MRI is short for magnetic resonance imaging. It is a very versatile medical imaging technique that is very popular in neuroscience. It delivers images of exquisite detail, which can be used to study brain anatomy. Furthermore, advanced MRI sequences exist such as functional MRI (fMRI), which can be used to study brain activity, and diffusion weighted MRI, which can be used to visualize nerve fibers within the brain. Both techniques can be used to study brain connectivity and brain networks.
In task fMRI, for example, a subject performs a certain task in the scanner while his/her brain is continuously being scanned (e.g. every 2s). The scanner parameters are tuned to the BOLD contrast (Blood Oxygenation Level Dependent), which means that the MR signal goes up and down in accordance with brain-activity-related changes in blood perfusion. By assessing in which voxels the BOLD signal follows the task design, so-called activation maps can be generated, see the figure above.
If you want to know more about how diffusion-weighted MRI can be used to reconstruct nerve fiber pathways in the brain, check out the section on brain networks.
MRI is actually based on a magnetic property of hydrogen nuclei called nuclear spin. Since soft tissue consists largely of water (around 70%), we typically tune into the spin frequency of hydrogen nuclei (H) in water (H2O). However, the spin frequency of hydrogen nuclei in other types of molecules is slightly different. This effect is exploited in MR spectroscopy, in which we assess the exact frequency content of the MR signal to find the presence and relative occurance of different neurotransmitters, i.e. brain chemicals that mediate the communication between brain cells.