fMRI, the basics

fMRI or functional MRI is technique that can map brain areas activated by a task or sensory input. fMRI can be used for clinical or research purposes and like most of the devices I am mentioning, is non invasive, though uncomfortable. fMRI like most neuroimaging uses an understanding of principles of electromagnetism to infer the brain's activity. Taking advantage of the known orientation of atoms and a constant magnetic field in the MRI scanner allows scientists to see changes in blood flow associated with brain activity, then infer the activations that must be the source. The intensity of an MRI signal is determined by the level of magnetic resonance, specifically what is called the BOLD effect (blood oxygenation level dependent). All materials have some effect on magnetic fields depending on it's susceptibility or "magnetic moment". Blood that is oxygen rich differs in how it effects a magnetic field from deoxygenated blood. Oxygenated blood is diamagnetic, it tends to take a position at right angles to lines of magnetic force. Deoxygenated hemoglobin (blood) takes a position parallel and proportional to the the intensity of whaever local magnetic field is present. The MRI, with its created and constant field is able to set up an environment where a difference in as little of 3% change in oxygen levels of the blood is detected and imaged.
Typically a high resolution MRI is done to be used as a backdrop before redoing the MRI with BOLD scans at lower resolutions and the participant engaging in an activity. A person in the MRI can wear special goggles or glasses to show the images of their brain. Images are made slice by slice, combined in the computer to form 3-D pictures of the brain. Final images will show the areas of activity during the experiment.