Functional magnetic resonance imaging (fMRI) is a non invasive technique for measuring brain activity with an excellent spatial resolution (few millimeters for an imager 3 T). It detects the changes in cerebral blood flow and the oxygenation of brain cells that is correlated to neural activity. fMRI is typically used to produce activation maps showing which parts of the brain are involved in a particular cognitive process.
What does the fMRI measure ?
Oxygen is delivered to neurons by hemoglobin in capillary red blood cells. When neuronal activity increases there is an increased demand for oxygen and the local response is an increase in blood flow to regions of increased neural activity.
Hemoglobin is diamagnetic when oxygenated but paramagnetic when deoxygenated. This difference in magnetic properties leads to small differences in the MR signal of blood depending on the degree of oxygenation. Since blood oxygenation varies according to the levels of neural activity these differences can be used to detect brain activity.
This form of MRI is known as blood oxygenation level dependent (BOLD) imaging.
What does the fMRI look like ?
The final results are images of the brain containing colored patches which correlate to the areas activated during the task performed by the subjects (left image below). These images, called activation maps, are in 3 dimensions (x for the left/right , y for the anterior/posterior, and Z for the superior/inferior dimension) or in some cases in 4 dimensions if the time is equally considered.
Researchers look at activity on a scan in voxels — or volume pixels, the smallest distinguishable box-shaped part of a three-dimensional image. The activity in a voxel is defined as how closely the time-course of the signal from that voxel matches the expected time-course. Voxels whose signal corresponds tightly are given a high activation score, voxels showing no correlation have a low score and voxels showing the opposite (deactivation) are given a negative score.
All fMRI projects belonging to Brain and Language Institute (BLRI) are conducted in the fMRI Center of Marseille (Timone Hospital, France, see image on the top of the page, Copyright©2016. Jean-Luc Anton).