MRI – a brief technical explanation
MRI is a diagnostic scanning technique which uses no radioactivity or X-rays and thus is why it's often described as being a safe modality.
The human body is predominately made of water molecules which contain hydrogen protons. When you are placed in a strong magnetic field, the tiny magnetic fields of the hydrogen protons (which normally move around randomly) are aligned to the magnetic field.
A brief radio pulse is rapidly switched on and off. This makes the protons' magnetic fields spin round in unison and emit a weak radio signal. By altering the timing of the radio wave applications it's possible to produce images which show up the various body tissues as shades of grey. The contrast on MRI images is very good and the operator can change the parameters to give images that demonstrate the anatomy of the area and also, in many cases, highlight common disease processes.
MRI and safety:
Do have an implant, a medical device or metal in your body?
An MRI must not be performed on people with certain implants in their bodies due to the powerful magnet that's used in the scanner.
Because of this, when you arrive at the scan department, you'll be asked to fill out a safety questionnaire to verify your personal safety. Please ensure that you answer the questions carefully and accurately. It's absolutely essential that you don't enter the scan room if you have a heart pacemaker.
It’s important that you tell us before the scan if you:
- are (or might be) pregnant.
- have a heart pacemaker or an artificial heart valve
- have any electro-mechanical devices used for drug delivery
- have any surgical clips
- have a cochlear implant, a neurostimulator or a programmable hydrocephalus shunt
- have had any operations on your head
- have any metal implant in your body
- have had an injury to your eyes involving metal or metal fragments
Could you have metal fragments in your eyes?
If there is a possibility that you might have metal fragments in the eyes – as a result of a penetrating injury, or from working with metal at high speed – you may need to have an X-ray to ensure that there are no particles present. This is because the MRI magnet can exert a pull on small fragments of metal. Such metal fragments can remain unchanged for many years – so we will need to be absolutely sure, regardless of how long ago a possible injury might have occurred.
Do you have reduced kidney function or allergies to medicines?
In some instances, you may need an injection to enhance the images. The injection is unlikely to have side effects but you should have a blood test (U&E) in advance with your GP to check your renal (kidney) disease. Please tell the MRI staff if there was any problem with this U&E test before any injection.
Traditional X Ray contrast media often contains iodine; MRI contrast media usually contains gadolinium or iron oxide combined with other products to reduce toxicity and make them safer to inject. However, if you have any concerns about your injection or have any serious allergies, particularly to previous injections of MRI contrast media (gadolinium or iron) please let us know in advance.
Before you arrive
- We make sure that it’s appropriate for you to have an MRI scan. We’ll ask you some basic questions when you book and you’ll be required to complete a questionnaire before your appointment. Unless we let you know otherwise, you don't need to make any special preparations before the scan. You can eat and drink as normal and take any prescribed medicine.
- Please wear clothing without zips or metal buttons, and leave jewellery and watches at home if possible.
- If requested, please confirm your appointment by phone before your scan and arrive in plenty of time.
- Please let us know if you have any disabilities so that we can ensure we are able to offer you the highest quality service. You’re welcome to bring a friend or a relative with you, but for safety reasons we won’t normally allow them into the examination room.
Preparation for the scan
- Once you’ve checked in at reception (don’t forget to bring your appointment letter), a member of the radiography team will meet you, explain the procedure, go through your safety questionnaire with you.
- We might ask you to change into a hospital gown. We’ll provide somewhere to store your personal possessions. You’ll be looked after by the radiography team throughout the procedure – they will explain what’s happening and will be there if you experience any discomfort.
- You’ll have the opportunity to ask any questions about the scanning process.
- We might need to give you an injection (known as a contrast medium) to increase the amount of information we can get from the scan, depending on the area we're scanning.
During the scan
- The examination consists of several scans, each lasting a few minutes with a short pause between each. The whole procedure will take between quarter of an hour and one hour – depending on which part(s) of the body we’re scanning.
- We’ll ask you to lie down on the scanner bed and we'll make sure you're comfortable so you can stay as still as possible. You won’t feel anything, but there is some mechanical noise from the equipment –, we’ll provide you with some ear defenders or ear plugs. For safety reasons, we don’t normally allow anyone accompanying you to come into the examination room whilst the scan is in progress. However if this is necessary we will have to carry out the same safety checks as we have done for you.
After the scan
- There are no restrictions on normal activity – you can eat and drink normally, drive and return to work immediately after the scan.
- If we’ve given you a contrast injection we will check you before you leave the scanner. A Radiologist will examine the images shortly after your visit and send a report to your Doctor or Consultant, normally within a few days.