by David Salazar
Being able to see what is going on within the human body without having to actually cut someone open is one of the greatest challenges of medical diagnosis and now one of its most useful tools. This is where non-invasive medical imaging comes in! Magnetic Resonance Imaging (MRI) is one such extremely powerful medical imaging technique. But how exactly does it work?
MRI is a fairly recent technique first proposed by Peter Mansfield and Paul Lauterbur in 1973 eventually earning them the Nobel Prize in Physiology or Medicine in 2003.
Similarly to X-rays, it also employs electromagnetic radiation to indirectly see within the body. But, this is achieved in a completely different way. In fact, it uses radio waves, which as the name implies are the same type of radiation used by radio stations to broadcast music and the news. Besides transmitting information and entertainment, radio waves can also interact with the positive central core of hydrogen atoms – known as the nucleus or nuclei for plural – which are present in large amounts in the body mostly as part of water molecules (H2O). The nuclei of hydrogen atoms behave like miniature magnets and interacting with radio waves essentially changes the direction in space that their north and south poles point. This interaction which can be used to produce detailed images of the body is known as magnetic resonance and gives MRI its name.
Nevertheless, in order to get useful information out of this the patient needs to be placed under a strong external magnetic field so that the hydrogen nuclei will try to align in the same direction as the field. Aligning the nuclei with a magnetic field means we can know on average in what direction they are pointing. This is done by lying the patient down inside a donut-shaped superconducting magnet. The direction that the nuclei are pointing can then be changed by applying a radio wave pulse. However, doing this means the nuclei will no longer be aligned with the magnetic field of the magnet. This is not an energetically stable state and they will therefore try to re-align with the magnetic field in a process known as relaxation. During relaxation the nuclei will also release radio waves that can be detected.
Most importantly, the speed at which the nuclei relax will depend on the molecular environment that they are in and the radio waves they produce will also depend on this. As a result, body tissue regions with different environments, such as different water concentration, will produce different signals. These can then be analysed by a computer to produce an MRI image with detailed information from within the body.