What an MRI scan is:

MRI stands for magnetic resonance imaging. This type of scan uses magnetism and radio waves to build up a picture of the inside of the body. 

MRI is completely painless, but the scanner is very noisy. The MRI scanner creates cross section pictures of the body. It can show up soft tissues very clearly and a single scan can produce many pictures from angles all round the body. The pictures can be affected by movement so they aren't used very often for some tumours because coughing, swallowing or breathing will make the scan less clear.
MRI can be used on most areas of the body. For some parts of the body and for some types of tissues, it can produce clearer results than a CT scan. For other situations, the CT scan is better. Your own doctor will know which is the best type of scan for you.
MRI is particularly good for some types of brain tumour, primary bone tumours, soft tissue sarcomas and for tumours affecting the spinal cord. 
Your doctor may suggest an MRI scan as part of an urgent referral. In some situations, your doctor may suggest MRI if a CT scan hasn't been able to give all the information they need. In some early cancers, such as cervix or bladder cancer, MRI is better than CT at showing how deeply the tumour has grown into body tissues. It can be particularly useful for showing whether the tissue left behind after treatment is cancer or not.
As well as being used to find or stage tumours, MRI can be used to measure blood flow. You may have an injection of a special dye before the scan to help make the pictures clearer.

The photo above shows an MRI scanner. It is a large cylinder with a couch that can move backwards and forwards through the cylinder. The pictures are taken inside the cylinder. The MRI is in a protected room to keep out radio waves as these interfere with the scan.

Preparation for the scan:

Some MRI scans need preparation beforehand. If so, you will be told about them in plenty of time. You can take all your medicines as normal beforehand. You may be asked not to eat or drink for at least an hour before your scan. Some stick on patches for medicines (transdermal patches) may cause burns if they contain metals because the MRI scan can make the patches overheat. So let the MRI staff know beforehand if you normally use patches for medicines. The staff can work out how you can safely take off the patches before the scan if necessary.

Having an MRI scan:

 Once you have checked in at the scan department, the receptionist will ask you to take a seat in the waiting room until the radiographer or an assistant calls you. At this point, you will have to complete and sign a checklist for safety. The checklist asks you about any medical conditions you may have or operations you have had, mainly those that may involve any metal implants. This includes pacemakers, surgical clips, pins or plates and cochlear implants (for deafness).
If you have your ears (or any other part of your body) pierced, you will have to take out your rings or studs before your scan. If you have a contraceptive coil (IUD) fitted, you must tell the radiographer as some have copper wire in them. You should also tell them if you think you may have any metal fragments anywhere in your body - this sounds odd but is common in people who do metalwork or welding for a living. If you think this is possible in your case, you'll need an X-ray before the MRI to check.
When it is time for your scan, you may first go to a cubicle to take off your outer clothing down to your underwear and put on a hospital gown. Don't forget to remove jewellery, watch and hair clips. If you are just having an MRI of your head, you may not have to undress.
The scan uses a very powerful magnet. If you still have your clothes on, it is best to empty your pockets. You need to remove all coins, keys and credit cards so it might be easier to take everything out. Because there is no radiation, it is safe to take a relative or friend into the scanning room with you if you like. They will also have to take off everything metal. Remember little things like hair slides, cufflinks and belt buckles.
When you are ready, the radiographer will take you into the scanning room. You will need to lie down on the MRI couch. Most MRI scans are done with you lying on your back. You have to lie as still as you can, but breathe normally. Your radiographer will explain any instructions beforehand. You may have to hold your breath at points during the scan so that you move as little as possible. Any movement can blur the scan.
The radiographer may offer you ear plugs or headphones to wear. MRI scans are very noisy. The noise is like a loud clanging inside the cylinder and it goes on throughout the scan. The radiographers prefer that you wear some sort of ear protection to protect your hearing and for your own comfort. And you may be able to listen to music while the scan is going on.
Once you are positioned in the scanner, the radiographer will go out to the control panel. They will be able to see the scanning room through a window or on a TV screen and you can talk to each other through an intercom. They will tell you that they are about to start the scan and remind you to keep as still as possible.
When the scan is over, the radiographer will come back into the room and help you down from the couch.
An MRI scan can take anything from 30 minutes to an hour and a half. Lying still for that long can be uncomfortable. If you are getting stiff and need to move, tell the radiographers through the intercom. It is very important to keep as still as you can, otherwise parts of the scan may have to be done again and may take longer. You should be able to go home as soon as the scan is over.
Some people feel a bit claustrophobic (closed in) when they are having a scan. If you think you are likely to feel this way, contact the MRI department before the day of your scan. If necessary, you can have a tablet or injection to calm you. If your radiographers know that you are nervous, they will take extra care in making sure you are comfortable and understand what is going on. Keeping your eyes closed sometimes helps. And in some centres it may be possible for you to take in some favourite music that the radiographers can play through your headphones to help pass the time.

Injections before the scan:

You may have an injection of a dye (called contrast medium) just before the scan. This helps to make the body organs show up more clearly on the scan. You have the injection through a small plastic tube (cannula) in your arm. This is usually kept in for about 15 minutes after your scan, incase you have any problems after having the injection. The radiographer removes the tube before you go home.
The contrast injection may cause side effects in some people, but these are usually mild and only last a short time. Some of the more common side effects include pain at the injection site, feeling or being sick, skin reactions, a headache or dizziness.

Is MRI safe?:

MRI is very safe because of the checklist you fill in beforehand. There are some people who cannot have an MRI but the checklist will make sure this is picked up. The scan uses magnetism, so metal will affect it. If you have certain types of metal surgical clips, metal pins or plates or a pacemaker inside your body, you cannot have an MRI scan.
Very rarely, someone has an allergic reaction to the injection of dye. The reaction most often starts with sweating, rash and difficulty breathing. The doctors and radiographers will know what to do if you have this type of reaction. 
Generally you won't be given an MRI scan if you are pregnant, particularly during the first 3 months, but it can be done if absolutely necessary. Contact the MRI department before your appointment if you are pregnant or breast feeding.

The results:

It can take time for test results to come through. How long will depend on why you are having the scan. Usually, a specialist in MRI sees the scan and dictates a report to be typed up. The report is then sent to your specialist, who gives the results to you. If your GP has sent you for the test, the results will be sent directly to the GP surgery.
Understandably, waiting for results can make you anxious. It usually takes a couple of weeks for the results to come through. If your doctor needs the results urgently, they will make a note of this on the scan request form and the results will be ready sooner. Try to remember to ask your doctor how long you should expect to wait for the results when you are first asked to go for the test. If it is not an emergency, and you have not heard a couple of weeks after your test, ring your doctor's secretary or GP to check if they are back.