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MRI scanThis factsheet is for people who are considering having an MRI scan or for people who would like information about MRI scans. A magnetic resonance imaging (MRI) scan is a special technique that uses magnets and radiowaves to produce two and three-dimensional pictures of the inside of the body. These images can help your doctor to make a diagnosis about a number of conditions. About MRIThe magnetic field produced by an MRI scanner allows radio receivers (placed close to the body) to detect tiny signals from inside the body. A computer turns these tiny signals into two or three-dimensional pictures. MRI may be used to make images of many parts of the body. Different types of tissue show up in different shades of grey on a computer-generated image. Abnormalities can be seen as darker or lighter areas in some tissues. Depending on the part of the body being examined, having an MRI scan may involve being enclosed in a fairly narrow space. If you are worried about this aspect of the scan you should speak to your doctor in advance. Having an MRI scan does not hurt. It does not use X-rays and you are not exposed to radiation.
Why have an MRI scan?An MRI scan can help a doctor make a diagnosis and has many uses including:
Who can have an MRI scan?Not everyone can have an MRI scan. The magnetic fields and radiowaves can seriously affect implants of metal in the body. This includes anyone who has:
Your doctor will discuss with you whether it is safe for you to have an MRI scan. Can pregnant women have an MRI scan?An MRI is not usually performed on pregnant women unless there is an urgent medical reason for the scan. This is because the long-term effects of strong magnetic fields on a developing baby are not yet known. What are the alternatives?Generally speaking, MRI provides a more detailed soft tissue image than other scans, such as a CT scan. In some cases, depending on individual factors such as your symptoms and the specific condition being investigated, there may be alternatives to having an MRI scan. These include:
What happens before the scan?An MRI scan is usually performed in a hospital. In many cases no preparation is required. However, if you are having an abdominal or pelvic, scan, you may be given special instructions in advance about eating and drinking. Before the MRI scan you will usually talk to your radiologist or radiographer about the scan and you may be asked to sign a consent form. This confirms that you understand the risks, benefits and possible alternatives to the procedure and have given your permission for it to go ahead. You may also be asked to fill out a questionnaire which will ensure that it is safe for you to have the scan. Some types of metal can interfere with the scan, so you will be asked to remove metal objects such as coins, keys, and jewellery. It is best to wear clothing that does not have metal zips, buttons or other metal parts such as underwired bras. Relatives or friends who wish to accompany a patient in the MRI room must also follow these guidelines. Contrast mediumDepending on the part of your body which is to be examined, a dye (contrast medium) may be used to make some tissues show up more clearly. These dyes are harmlessly removed from the blood by the kidneys and passed out in the urine. If needed, you will have the dye at the beginning, or sometimes towards the end of the scan. It will usually be given as an injection into a vein in your hand or arm. The scanAn MRI scan is performed as a day case, which means that you can have the scan and go home the same day. An MRI scan is painless so the procedure does not usually require an anaesthetic. However it can be uncomfortable lying still for this length of time, particularly if you have a painful condition, so you may be offered a sedative which will relax you. Small children may also be offered a sedative and sometimes a general anaesthetic. The scan itself can take from 15 minutes to an hour to perform, depending on the specific examination, but you should allow at least two hours for the whole appointment. You will lie on a table which will be moved inside a large "tube", which generates the magnetic field. You may need to enter the tube head-first or feet-first depending on the part of your body that is being scanned. Generally the part of your body to be examined will be placed in the middle of the tube. The tube is open ended so you will not be enclosed at any time. A radiographer operates the scanner from behind a window, and will be able to see and hear you throughout the procedure. You may be given a call button to hold during the scan which you can press to get the radiographer's attention at any time. It can take several minutes for each picture to be taken, and it's important to lie very still and breathe quietly during the process. The machine will make quite a loud knocking or buzzing sound so it may help to wear earplugs. In some units, music is played to help you relax. When the scan is complete, the table will be moved back out from the scanner. What to expect afterwardsOnce the examination is over, most people can resume their normal activities immediately. However, if you have had a sedative, you will need to arrange for someone to drive you home and then stay with you for the first 24 hours. If you have had a dye injection, it's a good idea to drink a lot of water for the next 24 hours to help flush the dye out of your body. The pictures taken by the MRI scanner will be interpreted by a radiologist, who will send the results to your doctor or specialist. Deciding whether to have an MRI scanAn MRI scan is a commonly performed and safe procedure. For most people, the potential benefit is greater than the disadvantages. However, in order to make a well-informed decision and give your consent, you need to be aware of the possible side-effects and the risk of complications. Side-effects are the unwanted but mostly temporary effects of a successful procedure. The MRI scan itself does not usually have any physical side-effects. However, if a contrast medium is used, you may experience a temporary flushing when you are given the injection. This usually passes within seconds. People who have metal fillings in their teeth may also feel a tingling during the scan. Complications are unexpected problems that can occur during or after the procedure. Most people are not affected. Complications that may arise from an MRI scan are related to the contrast medium, and include an allergic reaction to the dye. Different people may have different reactions to the dye - let the radiographer know if you feel uncomfortable or are short of breath during the scan. Allergic reactions happen only occasionally and can be treated immediately with appropriate medicines. Further information
Sources
Reviewed by Peter Hobden, Principal Neuroradiographer, Oxford University. Published by BUPA's health information team, healthinfo@bupa.com, September 2006.
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