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Magnetic resonance imaging (MRI) uses a very strong magnet and precisely programmed radio signals to distinguish between the different types of tissues in your body. A computer converts this information into pictures of your internal anatomy.
Unlike nuclear medicine and CT imaging, MRI does not use radioactivity or X-rays. It is a safe and completely painless procedure, but it may be a bit noisy.
MRI can also provide views of the body in different planes, such as this saggital view of the head (left). This view is not available using CT
MRI cannot be performed on certain people, such as those who have cardiac pacemakers or certain types of surgical clips, metal pins or plates. For these patients, exposure to the very strong MRI magnet would not be safe. If you have had any previous surgery (including eye surgery or brain surgery) or if you have a pacemaker, make sure that your doctor knows about it. Obese patients may be too large to fit into a standard MRI scanning machine, but a few centres have machines that can accommodate larger persons.
You will be shown to a cubicle and may be asked to take off your outer clothing and put on a hospital gown. If you are having a head scan, you may not need to remove your clothing. You must remove all metal objects such as belt buckles or watches as well as all jewellery.
The MRI is a large machine that has a circular tunnel built through it. You will be asked to lie on your back on the scanning bed, which can be moved back and forth into the MRI tunnel. When the MRI scan is being carried out, expect to hear some loud noises from the machine. Some MRI departments will offer you earplugs or a stereo headset to wear to block this noise.
In some cases, you will need to have an injection of a contrast agent or “dye” to make the images clearer and abnormalities easier to detect. This dye is usually gadolinium-DTPA and is different from the contrast dye used for CT scans. It does not cause any side effects.
The MRI scan typically takes between 30 and 60 minutes to complete. It is difficult for some who suffer from claustrophobia or who have trouble lying flat for that long a time. If you think you might be someone who would feel anxious about being closed in, tell the radiographer before you have your scan. It may be necessary to take a tablet or have an injection to make you feel calmer.