The MRI Scan

Frequently Asked Questions

What are the principles of MRI?

Magnetic Resonance Imaging (MRI) is a diagnostic scanning technique based on the principles of nuclear magnetic resonance. The term "nuclear" has long been dropped from the description of medical MR techniques because of its association with radiation and radioactivity. In fact, MRI uses no radioactivity or X-rays which is why it's often described as being such a safe modality. The basic principles are as follows:

• The human body is predominately made of water molecules which contain hydrogen protons
• When you're placed in a strong magentic 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. This is happening at a microscopic level, so you'll feel nothing at all
• 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 demonstate the anatomy of the area and also, in many cases, highlight common disease processes
• This contrast (grey scale) depends upon whether the hydrogen protons are linked to fat, muscle, water, etc. Depending on the area scanned - and, what your doctor wants to find out - the contrast can be altered slightly.  This is done by giving an injection of an agent into a vein that makes it easier to visualise certain structures
• During the scan, the main magnetic field strength is altered using electromagnets. This causes a wide range of sounds during the scan procedure including banging, buzzing and rumbling noises. You'll be offered ear protection during the scan and, in some centres, music is available

Tell me more about safety

MRI has been in use since 1977 and it has the major advantage that it doesn't use ionising radiation (X-rays). However, because the scanner contains a powerful magnet, it must not be performed on people with certain implants or metal fragments in their bodies.

What if I have an implant, medical device or metal fragments in my body?

If you have a device such as a heart pacemaker, aneurysm clip, a cochlear implant, electromechanical devices like drug delivery systems, surgical clips or certain varieties of metal artificial heart valves you should tell the MRI staff about this immediately. This is because 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 screening form to certify 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.

If you've ever had metal fragments in your eyes due to a penetrating injury or worked with metal at high speed, you'll need to have an X-ray of your eyes to ensure there are no particles still remaining. This is important because even small fragments of metal can be pulled strongly by an MRI magnet. And please remember that that the time elapsed since any metal related injury makes no difference - metal fragments can remain unchanged in the body for years.

What kind of noises does the scanner make?

MRI scanners use large electromagnets some of which are rapidly switched on and off during the scan procedure. Although these magnets aren't visible, the sound they make can be quite loud!

The sounds made by the scanner typically vary in volume and tone, depending on the type of procedure being performed. We've collected some of these noises for you to hear - just click on the boxes to the right.

Sincere thanks to John Talbot for the creation of our scan-sound jukebox!

 

What happens if I need an injection?

In some instances, you may need an injection to enhance the images, usually in parts of the body which have poor natural contrast. Most of the time, the injection is given in the vein in your elbow (the same place as a blood test). The injection is highly unlikely to have serious side effects.

If you have known renal (kidney) disease please tell the MRI staff before any injection. Please note that this isn't the same kind of injection which you may have had before during X-ray procedures such as CT (cat scanning). Traditional radiological 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. The composition and low dose of MRI contrast injections leads to very few adverse reactions.

For patients who already have kidney problems, there is an extremely small risk of developing a condition called Nephrogenic Systemic Fibrosis.  We undertake screening via a blood test to reduce the chances of this further.

Sometimes, the contrast agent may be given as a drink (in abdominal scan procedures) or as a very small injection into a joint (in orthopaedic scans). Again, there are usually no noticeable side effects to worry about. As always, 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.

How and when do I get my results?

During an MRI scan there may be literally hundreds of images taken; and remember that this procedure doesn't use X-rays so from a safety viewpoint there is no real limit to the number of images we can get.

These digital images are stored onto a computer hard drive and the operator will check the images for technical quality and to ensure that every necessary view has been taken during the procedure.

After all the images have been acquired and checked for technical quality, they'll be arranged into logical groups and printed onto a transparent film, similar to the ones used in conventional radiographic (X-ray) imaging. This allows the images to be reported or sent to clinic.

The reporting is done by a radiologist - this is a doctor who specialises in the interpretation of diagnostic images. The report is usually done soon after the scan, often on the same day, and the films may also be sent to the referring clinician or consultant.

The report will be sent to, and available at, your follow-up appointment with the consultant that referred you for an MRI scan. As you can appreciate, reporting the scan is a process which requires time and expertise. So please don't ask the radiographer for a result at the end of your scan, because they're simply unable to give a report at this time.

And remember, just because the radiographer can't give an answer, this doesn't mean that there is anything wrong with you or the scan.

What do the images look like?

We've collected some images from teaching libraries so that you can see the kind of images that the MRI scanner creates - take a look here to find out more.