How does MRI work?

MRI stands for magnetic resonance imaging and uses the application of a magnetic field and radio frequency photons to manipulate the spin state of protons (hydrogen nuclei). Hydrogen is one of the most abundant atoms in the body as it is a key component of many organic molecules and some inorganic ions.

Protons possess a property called spin, which can be in a particular direction. Inside an MRI machine a very strong magnetic field is applied to the body, and this causes all the protons to align their spin along the axis of the magnetic field. Applying radio frequencies at the same time as the magnetic field causes the protons to change spin and become orientated to a different position, for example a switch from spin up to spin down. When the radio frequency is switched off there is nothing forcing the protons out of alignment with the magnetic field, and as a result the protons will return to their original orientation along the axis of the magnetic field. When the protons return to this spin state they lose energy and this produces photons. Sensors that surround the part of the body being imaged detect these photons, and this detection can be used to generate an image.

It is important to note that photons within different environments behave differently, and this difference is what is used to generate the image.

     H                                                                    H

      |                             O                                      |

H -C – R                    /    \                       H – O – C – R

      |                         H     H                                   |

     H                                                                     H

The bonding partners of hydrogen atoms and the molecules in which they exist cause a difference in the time taken to realign to the axis of the magnetic field, and this produces specific delays for each tissue. The information based upon delays can be used to produce a weighted image in which tissues such as fat, muscle, and bone, and substances such as air and water, can be differentiated between. By using knowledge of anatomy the body structures can be identified and examined.