MRI

WHAT IS AN MRI SCAN

In simple terms, Magnetic Resonance Imaging (MRI) is a modern imaging technique that is useful in the diagnosis of many common illnesses and injuries. Unlike conventional radiography, MRI uses low energy radio waves to create the images rather than x rays. This makes the technique much safer to use, with far fewer restrictions on when and where it can be employed as a diagnostic tool.



The scanners themselves consist of large cylindrical magnets (housed in large plastic covers) that either surround the patient or are positioned above and below the patient in "open" scanner designs. The magnets used are very powerful - about 300 times stronger than a fridge magnet in fact!


More detailed/scientific description
If you are interested in the mechanism behind MRI, it is a fairly complex concept, but essentially it relies on the fact that a human body contains a lot of hydrogen. About half of your body weight is due to the water content of your cells - slightly less in women and rather more in men. Water contains hydrogen and oxygen atoms and as a result approximately 62% of your body is made up from hydrogen atoms.

The importance of a hydrogen atom in MRI imaging is the fact that it is a tiny single particle which has its own microscopic magnetic field. When you are positioned for your scan, these magnetic fields naturally align themselves to the magnet MRI6inside the scanner. You will not feel anything because the human body is not sensitive to magnetic fields of this magnitude, so you can lie back and relax. Interestingly, even when you are keeping still, the magnetic fields of your atoms are in constant motion - rather than just pointing in a particular direction they wobble about! If you have ever seen a spinning top or gyroscope you will be able to picture this very well. A spinning top rotates very quickly, but in addition to this you will notice that it wobbles slowly around on its own axis in a circular path. In the case of your hydrogen atoms, the speed of wobble can be calculated according to the field strength used by the system.

The second part of the process involves the radio waves mentioned earlier. Every scanner has a built-in radio transmitter that is designed to "broadcast" radio waves at exactly the right frequency to match the wobbling magnetic fields of the hydrogen atoms. This is where the term "resonance" comes in. A great explanation for what happens is to imagine a musical tuning fork. If you have ever used a tuning fork you will know that when you strike it against something it vibrates at a certain frequency. This is the exact frequency required to tune your musical instrument. An interesting thing happens when you have two tuning forks of the same size. If you hold the vibrating fork close to the other - they start to vibrate in unison. This transfer of energy is defined as "resonance".

The scanner broadcasts a radio wave at the same frequency as the wobbling magnetic fields of your hydrogen atoms and transfers a small amount of energy to them. The amount of energy is fairly small and carefully calculated by the system for your personal body size (weight), so you should feel nothing. You may feel a little warm, but this is quite normal.

The final part of the process is what happens after the radio waves are momentarily switched off. The hydrogen atoms lose the energy that they have been given, and by placing a radio antenna close to the part of the body being examined the scanner can detect a very weak signal created by the billions of atoms inside the area being scanned. The receiver coils used don't look like radio aerials, in fact they are specially configured to fit the part of the body being examined. They come in all shapes and sizes, cylinder shaped for around the knee, cup-shaped to fit over the shoulder wrap-around designs for imaging the abdomen and long flat receivers that are built into the patient couch for imaging the spine.

MRI scanners are surprisingly noisy, some being louder than others. The sounds range from loud knocking to a rhythmic vibration during the acquisition of your images. This is simply due to the way the system works and is nothing to be alarmed about. The noise level is exactly equivalent to that experienced in an aircraft cabin during a flight - so just like on a plane you will be offered ear defenders for your comfort. Some scanners also offer music via headphones during the procedure. Don't be tempted to tap your feet along with the music - during your examination you will need to keep nice and still - just like when you have a photo taken. If you move about the images will be blurred and will lose their diagnostic value.
Having an MRI scan
How do I prepare for my scan?

When you arrive, a member of staff will complete a safety checklist with you to make sure that you can be scanned, and will answer any questions that you may have. There is usually no need for any special preparation before your scan. On the day of your scan MRI1you will probably be able to eat and drink as normal and take any medicine you have been prescribed - if not, we will tell you when you book your appointment. You will need to remove all metal objects such as zips, jewellery, hair grips and so on before entering the scan room because of the powerful magnet that is used to scan. Credit cards are also affected by the magnet - you can leave these and other valuables in the lockers provided. Please do not wear mascara if you are having a scan of your head or eyes as it contains fine metallic particles which affect the pictures.

We will ask you to change into a gown if you are wearing clothes that contain metal. You may prefer to arrive for your scan in clothes which do not contain metal, for example you could wear a zipless tracksuit or an elasticated skirt. If you do this you will not have to change for your scan

What should I expect?
A specialist MRI radiographer will position you on a comfortable couch, which slides into the scanner. You will be asked to keep still while we scan you and produce your images. MRI is a completely harmless procedure, but the scanner is very noisy when it is taking pictures so we will provide you with ear protection. We will be able to play music during your scan so you may like to bring a CD with you to listen to.

There are no after-effects from your scan so you can carry on with all your normal activities (driving, for example) straight away.

How long will it take?

This depends on which part of the body is being scanned. Most scans of one area are completed in 20-30 minutes. Although we try to keep to appointment times, we don't always know beforehand exactly how long a particular scan will take. This means that occasionally an appointment time may be slightly delayed and we would appreciate your understanding if this happens to you.

Is it safe?

There is no potentially harmful radiation used for your MRI scan, so it is considered to be very safe. However, because the scanner uses a strong magnet it is important that you tell us if you have certain implants or if you have had certain operations. If you have a pacemaker, arterial clips, a neurostimulator, an artificial heart valve, brain clips, a cochlear implant or some sort of metal fragment in your eyes you CANNOT be scanned. Implants such as knee or hip replacements, dental work and gold/silver rings are perfectly safe

Will I need an injection?

Occasionally we will need to give you an injection of contrast (a dye which makes blood vessels and organs show up more clearly) to give better images. This is normally given into the arm or the back of the hand by a qualified member of staff.

What if I am pregnant?

There are currently no known dangers from MRI, although we do not carry out scanning in the first three months of pregnancy unless essential.

We can make arrangements for your individual needs and comfort, for example we can sometimes allow family members into the MRI area with you, or we may be able to find more comfortable positions for you to be in while you are being scanned.

Don't worry about your scan. We are here to help. If you have any fears or doubts, don't hesitate to talk to one of our staff. If you would like to visit the department before your scan to have a look around, please contact us.

Results

Your referrer should receive your scan images and report within a week. The referrer is the person who has sent you for your MRI (for example, your GP or Physiotherapist.) You should make an appointment with your referrer to discuss the results. Dr Udeshi will not be able to give you your results on the day of your scan unless you are visiting the one-stop mri service.
Frequently Asked Questions
Q. Does it hurt?
A. No

Q. Is MRI safe?
A. MRI is very safe when used properly. Powerful magnets are not harmful to humans, but of course they can forcefully attract any magnetic metal items which could be dangerous if they fly through the air. This is why you will be asked to remove metal items before the scan. MRI scanners can also cause problems with certain implanted devices in the body, i.e.pacemaker malfunction. It is important therefore that you follow the instructions given by our staff - even if you have been scanned before. Tell them if there is any chance that you might be pregnant, and if you have any metal inside your body - either due to surgery or accidental injury.

Q. Will you be able to hear me during the scan?
A. Yes, even though the procedure can be fairly noisy the scanner has a built in two-way intercom system. We can see you, hear you and speak to you at all times during the examination. You will also be provided with a special button that you can press if you need anything. In certain extreme cases you may be permitted to take a friend, parent or partner into the examination room. It is important that you don't chat during the scan however as this can sometimes cause movement, resulting in blurred images.

Q. Does the MRI scan give me a dose of X-ray radiation.
A. No, MRI uses radio waves and therefore there is no restriction or limitation in the case of repeat scanning. Radio waves are very low energy compared to X-rays, that is why they are safe to use in music and TV broadcasting, Wi-Fi, mobile networks, Bluetooth and radio-controlled toys. Your body is exposed to radio waves 24 hours a day for your entire life.

Q. My friend told me that the scanners make you feel rather enclosed, is this true?
A. Not really, modern MRI scanners are fairly open and brightly lit, especially the open scanner that we use at Cobalt. In the early days of MRI the scanners needed to be fairly large with long narrow tunnels, but happily those days are gone. Modern scanners have a comparatively short 70 cm aperture and we even have a scanner that is open on all sides. If you have ever had a CT scan (Computerised Tomography scan) you will hardly be able to tell the difference in terms of the equipment design. To put it into perspective MRI scanners are much less enclosing than the average tanning salon sun-bed!

Q. How long is the MRI examination?
A. That depends on the type of scan being performed. The shortest examinations take less than 10 minutes, for more detailed examinations or where there is more to cover the exam time can be 40 minutes or longer. The important thing to remember is that a longer scan-time does not mean that the scanner has found something wrong.

Q. I find it difficult to lie flat, will I need to keep still?
A. Yes, you will need to keep still, but let the radiographer know if you are unable to lie flat. If you move during the scan it can affect the quality of the images; meaning they may not be able to be used, but if the radiographers are aware that you find it difficult to lie flat or keep still they can help you.

There are sometimes short gaps between the individual parts of the scan and again this is normal. Dr Udeshi will need to look at the images taken to use as a landmark for the next set of images. This takes a little while each time - so relax and don't be tempted to move or change position as this may cause the following scan images to be in the wrong location.




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