More facts about MRI

MRI – a brief technical explanation

Magnetic Resonance Imaging (MRI) is a diagnostic scanning technique based on the principles of magnetic resonance. MRI uses no radioactivity or X-rays which is why it's often described as being such a safe modality.

The human body is predominately made of water molecules which contain hydrogen protons. When you're 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.

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 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.

MRI and safety

MRI has been in use since 1977 and it has the major advantage that it doesn't use ionising radiation (e.g.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.

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.

If you've ever had metal fragments in your eyes due to a penetrating injury or worked with metal at high speed, you may 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 the time elapsed since any metal related injury makes no difference - metal fragments can remain unchanged in the body for years.

MRI scanners can be noisy

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.

You might 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 unlikely to have 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 to reduce the chances of this occuring.

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.

The results of your scan

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 view required has been taken during the procedure.

The images are then reported by a Radiologist - this is a Doctor who specialises in the interpretation of diagnostic images. The report is usually completed within a few days of the scan and is sent to the referring Clinician.  Images are also sent if requested 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.

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.