What does MRI Measure?

The cylindrical tube of an MRI scanner houses a very powerful electro-magnet. A typical research scanner (such as the FMRIB Centre scanner) has a field strength of 3 teslas (T), about 50,000 times greater than the Earth’s field. The magnetic field inside the scanner affects the magnetic nuclei of atoms. Normally atomic nuclei are randomly oriented but under the influence of a magnetic field the nuclei become aligned with the direction of the field. The stronger the field the greater the degree of alignment. When pointing in the same direction, the tiny magnetic signals from individual nuclei add up coherently resulting in a signal that is large enough to measure. In FMRI it is the magnetic signal from hydrogen nuclei in water (H₂O) that is detected.

The key to MRI is that the signal from hydrogen nuclei varies in strength depending on the surroundings. This provides a means of discriminating between grey matter, white matter and cerebral spinal fluid in structural images of the brain.