A closer look - MRI
An MRI scan will not say for certain whether the person has epilepsy or not. But alongside other information, it might help the specialist to decide what the likely cause of the seizures is. Here we look at MRI in greater technical detail.
Magnetic Resonance Imaging - MRI
MRI is a technique used to create an image or scan of any part of the body. MRI scans look at the structure and function of the person’s brain (how their brain is made up and how it works). In people with epilepsy it can be used to see if there is an obvious reason for their seizures. This might be a scar or lesion on their brain that can be seen on the image. However, many people have brain lesions without having epilepsy, and many people with epilepsy do not have any scars or lesions on their brain.
Lesion – any abnormality or damage in the tissue of any organ, such as the brain.
How does MRI work?
Basically the MRI scanner uses magnetic fields and radio waves to create an image of the brain.
To understand how MRI works, we need to know a little bit about atoms. Atoms are the tiny particles which make up all types of matter and everyday objects. Atoms themselves are made up of three even tinier particles called protons, electrons and neutrons. The protons and neutrons make up the atom’s nucleus (centre), which is surrounded by the electrons.
The human body is mainly made up of water, and water contains hydrogen atoms. Hydrogen atoms have a special property known as ‘spin’, which is like a tiny magnetic field. MRI works by using this property of hydrogen atoms. This means that the nucleus of the hydrogen atom responds to radio waves that are produced by the MRI scanner, and this causes the nucleus to produce an MRI signal.
At the centre of the MRI scanner is a strong magnet, made up of coils of wire. An electrical current is passed through the coils to create a magnetic field. The coils are often ‘supercooled’ (cooled to a very low temperature) which allows the current to keep flowing through the coils (as long as they stay at this temperature). This means that the magnetic field created by the scanner is always ‘switched on’, which is why the MRI is so noisy.
At the start of the scan the person is given ear-plugs and lies on a couch with their head supported in a frame to help keep it still. The couch is then moved inside the scanner. The scanner is usually shaped like a cylinder or tube, and the person's upper body lies inside the tube, inside the coils of the magnet. At this point the nuclei of the hydrogen atoms in the body are not organised in any particular direction. They are randomly arranged.
Once inside the scanner, the magnetic field causes the nuclei in the body to ‘line up’ in the same direction as the magnetic field. This is called the ‘equilibrium position’.
Next, radio waves (pulses of electromagnetic energy) are created by the MRI scanner. The energy is absorbed by the nuclei in the body, which causes them to move away from the direction they were lined up in, becoming disturbed from the equilibrium position.
The radio signals are then switched off, and the nuclei return to line up with the magnetic field again, returning to the equilibrium position. To do this, the nuclei release energy as radio waves. Different types of tissue in the body (such as the muscles or brain) are made of different substances and have different densities. Because of this, the nuclei of different tissues return to equilibrium at different times. The radio waves produced by the nuclei are picked up and measured by the MRI scanner, and are used to create the picture of the body or brain.
How are these signals used to create a picture?
The MRI scanner uses a computer to make complicated calculations to generate a picture from the strength and location of the radio wave signals. The strength of the signal is shown as different shades of grey.
There are many different types of MRI scan, using different types of radio-frequency pulses, and these give different types of images.
How powerful is an MRI scanner?
At Epilepsy Society we have a 3 Tesla MRI scanner. The earth’s magnetic field is about 0.5 gauss (a unit of magnetic induction) and 1 Tesla = 10,000 gauss . So our MRI is about 60,000 times more powerful than the earth’s magnetic field!
Uses of MRI
The type of MRI technique described above is used for 'structural imaging' – where an image of the brain is created to see how it is made up. Structural imaging is used to look for a potential structural cause of someone’s epilepsy, such as a scar on the brain. However, for many people with epilepsy, no structural cause for their epilepsy can be found, and so their MRI results are said to be 'normal'.
Other MRI techniques have many other uses; showing us how our brains work, and what functions or activities each area of the brain is responsible for.
Another use of MRI is for functional imaging. Functional MRI (or fMRI) is used to look at the brain while the person is resting or doing a task, to see which parts of the brain are involved in the task and how they are working. For example, this might be when the person is looking at pictures, thinking of words, or making physical movements.
To do an fMRI scan, the normal MRI machine is used but in a special way. It is set up to look at how blood flows around the brain. The parts of the brain that are involved (or ‘active’) during a task have more blood flowing to them than other areas of the brain that are not involved in the task. During an fMRI scan, these areas of the brain that are active, and therefore have an increased blood flow, show up as bright colours on the scan images. This can show us which parts of the brain are involved in different types of tasks and activities, and how different parts of the brain work together.
It can also be used for people who are being considered for epilepsy surgery. In epilepsy surgery, the part of the brain that is causing seizures to happen is removed, to try and stop seizures from happening. With fMRI we can look at what this part of the brain does, and what effect removing it by surgery might have.
Spectroscopy is used to look at the amount of different chemicals in the brain. By studying these chemicals, we can see which chemicals are used when the brain works, and can also see how brain activity can be affected by seizures.
DTI and tractography
Diffusion tensor imaging (DTI) uses another different type of MRI technique. Normal MRI (described above) uses the presence of water in the brain to make images of the brain structure, and DTI looks at the movement or flow (known as diffusion) of water around the brain. This flow of water can show how the different areas of the brain are connected and whether any parts of the brain are damaged. For example, if a part of the brain is damaged, there may be ‘spaces’ between the neurones which are filled with water, and these will show up on the scan.
Neurones – the scientific name for nerve cells. The brain is made up of millions of neurones. Neurones control all of the body’s functions by communicating using electrical signals. These electrical signals can be picked up on an EEG.
Tractography is a technique which uses the information from DTI to work out how water flows through the brain by imaging nerve fibres - groups of neurones bunched together. So this shows how brain areas are connected and how information travels through the brain, with different parts of the brain working together.
MRI or CT scans – what’s the difference?
CT (Computerised Tomography) scans use X-rays (which are invisible radiation). MRI scans use magnetic and radio waves, not X-rays. MRIs tend to be clearer and produce more detailed pictures than CTs, and MRIs have no known side effects. However, some people are not able to have MRI scans, for example if they have a pacemaker but can have a CT scan instead. MRI scans, unlike CT scans, can also take pictures from different directions angles or ‘planes’ (for example from ear to ear, from the back of the head to the face, or from the top of the head towards the chin). The images can be either two-dimensional (like a square) or three-dimensional (like a cube).
Taken from our A closer look - MRI factsheet. Download this factsheet using the link below.