What is MRI?
Magnetic Resonance imaging (or MRI as it is known) is one of the most amazing developments in medicine of the 20th century. It relies on the fact that we have lots and lots of hydrogen atoms in our bodies and that the magnetic behaviour of the nuclei of those atoms depends ever so slightly on the environment of the atom – in other words whether it is a hydrogen atom in fat, or in water, or in brain tissue, and so on.
To do MRI it is essential to place the patient in a strong magnetic field. This is best created in the type of long tube that you saw Jem and Dallas going into in episode 3 of Bang Goes The Theory.
Patient going into an MRI scanner.
Once the subject is in the bore of the magnet, additional complicated sequences of smaller but rapidly varying magnetic fields are created by currents in coils of wire around the bore of the machine.
As the currents in the coils change the coils move, and this creates the noise that one can hear inside the scanner. These varying magnetic fields are used to create an image of the part of the body under investigation.
MRI is really effective for imaging soft tissue and has a wide variety of uses. One of the best uses is for diagnosing joint problems - as in this image of a knee.
MRI scans are excellent for showing up soft tissue such as ligaments and tendons in joints. This is an MRI scan of a knee. [Image courtesy of GE Healthcare]
What is the difference between MRI and fMRI?
Most of the images created in hospitals using MRI show structural features of the body, but it is also possible to show some information about the oxygen consumption of tissues as well – this is known as functional MRI, or fMRI for short. When the brain is working it needs a good supply of oxygen. The oxygen is carried in the blood in the form of a substance called oxyhaemaglobin. When the oxygen has been used up the remaining substance is called deoxyhaemaglobin.
Rather fortunately for brain researchers, oxyhaemaglobin and dexyhaemaglobin have different magnetic properties, so it is possible to see which parts of the brain are using more oxygen – or working harder.
And here’s a strange fact: one might think that there would then be more deoxyhaemoglobin in the regions of the brain that are working hardest but in fact the opposite is true!
The active regions of the brain need more oxygen, so the blood supply is increased and is increased by so much that there is extra oxyhaemaglobin in the active parts of the brain.
The sequences used in fMRI will pick up this extra blood supply and therefore give us a picture of the active regions of the brain. This technique is called Blood Oxygen Level Dependence, or BOLD, and is widely used by researchers such as those at the MRC, who are looking at the ways our brains carry out certain functions. Hence the lovely images of Jem’s brain solving problems better than Dallas’! .
An fMRI scan showing with areas of increased activity highlighted.