement, of water in the brain. Grey matter is relatively disorganized and water is less restricted than in white matter, where myelinated fibres constrain the direction of molecular movements. This is illustrated by a figure that I have made for an upcoming Elsevier textbook (full size image opens in new window).
Using this method it is possible to map out the major fibre tracts in the brain. Even better, it is possible to do tractography: following the white matter fibre bundles from a seed point and calculate the physical connections from this (see figure below)
Le Bihan and colleagues takes diffusion MRI one step further by applying it to study the brain's activation! The background assumption is: as neurons get (relatively more) activated, a lot of physical movement occurs in and around the neurons. The neuron consumes more oxygen and nutrients; internally in the cell, a lot of movement of these substances, and the movement of signal substances occur, inaddition to energy distribution throughout the cell; and the whole cascade of neurotransmitter release and effects is due to movement across the synaptic cleft. So neural firing causes movement both inside and outside the cell.
Now, what did diffusion MRI do again? It measures the movement (!) of water molecules in the brain. As a brain region gets activated, molecules move relatively more than at rest. As a reult, it should be possible to measure brain activity by looking at where in the brain that molecule movement increases. This is exactly what Le Bihan and colleagues have done. And they provide neat results that it actually works! And, as they argue, diffusion fMRI is a more direct measure of neural activation than the more used BOLD fMRI, which is an index of a complex and delayed mechanism of relative blood oxygenation in regions of the brain.
What remains to find out is what the signal really represents. While it is thought that neuroimaning tools such as BOLD fMRI and EEG measures the activation and energy consumption in the dendrites, we know little about the underlying neural mechanism in diffusion fMRI. Could it be mostly due to movement across the synaptic cleft, and hence be a measure of action potentials; could it be due to movement within the cells; or due to transport of molecules (oxygen + nutrients) across the cell membrane; or all at once? Today, this is an open question. But the mere idea of having yet another MRI tool for measuring the brain's activation, and with good spatial resolution plus the promise of better temporal resolution, is really worth noticing.
We'll be tracking the development of this tool closely!