Last update: September 17, 2022
By BrainMatters

MRI (magnetic resonance imaging) and fMRI are two methods based on the same principle. MRI is a method to map the structure and anatomy of the brain, and fMRI is a method to measure brain activity.

More than half of the human body consists of water. Water contains protons, small particles with a positive charge. Like all charged particles, protons have magnetic properties. So when the body is placed in a large magnetic field (as in the MRI scanner) the protons all lie in the same direction, namely the direction of the magnetic field. The protons do not lie still in this magnetic field, but they spin around their axis (this is called precession).

An electromagnetic pulse takes the protons out of the magnetic field and makes them perpendicular to it for a moment. However, as soon as this pulse disappears, the protons return to their alignment in the magnetic field. This movement of the protons releases an electromagnetic echo, which is received by an antenna around the head. The speed at which this happens depends on the type of tissue. By mapping this speed at which the protons return to their original state, an image of the head and brain can be obtained.

The nice thing about using MRI is that not all parts of the brain are affected by the electromagnetic pulses at the same time. Each 'slice' of the brain is affected by pulses of a different frequency. For example, the protons in the upper part of the brain are moved (these are not the real frequencies) by a signal with a frequency of 70 Hz, and in the lower part by a signal with a frequency of 60 Hz.

So far, the anatomical function of MRI. However, it is also possible to map the function of the brain using MRI. This is called fMRI, where f stands for functional MRI.

Author: Myrthe Princen (translated by Pauline van Gils)

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