Diagnostic tests - MRI

Diagnostic tests - MRI

In our last back2basics we looked at how an EEG is used to help diagnose epilepsy. Another test that is often used is the MRI or magnetic resonance imaging scan.

An MRI scan will not say for certain whether the person has epilepsy or not. But alongside other information, these might help the specialist to decide if epilepsy is a likely cause of the seizures.

Magnetic resonance imaging - MRI

MRI stands for magnetic resonance imaging, which is a technique used to create an image or scan of the brain. MRI scans can be used to look at the structure of the person’s brain (how their brain is made up). In people with epilepsy it can be used to see if there is an obvious reason (structural cause) 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.

How does MRI work?

Basically the MRI scanner uses magnetic fields and radio waves to create an image of the brain. But of course it is not that simple!

Atoms and protons – the science bit!

To understand how MRI works, we need to know a little bit about the tiny particles that make up the cells in our bodies. 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 each hydrogen atom responds to radio waves that are produced by the MRI scanner, and this causes the nuclei to produce an MRI signal.

Why is this important?

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’.

At the start of the scan the person having the scan is moved inside the scanner. The scanner is usually shaped like a cylinder or tube, and the person 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 strong?

AT Epilepsy Society we have a 3 Tesla MRI scanner. The Earth’s magnetic field is about 0.5 gauss (1 Tesla = 10,000 gauss). So our MRI is about 60,000 times more powerful than the earth’s magnetic field!

Other uses of MRI

The type of MRI technique described above is used for structural imaging – that is creating an image of the brain to see how it is made up. Structural imaging is used to look for the cause of someone’s epilepsy. But different 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.

Functional MRI

Another use of MRI is for functional imaging. Functional MRI (or fMRI) is used to look at the brain while the person is 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. The fMRI scan shows up these areas of the brain that have an increased blood flow (the areas that are active and involved in the task) as bright colours on the scan images. This means that we can look at which parts of the brain are involved in different types of tasks and activities, and also see how different parts of the brain work together.

For someone with epilepsy, this type of scan can be used to look at which part of their brain is affected by their seizures, and what happens to their brain activity when seizures happen. 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 will have.

Spectroscopy

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

DTI (or diffusion tensor imaging) uses another type of MRI technique. Normal MRI (described on the previous page) uses the presence of water in the brain to make images of the brain structure. 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.

Tractography is a method which uses the information from DTI to work out the directions of nerve fibres. It shows how individual nerve fibres are connected and how they, and parts of the brain, work together.

MRI or CT scans – what’s the difference?

CT (computerised tomography) scans use X-rays (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 or ‘planes’ (e.g. from ear to ear, from the back of the head to the face, or from the top of head towards the chin). The images can be either two-dimensional (like a square) or three-dimensional (like a cube).

Glossary

Atom: the smallest unit of matter, made up of protons (positively charged particles), electrons (negatively charged particles) and neutrons (uncharged particles).

Epilepsy surgery: an operation done to try and stop someone’s seizures by removing the part of their brain that causes the seizures.

Functional imaging: when scans are used to look at how the brain is working (how it functions).

Hydrogen: a chemical element (the most abundant element in the universe). Hydrogen atoms, along with oxygen atoms, make up water.

Magnetic resonance imaging (MRI): a type of brain scanning technique that uses magnetic fields.

Nerve fibres: groups of neurones, bunched together.

Neurones: the individual cells that make up the nervous system. Also called nerve cells.

Structural cause: a physical cause for a person’s epilepsy, such as a lesion, tumour or scar tissue.

Structural imaging: when scans are used to look at the structure of the brain.

© Epilepsy Society
July 2007