How does it work?
The powerful magnet of the instrument affects the body’s hydrogen atoms contained in body water. The hydrogen atoms line up in the magnetic field. Radio waves are then passed through the body and alter the position of some of the hydrogen atoms. When the radio waves are switched off the hydrogen atoms move back to their original position. When they do this they give out small radio waves of their own that are detected by the instrument. From these radio waves, the MRI instrument is able to produce a detailed picture of the body. Parts of the body with little water, like bone, show up dark while areas containing a lot of water, like fat, show up bright. The machine produces cross sectional pictures along the body and can produce pictures at all angles through the body. Usually two dimensional pictures are taken but it is possible to take three dimensional pictures. It can show up soft tissues very clearly.
What it is used for?
It is commonly used to look at the brain, spinal cord, pelvis and abdomen, and to investigate injuries to bone, joints and soft tissue. MRI scans are particularly useful for looking for tumours.
How is it carried out?
MRI scans are usually carried out as an outpatient procedure. The patient lies in a cylindrical chamber open at both ends. It is important that all metal containing objects are removed, such as watches, jewellery, belts, studs etc. Also the doctor needs to know if the patient has any surgical clips in the body or if the patient uses any electrical appliances like an internal hearing aid (cochlea implant) or pacemaker as an MRI scan will not be possible in such cases. It is important that the patient lies perfectly still during the test. Movement during the test will make images less clear. Because of this children may be given an anaesthetic. Some patients may find the chamber claustrophobic and need to tell the doctor, in advance, if they are anxious about this as the doctor may give a mild sedative. Sometimes an injection is given of a contrast medium, a substance that helps produce clearer images. You can have a person sit with you during the test but they also must have no metal objects on their person. The test takes from about half an hour to an hour and a half.
What are the risks?
There are no known side effects to MRI scans. The test is not painful and cannot be felt. The machine makes a banging noise that can be unpleasant but the patient is usually offered headphones to help reduce this unpleasantness. You can even bring in your own music for the radiographer to play through the headphones. The test can be repeated as radiation is not used. The test is not carried out in women less than 12 weeks pregnant.