A radiation-free, computer-assisted imaging technique that uses a strong magnetic field and radio waves

The technique of magnetic resonance imaging (MRI) has been used since the early 1980s to provide highly detailed sectional images of internal organs and structures. These images are created by a computer using information received from a scanner. MRI does not involve potentially harmful radiation; instead, it uses magnets and radio waves.

Although MRI is a relatively expensive procedure and MRI scans tend to take longer than other techniques, it does have several advantages. Images from MRI are similar to those produced by CT scanning, but MRI can distinguish abnormal tissue, such as a tumour, from normal tissue much more clearly. MRI scans can also be taken at a greater range of planes through the body than is possible with CT scanning and therefore can be used to image any part of the body. MRI is radiation-free and is considered to be one of the safest imaging techniques available.

MRI of the lower spine

This MRI scan of a healthy person’s lower spine clearly shows the different structures, including muscle, skin, and bone.

How does it work?

During an MRI scan, you lie inside a scanner surrounded by a large, powerful magnet. A receiving magnet is then placed around the part of your body that is to be investigated. If large areas, such as the abdomen, are to be imaged, the receiving magnet is fitted inside the MRI scanner; for smaller areas, such as a joint, a magnet may be placed around the part to be scanned (see Having an MRI scan).

Your body, like everything else, is made up of atoms. When the atoms in your body are exposed to a strong magnetic field from the large magnet in the scanner, they line up parallel to each other. Short pulses of radio waves from a radiofrequency source then briefly knock the atoms out of alignment. As they realign, the atoms emit tiny signals, which are detected by the receiving magnet. Information about these signals is then passed on to a computer, which builds up an image based on the signals’ strength and location.

MRI images can be enhanced by the use of a contrast medium to highlight particular structures in the body, such as tumours and blood vessels.

What is it used for?

MRI can provide clear images of any part of the body. This type of scanning is especially useful for looking at the brain and for detecting brain tumours. MRI is also valuable for looking at the spinal cord and may be used to investigate lower back pain. Sports injuries, especially in the knee (see Torn knee cartilage), are increasingly being examined using MRI. MRI may also be used to examine the breasts. MRI scans show the location of tumours within the breast tissue more accurately than plain two-dimensional X-rays (see Mammography). In addition, since MRI does not use radiation, MRI scans can be repeated often, allowing doctors to monitor a condition safely.

A particular type of MRI, called magnetic resonance angiography (MRA), enables doctors to look at blood flow by comparing the signals received from stationary tissue with those received from flowing blood. Another type of MRI, called functional magnetic resonance imaging (fMRI), can reveal areas of neural activity in the brain (“brain mapping”). This technique is relatively new and is currently used mainly for research but it could potentially be used diagnostically, for example, to assess the effects on brain function of stroke, tumours, or degenerative conditions such as Alzheimer’s disease.

What are the risks?

There are no known risks or side effects from MRI scanning. The scans do not use ionizing radiation and can be performed repeatedly. However, because the scanner uses a powerful magnet, it may interfere with the functioning of certain devices, such as pacemakers, cochlear implants, hearing aids, and implanted drug pumps. If you have any metal in your body, such as a surgical implant, you should tell your doctor before having MRI. You should also tell your doctor if you have an IUD fitted, because some have metal in them. During a scan, any magnetic metal items in your body will move around under the influence of the magnetic field and could cause serious damage. For this reason, your doctor may request X-rays to look for metal in your body before you have an MRI scan.

Although there is no evidence that MRI scanning poses a risk during pregnancy, doctors do not usually recommend MRI during the first three months, as a precaution.

Procedure: Having an MRI Scan

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

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