What is a MRI scan and how does it work?

MRI stands for Magnetic Resonance Imaging. MRI is a technique that is used to take high-quality images of the inside of the body by using short bursts of radiofrequency. These are directed at you while you are lying within a cylindrical magnetic field. The bursts of radiofrequency bounce against the tissues of the body and these are recorded as detectable radio signals in the machine. A computer then changes the radio signals into pictures, based on how strong each radio signal is. Different types of tissue make different images – bone usually shows as a dark or black image, while tissue that contains a lot of fat makes a brighter paler image.

How long does it take and is there anything special I must do?
A MRI scan is usually done as an outpatient appointment. It takes between 15 and 30 minutes and during this time you must lie still. There is no need to do anything special before your MRI scan. People who make involuntary movements and children may be given a general anaesthetic.

Are there any risks or problems?
There appears to be no risks with MRI scanning, unlike being X-rayed. However, if you have a pacemaker you shouldn’t have a MRI because pacemakers malfunction within the strong magnetic field. The same is true of a hearing aid, although obviously this can be removed before testing. Other things that might be affected by the magnetic field are small metal implants that might move, such as surgical clips of any sort or inner ear implants. Large metallic implants, such as those in the joints or the skull, are not usually a problem, although they can interfere with the picture that the scan gets.

If you are very overweight you may not be able to fit into the cylindrical magnet, and if you make involuntary movements, the scan will be difficult to do. Some people find it quite claustrophobic. Talk to your doctor if you have concerns about any of these.

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