Magnetic Resonance Imaging (MRI) is a diagnostic scanning technique that uses a strong uniform magnetic field (the large tube part of the scanner) and series of short bursts of radiofrequency waves to produce three-dimensional images. Generally MRI looks at the behaviour of the tiny magnetic moments present in the nuclei of almost all atoms. Clinical MRI specifically looks at the behaviour of the nuclear magnetic moments of hydrogen atoms: hydrogen atoms are everywhere in our bodies, mostly in the form of water but are also present in fat, muscles, tendons, bones, nerves, etc. The magnetic moments of hydrogen atoms behave ever so slightly differently in these different tissues and that allows one to distinguish the various different body tissues as shades of grey in MRI with often very high resolution.
How it works in principle
When your body, or parts of it, is placed in the strong uniform magnetic field, the tiny magnetic moments in the nuclei of the hydrogen atoms ‘feel’ the presence of the strong magnetic field and partially align with it (think of a strongly magnetic compass needle which aligns itself with the earth’s weak magnetic field). A short sharp burst of radiofrequency waves (a ‘pulse’) lifts this alignment, after the pulse the magnetic moments start re-aligning with the magnetic field. While doing so, they themselves emit weak radiofrequency signals which are picked up by antennae in the scanner. Now, that would give us a magnetic resonance signal but it would not be possible to read any information about the location of different tissues, etc, from the signal because the strong magnetic field is very uniform.
Something else needs to be done to also include information about location – a little bit like creating a giant three-dimensional jigsaw puzzle made up of thousands of small parts (voxels; think of your TV or digital camera – the two-dimensional images there are made up of thousands of small parts (pixels) too). In MRI, encoding information about location is achieved by switching on/off additional (weaker) magnetic fields, called gradients, which essentially label signals with information about the location from where the signal stems. The rapid switching of these gradients are the loud banging noises that you will hear during the scan. MRI scans typically follow recipes of complicated simultaneous series of radiofrequency wave bursts and switching of gradients, creating thousands of little pieces of information which afterwards are put together in a computer to construct a three-dimensional image.
In practice, the area of your body the doctor would like imaging will need to be placed in the centre of the big magnetic field. In addition, some hard plastic cover may have to be placed around the part of your body being scanned. This plastic cover carries additional radiofrequency wave equipment and gradients (‘coils’) to enable the creation of optimum images.
Is it safe?
Yes; the radiofrequency waves used in MRI have very low energy and thus do not damage body tissues (in contrast traditional X-rays or CT scans use high energy (ionising) radiation). However, MRI must NOT be performed on people with certain metallic implants in their bodies such as pacemakers, cochlear implants, some surgical clips and some artificial heart valves (such metallic implants may be strongly magnetic themselves or may be seriously disturbed by the strong magnetic field of the scanner and/or the scanning process).
Will I need an injection?
Some patients may need an injection of contrast medium, usually given in the vein in your elbow or back of your hand. The purpose of an MRI contrast medium is to selectively highlight certain aspects of the scans. An MRI contrast medium typically consists of molecules with magnetic properties (different from contrast media used in X-ray or CT scans).
These molecules are eliminated from your body by your kidneys, hence drinking plenty of water after the scan helps your body to remove the contrast medium swiftly from your body. The function of your kidneys will have been assessed by a blood test before your scan.
How long will it take?
Typically it takes about 20-30 minutes to complete a scan. The exact duration of a scan depends on the kind of tissue and the part of the body being investigated. There are two main reasons why it takes quite some time to collect the data for a complete MRI scan:
- the tiny magnetic moments of atomic nuclei only yield weak signals, and
- the response of these tiny magnetic moments in producing measurable signals is rather slow.
Magnetic resonance research laboratories all over the world currently search for methods to greatly boost the response of the tiny nuclear magnetic moments so that hopefully in the future MRI scans will take considerably less time to complete.
Does it hurt?
No, an MRI scan is a painless procedure. But you will have to be able to keep still and lie quite flat during the scan so that clear images can be recorded. You may find the loud banging noises of the scanner unpleasant but you will be offered earplugs or headphones to minimise the noise disturbance. Some people find confinement in the rather narrow scanner, together with additional equipment placed close to the head, a little claustrophobic.
The best advice really is to relax and ignore this environment as best you can – it is not in any way dangerous, just not particularly ‘nice’. Recently MRI scanners with open magnets have become available. These scanners may feel less claustrophobic but they deliver lower quality images.
Next section: MRI