All you need to know about MRIs
MRI stands for Magnetic Resonance Imaging. An MRI machine is used to create detailed images of the inside of the human body from almost any angle. Using a combination of powerful magnetic fields and radio waves, an MRI scan can help doctors see and examine a body’s organs, bones and soft tissues. The procedure is painless and harmless, and can be used to detect the exact location and nature of everything from brain tumours to torn ligaments.
How MRI scanners work
An MRI machine is a long horizontal metal tube. A narrow table slides in and out of the tube and this is what the patient lies on to have a scan.
The metal tube contains a series of powerful magnets, which are switched on and off during a scan to create a collection of magnetic fields. A powerful computer interprets the data collected during the scan to create 2D and 3D images of the body.
An MRI machine is incredibly complex and, to understand how it works, you’ll first need to know a bit more about the human body. Our bodies are mostly made up of water molecules – two parts hydrogen, one part oxygen. Hydrogen atoms are very sensitive to magnetic fields and, in understanding how the scanner affects them, you’ll understand how the machine works.
Imagine a patient lying on the table in an MRI scanner. Before the main magnet is switched on, the millions of hydrogen atoms within our patient’s body are spinning randomly. As soon as the MRI scanner is turned on, its powerful magnetic field makes all the atoms ‘line up’. Because the main magnet runs along the length of the tube, each hydrogen atom either points north (towards the patient’s head) or south (towards their feet).
About the same number of atoms point north or south, but a few every million or so are ‘odd’. Those unmatched atoms are what the MRI scanner uses to create its images.
The secondary magnets within the machine now send out radio wave pulses. The unmatched atoms use the energy of those waves to spin and point in the opposite direction. Once the radio wave pulse is switched off, those odd atoms spin back to their original position and release the energy they just absorbed – this is known as resonance.
As the full name of an MRI suggests (Magnetic Resonance Imaging), the reception of this resonance is crucial. A couple of important factors affect hydrogen’s resonance: its surrounding atoms (which informs the scanner of the type of tissue the hydrogen is part of), and the magnetic fields through which it’s travelling (because the scanner knows where those fields are, it can work out where the resonance is coming from).
Depending on what the doctor is looking for, injectable contrasts or dies are used to show differences in tissue type. These dies affect normal and abnormal tissues slightly differently, making those hydrogen atoms send out slightly different signals.
What MRI scanners are used for
The first MRI scan was performed on a human being on July 3rd 1977. Since then, MRI scanners have become much cheaper to buy and run, and are used in hospitals around the world.
They are commonly used to provide very detailed images of the brain and spinal cord, helping doctors diagnose injury and infections, such as haemorrhages, meningitis and encephalitis, as well as diseases such as multiple sclerosis and Parkinson’s.
MRI scans are also often used to look at other organs in the body, for example the heart, kidney and liver. In particular, these scans are extremely useful when it comes to determining the spread of cancerous cells; that is, to find out if treatment has been successful.
The third main use for MRI scanners is to get a detailed image of bones and joints, in particular the soft tissues that would not be picked up in an X-Ray.
How MRI scanners are different to other imaging equipment
MRIs provide much more detailed scans than CAT scans or X-Rays. They are also much safer because they don’t expose the body to any radiation. However, MRI scanners are more expensive to run and require a patient to lie still for a long time in an enclosed and noisy space.
Due to the strength of the magnets used in an MRI, there are strict precautions. This means, for example, people with pacemakers or metal implants or plates are unlikely to be suitable for this particular procedure.