MRI: the long version

MRI: the long version

 

Science behind the images

Atoms are the basic building blocks of molecules, and so in turn your entire being. The body contains many different types of atoms, but most of you is hydrogen (about 95%, with much of it in the form of water). Hydrogen is the first of the elements and has the most simple structure with a small nucleus. All hydrogen nuclei, including the ones in your body, constantly rotate like little spinning tops. The direction in which they spin is usually random, but when placed in a strong magnetic field they all line up. Their spinning can then be altered by passing radio waves through the body and the scanner detects the signal that this produces. Disturbingly powerful computers then reconstruct this information into a picture that we can interpret.

How does the scanner work?

An MRI scanner is therefore, in essense, a very strong cylindrical magnet combined with a radiotransmitter. Most of the scanners in use have a super-conducting electromagnet cooled with liquid helium, all housed in a vaguely ergonomic tube. The strength of the magnet varies with the type of scanner, typically ranging from 1 to 3 Tesla. This means that the magnet is several thousand times stronger than the Earth's magnetic field.

 

The magnetic field needs to be really uniform, but this is only possible at the centre of the magnet. Unfortunately this means that you have to go inside the magnet, making it seem less ergonomic than first thought. In addition to being close to the magnet, a radiotransmitter loop needs to be near to the area under investigation, so you may find your head needs to be partially enclosed. Once set, the scanner works its magic, detecting the subtly different signals that arise from various different tissues.

More than just anatomy...

There are a number of different types of scans that an MRI machine can perform, many of which are used in our research. The type of scan depends not on changes to the magnet, but on the sequence and frequency of the radiowaves delivered.

 

Structural

Detailed pictures of the human body can be aquired in a relatively short time and viewed from any angle. You don't even have to say cheese.

Functional

When the brain is active it uses more oxygen and so needs more blood in the places that are doing the work. Blood contains iron, which is magnetic, and so the signal detects changes with this increased blood flow. We can't tell what you're thinking, but we can see which part of your brain is thinking it!

 

Spectroscopy

We can use the scanner to analyse the chemical composition of the brain. In our lab we have retuned the scanner to be able to detect lithium as well as hydrogen.

What else does an MRI examination involve?

Coming for a scan can seem daunting, but ultimately most people find the experience to be really interesting. It helps if you know what is likely to happen, and come prepared. As the scanner creates a strong magnetic field you will be asked to remove all metal objects, including watches, credit cards and jewellery (piercings too). It helps if you dress for the occasion, something comfortable without zips or belts.

If you wear glasses these will need to be removed. You can wear contact lenses, but not if they are tinted. We have MRI compatable glasses in most prescriptions, so don't worry about your vision requirements.

You will be shown the MRI suite and asked to lie on a comfortable bed which will then be moved into the scanner. We check the alignment using lasers and it helps if you make a joke about Bond movies at this point. It's unlikely that you will feel anything untoward during the scan, though be aware that the scanner creates a loud rhythmical banging noise when active. This is normal and does not indicate a fault or impending doom. You will be given ear protectors and a choice of music to distract you.

If you are taking part in a functional MRI study, a mirror will be placed above you within the scanner so that you can see the projection screen. You will also be given a response box/finger button type device to record your participation in the various tasks.

 

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