Virtual Medical Centre

MRI (Magnetic Resonance Imaging)

• How does MRI work?
• Preparing for an MRI Scan
• During an MRI
• Benefits of MRI
• Risks of MRI
• Limitations of MRI

How Does MRI Work?

An MRI scanner is a cylindrical machine, used to get images of the human body. An MRI machine consists of a round tunnel within where the patient lies on a narrow table. An image of this is seen to the right. Surrounding the tube is a large cylindrical magnet. During an MRI Scan, the patient is within a stable magnetic field which is 10,000 - 30,000 times stronger than the earth's magnetic field. Protons are tiny particles that are present in water molecules throughout the body. These are aligned by the incredibly strong magnetic field, noting that there are no water molecules in the human skeleton, only in bodily tissue. Radio waves are transmitted in pulses, and these protons produce echoes that are emitted out of the body. These echoes are received by the MRI scanner, and are then reconstructed into images of the body by a computer. These images are very precise and give a clear anatomical view of the body from any angle.

Preparing for an MRI Scan

The presence of a strong magnetic field means the metal objects of any kind are not permitted within the scanning room during an MRI Scan. All jewellery and clothing containing metal, particularly objects containing iron need to be removed. Internal metal objects such as metal clips, medication pumps, or any internal metal items such as shrapnel or metal particles also present a considerable risk and must be made known to the doctor. Other equipment that may cause a risk include cardiac pacemakers or defibrillators; a catheter with metal components; aneurysm clips and cochlear implants. Some metal implants do not rule out using an MRI scanner, but some do and it is important to tell a doctor about any implants or metallic objects. If the patient suspects they may be pregnant, the doctor must be informed as little is known about the effect of MRI scans on an unborn baby. To improve the experience of the MRI, it is advised that the patient not drink for several hours before the scan. This is especially true of coffee and tea, as going to the toilet is not possible without interrupting the scan and beginning again.

During an MRI

Once you are dressed in a medical gown, you will then be asked to lie down on a narrow table which then moves into the tunnel. An MRI can last from 30 minutes to an hour, depending on the scans being performed. The scan consists of sequences, lasting from 2-15 minutes, during which the machine makes knocking noises, which can be loud. Patients are often provided with earphones to listen to music to distract them from these noises during the procedure. It is essential that the patient must remain motionless in order to produce a clear picture. For some patients claustrophobia may be an issue as the space within the MRI can be confining. Subsequently those with claustrophobic anxiety and children may need light sedation. Through out the procedure imaging technicians are able to communicate with the patient via intercom to ensure that the patient is informed and comfortable.

Benefits of MRI

The MRI scan is a painless and safe scan that produces clearer images of the body and its tissues, at any angle. This is particularly useful in detecting soft tissue tumours throughout the body. An MRI is nearly twice as sensitive as X-ray mammography in detecting breast cancer in women that have a high genetic risk of the disease. It uses no radiation for scanning and therefore eliminates the health risk of x-rays that do use radiation.

Risks of MRI

While an MRI scan is a relatively safe procedure as there is no damaging radiation involved, there are still several risks. If the patient is pregnant, or suspects they may be, a doctor should be informed because the effects on an unborn baby are poorly understood. There is also the risk of patients being injured if they forget to remove pieces of metal from their body or their clothing. There have been cases where patients have been injured due to metal left behind by the previous patient. If sedation is required due to claustrophobia, then there are associated risks of over-medication. If a contrast dye is used, which helps to show up some parts of the body more clearly, there is a small risk of allergic reaction.

Limitations of MRI

An MRI is a very expensive and time consuming investigation compared to other methods such as x-ray and CT scan. Some parts of the body, like bone, are better examined using simpler techniques such as an X-Ray. An MRI may not always be able to tell the difference between some disease processes. It is also not a very good investigation for emergencies or accidents because of the long time it takes and the fact that all equipment has to be removed from the room while the machine is running.